Introduction

Pyrus calleryana is often called the Bradford Pear after the cultivar that was planted widely in the US in the 1960s and 1970s and still sometimes is today. However, the Bradford Pear itself is an infertile cultivar, hence the more appropriate common name of the Callery Pear. The Bradford Pear cultivar can neither self-pollinate or cross-pollinate with the same cultivar as its seeds are infertile. However, if different cultivars of Pyrus calleryana are grown within 100 m of each other they can cross-pollinate and produce fertile offspring. The resulting Callery Pear is a fast-growing aggressive tree with a broad ecological tolerance, rapid growth, few pests and early sexual maturity. It produces abundant tiny fruits with abundant tiny seeds. The fruits are hard and woody but soften after a hard frost and become appealing to birds which then spread their seeds around. It quickly replaces native vegetation wherever its seeds have a chance to grow. Once established, removing or controlling Callery or Bradford Pear can be challenging.

Do not buy any cultivars of Pyrus calleryana because they may cross-pollinate and spread in the wild. The Bradford / Callery Pear story is a perfect example of why we should not buy supposed “sterile cultivars” of invasive species because we never know how they will behave until they have already escaped into the wild. Supposed sterile cultivars of Scotch Broom and Purple Loosestrife have also done a similar thing. Instead, research plants that are native to your area and plant those. Native plants tend to be naturally resistant to pathogens and insects, require little watering or feeding, and promote biodiversity and wildlife values. Also, there are always lovely native plants everywhere that are just as beautiful as invasive cultivars, if we just look for them we will find them.

Description of Pyrus calleryana

Leaves & Stems

The Callery Pear is a deciduous tree that can grow 5 – 20 m tall and has a trunk that is up to 0.6m in diameter. The trunk, however, often splits into multiple branches that tend to break easily in storms damaging the tree and shortening its usefulness as a landscape tree. Its branches are reddish-brown when young and become grayish-brown with age with vertical grooves in the older bark.

Pyrus calleryana simple leaves are arranged alternately on the stem. They are 4 – 9 cm long and are on 2 – 4.5 cm long petioles. Leaf blades are ovate, broadly ovate or oblong–lanceolate. The surface is shiny green glabrous (hairless) with undulate (wavy) and slightly toothed margins. In fall the leaves turn yellow, orange, and red before falling off for the winter.

Leaf shape can be variable and these indicate the different varieties of Pyrus calleryana. For the purpose of invasive species removal or control, identification to the species level alone is sufficient. There will always be variations in morphology because the wild species are all hybrids of various cultivars and as such will generally exhibit intermediate characteristics between its two parent varieties.

Flowers & Fruits

Callery Pear flowers in April and May just before the leaves appear. The flowers are malodorous smelling somewhat of rotten fish. Flowers are 2.5 cm wide with 5 white obovate petals 6 – 7 (–13) mm long and × 6 – 7 (–13) mm wide.

Pyrus calleryana has numerous stamens with white filaments and topped with reddish or brownish anthers. Its ovaries are 2 – 3 (–4)-locular and they are topped with 2 or 3 styles.

Fruits are small pomes (like an apple), around 1 cm in diameter and globular (round) in shape. They are green when young in late spring and summer but turn brown when ripe in the fall and winter. They contain numerous cyanide-laced seeds inside.

Similar Species Frequently Confused With

There are numerous species of mostly Pyrus and Prunus (plum and cherry) species that Pyrus calleryana could be confused with as well as some other genera. All genera it can be confused with are part of the Rosaceae family. Sometimes people confuse it with Populus species as well but the appearance of the flowers in the spring will easily distinguish it from those.

• Pyrus communis – the Common Pear has very similar flowers and leaves but its mature bark has horizontal and vertical grooves in it making it more of a checkerboard-like pattern than the vertical striations of Pyrus communis. Its leaves are usually smooth-margined rather than undulate. Also, the fruits grow much, much larger, are pear-shaped and edible.
• Crataegus spp – there are multiple Crataegus species in its range in North America that, like many Rosaceae, have very similar flowers. However, Crataegus species always have variously lobed leaves compared to the simple unlobed leaves of Pyrus calleryana.
• Prunus avium – the Wild Cherry has a similar range, similar leaves and flowers. Its bark is smoother and shinier, lacking any longitudinal or horizontal grooving. The smooth bark does, however, possess numerous horizontal lenticels (slightly raised striations). Its fruits are also larger and of varying shades of red or sometimes yellow when ripe. The fruits have a single pit and are edible.
• Prunus mahaleb – the St. Lucia Cherry or American Black Cherry has a similar range, leaves and flowers as well. However, it never grows to more than a large shrub up to 5 m tall. Its bark lacks the grooves of Pyrus calleryana and it has horizontal striations. Its bitter fruits are also black when ripe and possess a single pit.
• Prunus mexicana – the Mexican Plum has a similar range, leaves and flowers as well. However, its filaments on its many stamens are often pink, the flowers are often pinkish-white, and its fruits are larger, plum-like and pinkish to purple when ripe rather than brown.
• Prunus cerasifera – the Myrobalan Plum or Plum-Cherry has similar flowers, leaves and range. However, its flowers are either white or pink and the anthers are typically yellow rather than reddish or brownish. The fruits are yellow to reddish in color when ripe and are edible and larger, to 2-3 cm in diameter.
• Prunus serotina – the Capulin or Black Cherry has a similar range, leaves and flowers too but the flowers grow in elongated racemes instead of umbels. Also, its bark is more smooth and covered with horizontal striations. The somewhat bitter fruits are a bit larger and are reddish-black to black in color with a single pit as opposed to brown with multiple seeds.
• Viburnum prunifolium – this Rosaceae member has similar leaves and range as Pyrus communis. However, its flowers are much smaller, to 0.9 cm in diameter, they only have 5 stamens, and they are arranged in tighter umbels with more flowers.

Native Distribution of Pyrus calleryana

The Callery Pear is native to Asia in China, Japan, Korea, Taiwan and Vietnam.

Habitat Types Where Bradford Pear is Found

Callery Pear can be found growing in empty lots, mixed forests, forest edges, thickets and plains. It can grow from 0 – 800 m in elevation. It prefers full sun but can tolerate part shade. Pyrus calleryana does not grow in full shade.

Pyrus calleryana is tolerant of a very wide range of soil types and pH. It easily tolerates the poor soils and polluted conditions of living in urban environments. It can grow in poor and rich soil alike. Callery Pear tolerates a range of soil conditions and will tolerate temporarily water-logged soils and temporary droughts. However, it will not survive extended periods of flooding or extended periods of drought.

Human Uses of Bradford Pear

Callery Pear Pyrus calleryana as Ornamental or Tools

The Bradford Pear was widely planted in the USA in the 1960s and 1970s as a fast-growing and low-maintenance landscape tree. As a landscape tree, it produces abundant white flowers in spring and has red foliage in the fall making it a desirable ornamental despite the malodorous flowers. Callery Pear is often used as a rootstock to graft fruit cultivars onto.

Callery Pear wood, like all Pyrus wood, is very fine-textured and sometimes used for making musical instruments, veneer, woodcuts, or other fine wooden products.

Distribution of Pyrus calleryana in North America

The species was first brought to North America in 1909. It was later widely planted in the 1960s and 1970s in the USA.

In Canada, Pyrus calleryana has not yet been recorded on Canadensys. However, there have been reports on iNaturalist in southern Ontario, Nova Scotia and possibly in southern BC. It is not clear if those are all planted or wild specimens. Given that it is invasive just south of the border in these locations it seems likely that it may start spreading more into Canada soon.

In the USA, Callery Pear has been introduced in Oklahoma, Texas, Arkansas, Louisiana, Mississippi, Alabama, Georgia, Florida, South Carolina, North Carolina, Tennessee, Kentucky, Illinois, Indiana, Ohio, Pennsylvania, West Virginia, Virginia, Delaware, Washington DC, Maryland, New Jersey, New York, Connecticut, and Massachusetts. On iNaturalist it is also reported in Colorado and all along the west coast but most of these may be intentional plantings of cultivars that may or may not yet have escaped into the wild.

In Mexico Pyrus calleryana so far has only been reported in Tamaulipas. Given its close proximity to the northern Mexican border on the US side, it is likely that it will spread more in the near future.

How Callery Pear Spreads

Callery Pear is primarily spread through long-distance by deliberate human introductions as garden and landscape ornamentals. While the Bradford Pear itself is infertile it can cross-pollinate with other cultivars of Pyrus calleryana and the Asian Pear Pyrus betulifolia which can then produce fertile seeds and spread in the wild. Also, fertile varieties are often used as the rootstock when grafting. If the infertile crown of the tree is damaged sometimes the fertile rootstock will then grow and produce fertile fruits.

Short-distance dispersal occurs primarily through birds and humans. The hard fruits become soft after a frost and are a popular winter food source for winter birds in North America. The birds eat the winter fruits and spread the abundant fertile seeds in their feces. Even without fruits Callery Pear also reproduces short distances vegetatively through suckers.

Humans can also spread the seeds short distances unintentionally on their shoes or clothes. Often the disposal of unwanted trees or branches removed from pruning while in fruit results in infestations from yard waste piles when they are not burned or properly disposed of.

Habitats at Risk of Invasion in North America

Callery Pear often forms dense patches in abandoned fields, empty lots, disturbed areas and on roadsides. It can invade any riparian area, open mixed forests, meadows, thickets, mountain slopes and anywhere else its fertile seeds land and grow. Since it is tolerant of such a wide range of soil conditions this means that many habitats are at risk.

It generally will not invade dense forests, particularly coniferous ones. Also, it typically does not invade permanent wetlands except where drier conditions exist. It does not tolerate severe drought so will not invade desert areas.

Impacts of Invasion by Pyrus calleryana

Pyrus calleryana often creates dense near monoculture stands, particularly in disturbed areas, roadsides and old fields. It can also gain a significant foothold in open mixed forests. They out-compete the native vegetation and significantly reduce the biodiversity in the area upon infestation. Once a dense stand is created it can be difficult to remove.

Potential Benefits of Invasion

Birds enjoy Callery Pears abundant fruits throughout the winter, including many native birds. However, the birds are the primary dispersal method and would likely have eaten native fruits left on trees that were displaced by the Callery Pear.

Methods to Remove Bradford or Callery Pear

As always prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in most local garden stores. Do not buy or transport any Pyrus calleryana cultivars. Even though Bradford Pear themselves may be infertile they can cross-pollinate with other cultivars and escape into the wild. Do not plant it in your yard. Instead, research native species to your region. Native species require little or no maintenance, are very beautiful and provide wildlife and biodiversity values.

If you see them being sold online or in your local garden stores please inform them of their invasive status and ask them to do their part and cease selling them. Inform them that though supposedly infertile they are still able to cross-pollinate with other cultivars and they are spreading in the wild. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species.

Physical Control of Bradford or Callery Pear

Once already established physical control is always the most effective means to remove Callery Pear. Physical control is labor-intensive and time-consuming but it causes the least amount of environmental damage.

The best time to remove Callery Pear is in the early spring either before or during flowering but before the plant sets fruit to avoid dispersing the seeds around.

Physical methods to remove Callery Pear depend on the size. Young seedlings or trees can simply be pulled out. When the ground is moist it makes removal easy as the roots cling less tightly to the soil when it is moist. Young trees can be pulled out with a weed-puller like the one shown in the affiliate ads below.

Mature trees on the other hand will need to be cut down and then the root pulled out by digging it with a shovel or machine, depending on the size. If it is too difficult to remove the root then you can deal with it one of three ways. First, you can solarize the stump with a plastic tarp by laying the tarp over the stump and weighing it down with rocks. Leave it like that at least until the following year. The second method is to return to the site every year and cut any sprouts that try to grow. Eventually, it will starve out the root. The final method is to chemically treat the stump with a herbicide.

Another method to control large trees in areas that make them difficult to remove is to ring the bark. Using a very sharp knife cut into the bark and remove a horizontal patch at least 5 cm wide all the way around the tree. Be sure to remove all bark in this patch down to the heartwood. This prevents the tree from transporting nutrients and water up and the products of photosynthesis down. This effectively starves the tree so that it will eventually die. It would still need monitoring however to be sure that no new sprouts try to start from its stump.

Disposal of the Trees Once Removed

If you have plants that have seeds on them they must either be burned or solarized. Do not dispose of them in yard waste if they have mature fruits. Large trees can also be cut into firewood. If you do not have a fireplace and are not allowed yard burning in your area then you will need to solarize them before disposal. To solarize put the trees under a thick black tarp and leave them in the full sun for a good 6 – 10 weeks at least to be sure that all seeds are no longer viable. If there are no fruits on the trees you can dispose of them in yard waste.

Chemical Control of Bradford or Callery Pear

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. It is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. Furthermore, there are no chemical control methods that effectively target only Pyrus calleryana. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used species riches significantly increases.

However, the one suitable exception is the chemical treatment of large root stocks that you are unable to dig out mechanically or by hand. Ideally solarize or return to the area as part of an ongoing monitoring process. If you are unable to do either then chemical control of Callery Pear stumps can be a good alternative as you are only treating the stump to prevent re-sprouting. Research herbicides safe for use in your area and treat only the stump. If it is in a riparian area you may not be able to use this method due to proximity to water. Ideally you should still return the following year to be sure that nothing survived.

Biological Control of Callery Pear

Biological control involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800s to help control the rat. To this day there are still rats in Hawaii but the mongoose has helped to decimate many native bird populations.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. This is because the control agent would need to effectively destroy over 99% of plants and their seeds to actually control the Callery Pear on their own.

Unfortunately, Pyrus calleryana seems resistant to pests of all kinds. It is part of what made it desirable as a landscape plant. Also, finding suitable pathogens is tricky because if they could be found even testing them in the field could put commercial Pyrus and Prunus crops at risk. While insects and grazers alike will feed on the pear, particularly when young, they are ineffective as a control measure on their own. And once Pyrus calleryana reaches a significant size large herbivores will no longer be able to reach the foliage.

However, young stands of Callery Pears could be controlled with the use of goats which are known to eat anything. Provided that all the trees are young and can be reached by the goats they can be penned in. They will continuously feed on the young trees and over time will destroy any sprouts or seedlings that emerge. This is only suitable in dense young stands, however, as the goats will also eat any native species in their vicinity. The goats would need to remain in the patch for a couple of seasons.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form this involves physical removal methods, possibly biological control methods, replanting and ongoing monitoring.

Replanting With Native Species is Crucial

In most cases of removal, the site would ideally be replanted with native species to prevent invasive species from re-establishing in the bare soil that was left behind. A replanting program should already be planned and ready to implement immediately upon the removal of Callery Pear. You would need to research plants native to your area and study the site to determine which species would be most suitable. Ideally, gather seed stock from the surrounding area to ensure you have local ecotypes suitable to the area. If this is not possible then buy from a nursery specializing in native species to ensure that you are not planting cultivars of native species.

Ongoing Monitoring is Essential

In all cases of invasive Callery Pear removal, ongoing monitoring is absolutely essential. Yearly monitoring programs should be put in place to ensure that any surviving individuals are removed. This is required whether the area is replanted or not. Callery Pear is aggressive and prolific and can out-compete planted vegetation. Yearly monitoring to remove young plants before they have a chance to become established can prevent this. Yearly monitoring in late spring followed by removal of all sprouts and seedlings will be sufficient for a monitoring program.

References and Resources

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

Eflora of North America http://www.efloras.org/browse.aspx?flora_id=1

iNaturalist Plant Search https://www.inaturalist.org/home

USDA Plants Database https://plants.sc.egov.usda.gov/home

Wikipedia on Pyrus calleryana https://en.wikipedia.org/wiki/Pyrus_calleryana

Willis, Lyrae (2022).  Plant Families of North America. Not yet published.

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If you are able to donate so that I can continue this non-profit work of supplying people with scientific information on the plant families, native plants, and invasive species found throughout North America, please donate using the GoFundMe link below. Thank you!

Introduction

Kudzu is the collective term for several Pueraria species, members of the Fabaceae family. There are multiple species and varieties that can be difficult to distinguish and there is some debate as to whether we even should, or just lump them into one or two more variable species. Regardless of the species debate Pueraria species are highly aggressive and invasive weeds in most places where they are introduced. I have only seen them a few times myself when I was in the eastern USA but there they grow over top of everything, including the other aggressive native and non-native vines found there. Kudzu was encouraged to be planted by the US government in the 1940s for erosion control. Now millions of dollars are spent annually trying to control or remove Kudzu. It is often referred to as “the vine that ate the south”.

Description of Pueraria montana

Leaves & Stems

Kudzu vines are part of the Pueraria genus in the Fabaceae (legume) family of the Fabales order of dicot flowering plants. The most common species in North America is Pueraria montana var lobata, but many people still refer to it as Pueraria montana. The distinctions are subtle, and the vines in North America of all Pueraria species are able to hybridize and distinctions between them become hazy. Once identified to the species level, for the sake of treating it as a noxious weed at least, further classification is rarely necessary and may be left to a professional. The characteristics described here define Pueraria montana in all its variants and subspecies forms.

Pueraria montana is a herbaceous perennial vine that will climb over anything when it is able or sprawl across the ground where there is nothing to climb. It has strong stems that are 0.6 – 2.5 cm in diameter and can be up to 30 m in length. They are incredibly fast-growing, up to 25 cm per day or 18 m in the growing season where the conditions are right.

Kudzu grows from enormous tubers that are up to 2 m long and 18 – 45 cm wide that can weigh up to 180 kg on mature individuals. Like many Fabaceae, it is capable of nitrogen fixation through its root nodules allowing it to grow in poor soil conditions.

Kudzu has pinnately arranged trifoliate leaves 8 – 20 cm long and 5 – 19 cm wide. Its leaflets are ovate to orbicular and are either entire or lobed. Leaflets are pale green above and paler to grayish-green below. They are usually hairy on the abaxial (under) side.

Flowers & Fruits

The fragrant pea-like flowers are reddish, purplish or bluish in color and frequently have yellow patches. Flowers grow from the leaf axils in elongated mostly unbranched inflorescences 10 – 25 cm long. Each flower is 2 – 2.5 cm wide.

The fruits are flattened oblong pods that are 4 – 13 cm long and 0.6 – 1.3 cm wide. They are green when young but turn brown with golden hairs when they mature. The flattened ovoid seeds are visible through the pods and are 4 – 5 mm long by 4 mm wide. They are reddish-brown when mature. While Kudzu is fully capable of pollinator-assisted sexual reproduction it relies mostly on vegetative reproduction to spread.

Similar Species Frequently Confused With

It is difficult to confuse Kudzu with other genera due to the large ovate trifoliate leaves. Sometimes people mistake English Ivy Hedera helix or Poison Ivy Toxicodendron radicans because they have a similar range. However, neither of these are from the Pea family and will have very different flowers. Also, their leaflets in general are much smaller and seldom to never ovate in shape. The only other vine of the pea family that it could be mistaken with is Amphicarpaea bracteata described below.

There are 2 Kudzu species in the Americas, and one of the species has a variant that is similar enough to simply be a variant and in most cases identification down to the variant level is unnecessary. Some people believe Pueraria montana var lobata is the one whose leaflets are lobed, but Pueraria montana itself may also have lobed leaves. Differences are subtle, and many hybridize, so we will leave Pueraria montana identification to the species level only here. The other species is often referred to as Tropical Kudzu and it has a much more limited distribution. It can be differentiated as follows:

• Tropical Kudzu Pueraria phaseoloides – is only found in a handful of locations in the eastern USA and in Veracruz, Mexico. Its flowers are white and purple instead of red, blue or purple with yellow. Its leaflets tend to be much smaller, but can sometimes grow as large, and they tend to be triangular but occasionally may be ovate. And they can also hybridize with the other Pueraria species, further blurring the lines of identification.
• American Hog-Peanut Amphicarpaea bracteata – native to the eastern USA and grows in similar habitats as Kudzu and has trifoliate leaves that look similar, but are usually much smaller. Its flowers are smaller and fewer and are usually white or light pink, never red, blue or purple. Its fruit is usually much smaller and is never golden-hairy.

Native Distribution of Pueraria montana

Pueraria montana is native to East Asia, southeast Asia and some Pacific Islands. It is common in China and Japan.

Habitat Types Where Kudzu is Found

Kudzu can grow in most habitats including forests, riparian areas, roads, fields, along fence rows, and any abandoned lots, buildings, waste areas etc. It is often seen hanging off of trees it has killed, telephone poles, abandoned buildings, vehicles, etc. In China it is often seen on road embankments and in mountains where the land cannot be cultivated.

Kudzu is an opportunistic vine that grows in almost any soil type excluding very wet soils or those with high pH. It can easily grow in nutrient-poor sandy or clayey soils though it prefers well-drained loam.

Human Uses of Kudzu

Kudzu as Food

The large tubers of Kudzu are frequently eaten as a starchy root vegetable. Kudzu leaves, shoots and flowers are often steamed or pickled and eaten as a vegetable.

Kudzu vines are often used as a nutritious forage for livestock.

Kudzu as Medicine

Kudzu root, flower, and leaf have all been used to make medicines, especially in China since at least 200 BC. It is used to treat a variety of ailments including heart and circulatory problems, high blood pressure, chest pain, sinus infections, colds, hay fever, flu and skin problems including allergies, itchiness and psoriasis. It is also sometimes used to treat menopause, muscle pains, measles, dysentery, gastritis, fever, diarrhea, stiff neck, polio, encephalitis, migraine, diabetes and traumatic injury.

Kudzu is a common treatment for hangovers from too much alcohol and has been used in China and Japan for this since 600 AD. It is still widely used today to reduce the symptoms of hangovers including headaches, upset stomach, vomiting, and dizziness.

Kudzu as Ornamental

Kudzu is often used as a garden ornamental. It is sometimes also used to stabilize slopes. Due to its noxious nature, this is done less often now, but still occasionally happens.

Distribution of Pueraria spp in North America

Kudzu was first brought to the USA from Japan at the Japanese pavilion in the 1876 Centennial Exposition in Philadelphia. It was extensively planted in the US in the 1930s and 1940s to control erosion. While multiple species have been introduced and have spread the most abundant species in North America is Pueraria montana var lobata.

In Canada, Pueraria montana has so far only been recorded in Ontario and was only discovered in 2009. Given that it is in Washington state just south of the British Columbia, Canada border it is likely only a matter of time before it appears in British Columbia.

In the USA, Kudzu is found in Washington, Oregon, Nebraska, Kansas, Oklahoma, Texas, Missouri, Arkansas, Louisiana, Illinois, Indiana, Ohio, Kentucky, Tennessee, Mississippi, Alabama, Georgia, Florida, South Carolina, North Carolina, Virginia, Washington DC, West Virginia, Maryland, Delaware, Pennsylvania, New York, New Jersey, Connecticut, Massachusetts, and Maine. It is also found in Hawaii, the Virgin Islands, and Puerto Rico.

In Mexico Pueraria spp so far has been reported in Pueblo, and possibly in Oaxaca, Guerrero, Nayarit, Jalisco and Veracruz but the identifications could not be confirmed without flowers.

Kudzu has now been introduced on every continent except Antarctica.

How Kudzu Spreads

It is primarily spread through long distance by deliberate human introductions as a garden ornamental or landscape plant. It can be ordered online or purchased in local garden nurseries in some states. It is regulated as a pest in Canada and more difficult to purchase, though online sales are seldom regulated.

Seed dispersal is responsible for a small fraction of its spread. Seeds transported in soil will result in some short distance dispersal. However, short distance dispersal occurs mostly through vegetative reproduction as it is capable of rooting from its stem anywhere it touches soil of any quality. It can grow substantially in subtropical areas, up to 18 m in a single growing season. Yard debris piles where careless discarding of vines removed from yards is a significant source of introduction into the wild as is escaping from the yard itself to adjacent land.

Habitats at Risk of Invasion in North America

Any open forest, forest edge, field, abandoned lot, riparian area, etc is at risk of invasion by Kudzu, particularly if they are located next to a source of Kudzu. Because Kudzu primarily reproduces by vegetative spread this slows the rate of invasion quite significantly in terms of distance. In local areas, however, it can rapidly overtake everything.

Kudzu so far has spread widely throughout eastern USA but it will likely spread more north into southern Canada, though so far its range is limited to southern Ontario. Southern Quebec and the Atlantic Provinces are also at risk. It will not invade wetlands or deserts as it cannot tolerate permanently wet soils or extended periods of drought. It can handle drought, however, as long as they are not too severe. It may not invade much of the central North America due to harsher winters. However, with climate change that is unpredictable.

There is still a lot of habitat suitable for it particularly on the west coast of North America including all of the western USA as well as British Columbia, Canada, and parts of Mexico. So far early detection has controlled its spread in these regions.

Impacts of Invasion

Kudzu is a large and aggressive vine that grows over all other vegetation it encounters smothering it and killing it creating monocultures. Even trees are eventually smothered and suffer from loss of photosynthetic capacity until eventually they die. Kudzu, therefore, results in dramatic losses of biodiversity in the area of infestation, negatively affecting native vegetation as well as wildlife that would otherwise feed, nest, or grow on the native plant species it displaces. Once introduced it is notoriously difficult to control.

Potential Benefits of Invasion

Kudzu has often been used to stabilize slopes and does a good job of it. Also, it fixes almost all of its nitrogen and because of the die-back of its leaves it contributes a lot of nitrogen to the soil. After Kudzu is removed, providing you are successful in removing it, the soil is often richer as a result. However, the fact that it smothers and kills all other plant life growing in its vicinity outweighs its benefit as a soil stabilizer and enricher.

Methods to Remove Kudzu

As always prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in some local garden stores. Do not buy or transport any Kudzu plants or roots and never plant them in your yard. It is illegal to purchase in many states and provinces. However, online sales are rarely properly regulated.

If you see them being sold online or in your local garden stores please inform them of their invasive status and ask them to do their part and cease selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species.

Once established in an area Kudzu is notoriously difficult to eradicate. Its extremely fast growth and its ability to smother everything in its path combined with its rapid ability to produce roots at any node which touches the soil all makes it difficult to control. Intensive ongoing effort and an integrated management approach are required in order to control or remove Kudzu infestations.

Physical Control of Kudzu

Once already established physical control is very challenging. Physical control of Kudzu is labor-intensive and time-consuming but will cause far less environmental damage than chemical control.

Since Kudzu spreads mostly vegetatively it can be removed in spring, summer, or fall without having to worry much about the seeds. If there are mature seeds on the plants clip them off into a paper bag to keep them contained in case they are fertile and can produce new plants.

The vines should be cut with hand cutters or loppers and pulled down from trees and structures they are climbing on. Then the tubers and all new roots will need to be carefully removed. Since it grows so quickly the biggest challenge in Kudzu removal is that every root crown must be destroyed or the population will recover. Physical removal must be part of an integrated management approach with continuous ongoing monitoring in order to be successful. The area will need to be revisited multiple times the first year to out compete its rapid growth and re-vegetation rate. Then the site will need to be revisited for 4 – 10 years afterward to ensure that every vine that tries to re-establish is destroyed. Even if a single root crown survives and is left to grow within just a few years it could take over the entire area once again.

Mechanical mowing of ground cover patches in flat open areas is a somewhat less labor-intensive approach that can be successful. Mowing repeatedly throughout the growing season for 3 – 5 years in a row will starve out the roots. Eventually, the plants will die without having above-ground material to photosynthesize in order to keep the roots alive.

Prescribed burning can be used in early spring before fire season if your area allows it. This will kill small kudzu plants and sever the climbing stems that are in the trees, poles, etc. Burning may not be effective for larger plants with intensive root systems as the fire may not ever get hot enough. Also, fire alone will not work. It will need to be followed up with physical removal and of course ongoing monitoring.

Disposal of the Shrubs Once Removed

Plants including roots, stems and seeds all must be either burned or solarized before disposal to prevent new populations from beginning at the disposal site. Burning is an effective method to destroy the plant matter, but it is not allowed in some areas or in certain seasons. In this case, solarize them instead. To solarize put the vines into thick black garbage bags and leave them in the full sun for a good 6 – 8 weeks at least to be sure that none of the roots, stems or seeds are no longer viable. Do not try to solarize them on the ground under a tarp. Due to its extremely fast growth rate and rapid rooting at nodes the vines may be able to grow out from under the tarp and start new plants before the ones under the tarp are properly destroyed.

Once solarized the vines can be disposed of at your local garbage dump. Be sure to still inform them that they are an invasive species so that they can be disposed of accordingly, just in case there are still viable stems.

Chemical Control of Kudzu

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used species riches significantly increases.

Furthermore, there are no chemical control methods that effectively target only Kudzu. And due to the highly aggressive nature, large tuberous roots, and extremely fast growth rate of Kudzu multiple applications are always needed in a single growing season. If even a single root crown survives the treatment and goes undetected within a few years the Kudzu will again take over the area. Therefore applications must continue for 4 – 10 years after the initial treatment. If chemical control alone is used there will be an enormous amount of toxic chemicals being dumped into the environment, making it less suitable for native species.

Chemical control is not recommended.

Biological Control of Kudzu

Biological control involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800s to help control the rat. To this day there are still rats in Hawaii but the mongoose has helped to decimate many native bird populations.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. This is because the control agent would need to effectively destroy 100% of the vines, their stems, their roots, and their seeds in order to remove Kudzu on its own.

However, with Kudzu biological control is possibly going to be the most effective means with the least amount of effort due to its rapid growth rate and difficulty controlling it by other means. Particularly with large infestations or those near a large source of Kudzu, biological control may be the most desirable. It still of course must be done as part of an integrated management approach that uses physical removal of those unaffected by the biological treatment along with essential ongoing monitoring for multiple years.

Following is a list of biological control methods that have been used in North America in an attempt to help reduce the population densities of Pueraria montana and varieties.

Pathogens both native and non-native to North America have been isolated from Kudzu populations.

• Pseudomonas syringae pv. phaseolicola a plant pathogen native to North America that causes halo blight of legume plants was tested to control Kudzu. It caused high mortality on young and old growth. However, it performed much less well under dry conditions in the field.
• Myrothecium verrucaria is another native fungus that showed greater potential success in the field as it did not require such moist conditions. High mortality rates were observed in the field, and they even performed even better in the heat of summer. Since the application requires the use of a surfactant it keeps the risk of the fungus spreading beyond the application area minimal. However, the fungus did show high toxicity to mammals so extreme caution by professionals is required for application in the field.
• Alternaria and Fusarium fungal species are currently being developed in the USA for potential biological control of Kudzu and other weeds.

Heavy grazing by livestock such as by cows, pigs, horses or goats can also be used to remove Kudzu as part of an integrated management approach. This would only work in groundcover patches though as the animals cannot eat the vines in the trees or on buildings. The animals would need to be fenced in the Kudzu patch for long periods of time to be effective at controlling Kudzu on their own.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring.

Replanting With Native Species is Crucial

In all cases of removal, the site should be planted after the Kudzu is removed. This cannot be done immediately upon the first removal, however, due to the extreme growth rate of Kudzu. But replanting should be commenced in years 2 – 4, depending on the patch dynamics. Replanting with aggressive native species will help keep the Kudzu at bay. However, ongoing monitoring to remove any new seedlings or sprouts of Kudzu will still need to be intensely carried out. If a single root crown survives it will still grow over and smother any vegetation that is planted.

Ongoing Monitoring is Essential

In all cases of invasive Kudzu removal, ongoing monitoring is absolutely essential. Monitoring is important for all invasive species removal, but perhaps Kudzu most of all. The extreme growth and survival rate of stems and roots requires extreme diligence in monitoring. Repeated monitoring for the first couple of years should be carried out on a monthly basis during the growing season to remove any new growth. Then yearly monitoring programs should be put in place for a minimum of 4 years after the initial removal. Monthly monitoring can be reduced to every couple of months in years 2 and on, presuming there are few new individuals being found. An aggressive monitoring program is the only thing that will ensure the success of any biological, physical, or chemical removal or control of Kudzu.

References and Resources

CABI on Pueraria montana https://www.cabi.org/isc/datasheet/45903

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

iNaturalist Plant Search https://www.inaturalist.org/home

Kudzu Photo Smothering Trees in Georgia by Scott Ehardt, Public domain, via Wikimedia Commons https://commons.wikimedia.org/wiki/File:Kudzu_on_trees_in_Atlanta,_Georgia.jpg

RxList.com for Medicinal Uses of Kudzu https://www.rxlist.com/kudzu/supplements.htm

USDA Plants Database https://plants.sc.egov.usda.gov/home

Willis, Lyrae (2022).  Plant Families of North America. Not yet published.

Currently Seeking Funding To Continue This Non-Profit, Ad-Free Work

If you are able to donate so that I can continue this non-profit work of supplying people with scientific information on the plant families, native plants, and invasive species found throughout North America, please donate using the GoFundMe link below. Thank you!

Introduction

Reynoutria japonica, or its synonyms Fallopia japonica and Polygonum cuspidatum are commonly known as Japanese Knotweed. Sometimes it is called False Bamboo due to its superficial appearance to Bamboo though it is of a completely different and unrelated family. This was another one of my early discoveries of invasive species after Scotch Broom. On the Sunshine Coast of British Columbia where I grew up, False Bamboo infestations were particularly common along wet roadsides, riparian areas and wetlands. It is notoriously difficult to control Japanese Knotweed due to the vegetative reproduction that allows them to easily spread to nearby wild areas when left unchecked. I have seen firsthand the biodiversity loss that results when they invade a habitat. There are so many gorgeous native shrubs in North America, there is no need to risk planting this one in your yard. If you do, you may spend many years trying to get rid of it later.

Reynoutria japonica is on the IUCN’s list of the world’s most 100 invasive plants. There are two other plants in the same genus that look extremely similar, grow in the same habitats and ranges and can be equally invasive. For the most part, being able to distinguish the three species is not important since collectively all of the Reynoutria species in North America are invasive. That means that they should all be treated the same in terms of reporting infestations and implementing the control or removal of Japanese Knotweed species. This article discusses Reynoutria japonica but in the description section there are instructions on how to differentiate the species. There is one native species in the eastern part of North America that is sometimes misidentified. Please be sure that you have identified the plant as one of the Japanese Knotweed species before you remove it. It is also described below.

Description of False Bamboo Reynoutria japonica

Leaves & Stems of Reynoutria japonica

Reynoutria or Fallopia japonica is part of the Polygonaceae family in the Caryophyllales order of dicot angiosperms. It is a herbaceous perennial that dies back each fall and grows again every spring from its spreading rhizomes. Japanese Knotweed has a horizontal root system of rhizomes that can grow aggressively and spread 10 – 20 m away from the plant and up to 3 m deep. It then produces additional stems from its spreading rhizomes creating clonal colonies. Its primary mode of reproduction is this vegetative spreading of its rhizomes, making it a very aggressive and rapid invader.

Its purplish-green stems are slightly woody but hollow and jointed, similar to bamboo. It grows to 2 – 3 m tall each year, though it may be shorter with thinner and more solid stems in areas where it gets mowed back periodically. The rapidly growing stems can grow up to 8 cm per day or 1 m in 3 weeks. This allows it to rapidly produce large bamboo-like clumps of stems.

Reynoutria japonica has alternate leaves arranged in a characteristic zigzag pattern. The leaves are ovate to triangular in outline with an acute apex and a truncated base. Leaves possess a long petiole with a pulvinus, a swelling at the base of the joint where the petiole attaches to the stem. False Bamboo leaves are from 7 – 15 cm long and 5 -12 cm wide. They are red when young but unroll into a green leaf with reddish veins that fade with age. Its margins are entire and not wavy or toothed.

Flowers & Fruits of Reynoutria japonica

Japanese Knotweed flowers in July and August producing 6 – 15 cm long erect panicles. Each panicle consists of numerous small white-green flowers near the end of the stems and in the leaf axils. It is a dioecious plant meaning that it produces viable male and female flowers on separate plants and requires both sexes to cross-pollinate to produce viable seeds. Upon superficial examination, however, the flowers will look bisexual because the male and female flowers both have vestigial parts of the opposite sex. Vestigial parts are evolutionary remnants of the sex organs that still grow in the flower but are no longer functional. However, Reynoutria japonica spreads much more often by its aggressive rhizomes and relies far less on the sexual reproduction of viable seeds.

It produces fruits of winged seeds that are triangular, shiny, and very small, suitable for both wind and water dispersal.

Similar Species Frequently Confused With

There are multiple plants that are often mistaken for False Bamboo including docks, lilacs, dogwoods and others based on the appearance of the leaves. To make identification much more simple, if you are uncertain at all, wait for flowering. All those other species have different flowers and can easily be differentiated at that time.

In flowering, there are four species that Reynoutria japonica is often confused with. Two are from the same genus, look very similar, grow in the same habitats, are equally as invasive and are often called by the same common name. For the purpose of dealing with them as an invasive species all Reynoutria in North America is invasive and can be treated accordingly. The other two species are related being in the same family, but one is native and one is not. They can all be differentiated as follows:

• Reynoutria × bohemica is a hybrid species formed from a cross between Reynoutria sachalinensis and Reynoutria japonica. It is found in much the same range in North America. They look very similar with the hybrid having features that are intermediate between its two parent species. The hybrid is often misidentified as Reynoutria japonica but can be distinguished by the short pubescence on the veins on the abaxial (under) surface of young leaves in the spring. R. x bohemica hairs are unicellular, acute and less than 0.1 mm long. Hairs of R. sachalinensis are multicellular, twisted, acute to acuminate, and 0.2 – 0.6 mm long. Hairs on R. japonica are unicellular, blunt, and barely raised, giving the veins a scabrous appearance as opposed to hairy or pubescent.
• Reynoutria sachalinensis sometimes called Giant Japanese Knotweed is also found in much the same range throughout North America and also looks very similar. They can be differentiated by its very large cordate (heart-shaped) leaves that grow 15 – 40 cm long and 10 – 28 cm wide with undulate (wavy creased) margins. It also grows to 4 m tall as opposed to 2 – 3 m tall.
• Fallopia baldschuanica the Russian Vine is introduced in North America and so far has limited but scattered distribution here. It appears superficially similar but it is a climbing vine that does not possess the erect stems of Reynoutria spp. Its leaves are typically much more narrowly ovate than the broad ovate of Reynoutria japonica. Also, its flowers that also appear in panicle-like inflorescences are usually pink or pinkish instead of greenish-white.
• Fallopia cilinodis is native to eastern North America. It is superficially similar but can be easily differentiated by its arrow-shaped sagittate leaves with a deep cordate base and reddish veins. It also has a scandent and sprawling habit as opposed to the erect shrub of the Reynoutria genus.

Native Distribution of Reynoutria japonica

Reynoutria japonica, Polygonatum cuspidatum or Fallopia japonica is originally native to the eastern Asian countries of Japan, Korea, China and Taiwan.

Habitat Types Where Japanese Knotweed is Found

Japanese Knotweed in its native environment prefers rich, moist soil types in wetland and riparian areas. It also grows in moist fields, meadows, hillsides, and forest edges.

It prefers full sun and while it can grow in shade, its growth and reproduction is reduced there. Due to its reduced growth in shade, it is never able to dominate in forest habitats. It has a wide tolerance to soil type, pH, and salinity. The roots can survive in temperatures down to -35 C so it is capable of invading warm and cool temperate zones alike.

Human Uses of Japanese Knotweed or False Bamboo

Japanese Knotweed used to be widely used as an easy-to-grow ornamental plant and still sometimes is used this way. It can be used as a hedge plant due to its rapid growth.

The young stems are eaten in spring and taste similar to rhubarb. To eat them peel the fibrous skin off and soak them in water for half a day before cooking them to make them less bitter.

Beekeepers use the plants for their abundant nectar to make honey.

Medicinal Uses of Reynoutria japonica

Japanese Knotweed is used in traditional Chinese and Japanese medicine to treat various disorders through the actions of resveratrol, a secondary metabolite produced by plants in response to disease or injury. The levels of resveratrol are highest in the roots of Reynoutria japonica and this is usually extracted at the end of the growing season. If you want to harvest root for medicine then harvest it in the fall and remember that when dealing with aggressive invasive plants like this one the rules of ethical wildcrafting do not apply. Harvest to your heart’s content.

Extracts are used to treat a number of ailments including as a diuretic, as a laxative, to treat flus, acute hepatitis and appendicitis and gynecological problems like menstrual irregularities. They are also used to treat injuries, infections, and poisonous snakebites. There is research being done into its possible anti-tumor activity.

Leaves can also be crushed and applied externally as a poultice to treat abscesses, cuts, burns and boils.

Distribution of Reynoutria japonica in North America

The species was first brought to North America in the late 1800s as an ornamental plant for gardens.

In Canada, Reynoutria japonica has been recorded in British Columbia, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, Prince Edward Island, and Newfoundland Island (but not Labrador).

In the USA, Japanese Knotweed is found in most of the continental USA as well as Alaska. The states where it has so far been reported include Washington, Oregon, California, Montana, Idaho, Utah, Colorado, South Dakota, Nebraska, Kansas, Oklahoma, Minnesota, Iowa, Missouri, Arkansas, Louisiana, Michigan, Illinois, Kentucky, Tennessee, Mississippi, Georgia, South Carolina, North Carolina, Virginia, Washington DC, Maryland, Delaware, West Virginia, Indiana, Ohio, Pennsylvania, New Jersey, New York, Connecticut, Rhode Island, Massachusetts, Vermont, New Hampshire and Maine.

In Mexico Reynoutria japonica so far has only been confirmed in Oaxaca. It is possibly also present in Chihuahua, Nuevo Leon, Jalisco and Guanajuato.

Japanese Knotweed has been introduced to every continent except Antarctica.

How Japanese Knotweed Spreads

Japanese Knotweed is primarily spread through long distances by deliberate human introductions as garden ornamentals. It is still sold in garden stores and online despite widespread knowledge of its invasiveness.

Short distance dispersal occurs primarily through vegetative reproduction of its rapidly growing and spreading rhizomes. Short distance dispersal is also common in carelessly discarded “yard waste” piles which frequently are seen to sprout False Bamboo from both stem and rhizome fragments that were not properly solarized before disposal. See Physical Removal of Japanese Knotweed below on how to properly dispose of stems and rhizomes.

Transportation of seeds by wind, water, pets, livestock, humans etc can also be a source of spreading both long and short distances. However, by far most spread is from the aggressive vegetative reproduction of Reynoutria japonica.

Habitats at Risk of Invasion in North America

False Bamboo or Japanese Knotweed will easily invade any riparian areas, wetlands, roadsides, ditches and wet or moist forest edges it gains a foothold in. It is common in disturbed sites, empty lots and abandoned gardens. It is very common in waste areas, especially near yard waste piles.

Due to its preference for sun, it is not able to grow well in dense forests, though forest edges are at risk. It also seldom grows in arid and semi-arid areas unless in a riparian habitat.

Impacts of Invasion by False Bamboo

Japanese Knotweed forms dense thickets of bamboo-like vegetation that easily and aggressively out-competes native vegetation. This reduces biodiversity, wildlife values and causes a multitude of other negative impacts in wetland and riparian areas. The water carrying capacity of wetlands and riparian areas is often reduced in areas of False Bamboo infestation. The dense thickets reduce sunlight around them by 90% or more, shading out native plants and out-competing them for sunlight. Its thick mats of decaying vegetation it produces every fall when its abundant herbaceous stems and leaves die back build a thick mulch layer that also prevents other plants from growing with it.

Reduced plant and invertebrate densities are significant in established populations of Japanese Knotweed. Furthermore, native amphibian, bird, reptile and mammal populations are also much reduced in the dense stands of False Bamboo. It is also suspected that the aggressive roots of Japanese Knotweed are allelopathic meaning that they produce compounds that may alter soil chemistry and further negatively impact the growth of other nearby plant species.

Japanese Knotweed is capable of growing through cracks in cement, damaging sidewalks, driveways, roads, and house foundations. It can puncture its way through cement and asphalt 8 cm thick.

Japanese Knotweed roots are aggressive and strong but they are not as dense as those of native plants and they do not hold onto the soil nearly as well. As a result in riparian areas where Japanese Knotweed invades it can make the stream banks unstable and more vulnerable to erosion and flooding. This causes a loss of soil and space for vegetation, making the area further susceptible to future flooding and erosion.

Due to its impact on riparian areas False Bamboo also threatens salmon streams and stream reclamation projects.

Potential Benefits of Invasion

Wild birds are known to eat the seeds and bees use the flowers for their abundant nectar.

Methods to Remove Japanese Knotweed

As always prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in most local garden stores. Do not buy or transport any Japanese Knotweed. Do not plant it in your yard. Plant native species instead that will require little to no maintenance once established, provide wildlife and biodiversity values and likely will not need to be ‘controlled’.

If you see them being sold online or in your local garden stores please inform them of their invasive status and ask them to do their part and cease selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species.

Once Japanese Knotweed is established it is one of the most notoriously difficult plants to eradicate. There are no methods known today that will remove Japanese Knotweed in the short term. Long term removal of Japanese Knotweed is labor-intensive no matter what the method used. And long term eradication is seldom successful due to the aggressive nature of Japanese Knotweed. Diligence and hard work, integrated management and rigorous ongoing monitoring is the only possible ways to remove Japanese Knotweed.

Physical Control of Japanese Knotweed

Once already established physical control is notoriously difficult. While physical control is labor-intensive and time-consuming it still usually causes the least amount of environmental damage. Physical control is particularly difficult with False Bamboo due to the way it spreads vegetatively through its rapidly spreading rhizomes that grow up to 20 m long and 3 m deep. This makes removal particularly challenging and many people still turn to chemical methods. All methods however require multiple treatments over multiple years so physical removal of Japanese Knotweed is the least environmentally damaging option.

Physical methods to remove Japanese Knotweed generally involve the repeated cutting of the above-ground stems throughout the growing season for multiple years in a row. Cut it first in the spring after it reaches 0.5 – 1 m tall and keep going back to cut it again each time it reaches this height. It is a rapid-growing plant that can grow 1 m tall in three weeks if the conditions are right so in spring and early summer this will require multiple visits. As the soil dries and it gets hotter later in the summer it will not grow as rapidly and cutting frequency can be reduced. This will eventually weaken the aggressive root system as the plant is unable to conduct photosynthesis to store energy in its roots for subsequent years’ growth. This is very time-consuming but it will eventually kill it after several years of treatment.

Another method some people have tried with smaller patches is the cut all the above-ground growth and then solarize the entire patch with a heavy black tarp. This works for small infestations where the tarps can extend far beyond the stems. This is because the rhizomes can grow up to 25 m so the plants will tend to regrow just outside the edges of the tarp. This can still be done, however, as you can simply keep returning to the spot to cut any growth attempting to start outside the tarped area, as described above for stem removal.

Where land is to be cleared the best method may be to dig up the entire patch to remove all the rhizomes. Use caution however as any piece of rhizome left can grow and start an entirely new infestation. Proper disposal is also an issue in this case as there will be large volumes of soil needing to be properly disposed of so that it does not create a new infestation. Bringing it to a garbage dump and informing them of its aggressive invasive nature is one option. Phone ahead to make sure they have the means to bury it immediately, and deep enough (more than 5 m) to ensure that it does not grow again.

Disposal of the Shrubs Once Removed

Stems and rhizomes should never simply be disposed of as they will rapidly grow and invade any site they are delivered to. They can be burned on the spot if there are open fires allowed in your area. Burning is usually a good option in spring before fire bans have begun.

In the summer, however, solarization may be the only option. Place stems and rhizomes either under a heavy thick black tarp or into thick black garbage bags and leave them in the full sun for a good 10 weeks to be sure that all roots, stems and seeds are no longer viable. Many people recommend shorter solarization periods but due to the differential heats achieved in garbage bags and under tarps I recommend solarizing for as long as possible. Even after solarization you should bring them to the garbage dump and inform them that it is an invasive species so that they can be disposed of appropriately.

Chemical Control of Japanese Knotweed

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used species riches significantly increases.

Furthermore, there are no chemical control methods that effectively target only Japanese Knotweed. And due to the aggressive nature and deep root system of Japanese Knotweed, multiple applications during the year and in subsequent years are always needed.

However, if it is a very large patch of Reynoutria japonica and physical removal is not a viable option sometimes chemical control of Japanese Knotweed may be necessary. If it is near a riparian area or wetland, however, chemical application cannot be used due to the proximity to water. In those cases, physical removal is the only viable option.

Chemicals registered for use vary from area to area so if you are considering this method please research what is legal to use in your area and take into consideration proximity to water.

Biological Control of Japanese Knotweed False Bamboo

Biological control involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800s to help control the rat. To this day there are still rats in Hawaii but the mongoose has helped to decimate many native bird populations.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. This is because the control agent would need to effectively destroy over 99% of the plants in order to be an effective control method. Results this high are rarely seen in the field. However, using biological control in conjunction with physical control and ongoing monitoring can be very effective. Following is a list of biological control methods that have been used to help control Japanese Knotweed.

• Aphalara itadori the Japanese knotweed psyllid insect feeds only on Japanese knotweed. It has been tested in the United Kingdom with some success.
• Aphalara itadori leaf fleas that suck up the sap out of the plant have been recently released in Amsterdam. Results are not yet available.
• Mycosphaerella leaf spot fungus is currently being researched as a mycoherbicide due to its effect on plants in Japan. Research, however, is still preliminary and no results are available for its use outside of Japan.
• Goats are a great method to control Japanese Knotweed as they will feed on all the above-ground growth. If you have a small patch that you want to get rid of simply build a goat containment area around it and let the goats live there for several years. Eventually, the plants will cease to grow as the goats continually graze on all the above-ground growth. Some people recommend the use of pigs following the goats as the pigs will root out and eat the rhizomes. However, considering that the rhizomes can grow to 3 m below ground it is unlikely that the pigs will be able to deal with all the roots. Again, leaving them there for several years to keep removing any above-ground growth that appears can be a successful method.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form this involves physical removal methods, possibly biological control methods, replanting and ongoing monitoring. Integrated management recognizes the fact that when dealing with invasive species one method alone is almost never successful, especially in the long term.

Replanting With Native Species is Crucial

After removing any native species from an area replanting with native species is important to prevent the area from becoming reinfested with that or another invasive species. This is somewhat complicated in the case of Japanese Knotweed removal. This is due to the aggressive nature of the deep and extensive rhizomes which are its primary method of reproduction and spread. If the area was heavily infested and replanted immediately it will seldom be successful as the new growth of Japanese Knotweed from its extensive rhizomes will quickly take over. Replanting of native species should instead be planned for the following year(s) once the patch truly seems under control.

Ongoing Monitoring is Essential

In all cases of invasive Japanese Knotweed removal, ongoing monitoring is absolutely essential. More so for Japanese Knotweed removal than almost any other invasive species. Japanese Knotweed is one of the most aggressive invasive species known and eradication is notoriously challenging.

Yearly monitoring is not adequate due to the rapid and aggressive growth. Instead, monitoring should be done every 3 – 4 weeks during the growing season which would then include physical removal of any sprouts or seedlings found. Then this would need to be repeated yearly for at least 4 – 7 years, depending on how many individuals grow up each year. Yearly monitoring programs should be put in place after that for a few more years just to ensure that nothing has returned.

References and Resources

CABI on Fallopia japonica https://www.cabi.org/isc/datasheet/23875

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

eflora.org Flora of North America http://www.efloras.org/browse.aspx?flora_id=1

iNaturalist Plant Search https://www.inaturalist.org/home

Invasive Species Center on Japanese Knotweed https://www.invasivespeciescentre.ca/invasive-species/meet-the-species/invasive-plants/japanese-knotweed/

USDA Plants Database https://plants.sc.egov.usda.gov/home

Wikipedia on Reynoutria japonica https://en.wikipedia.org/wiki/Reynoutria_japonica

Willis, Lyrae (2021).  Plant Families of North America. Not yet published.

Currently Seeking Funding To Continue This Non-Profit, Ad-Free Work

If you are able to donate so that I can continue this non-profit work of supplying people with scientific information on the plant families, native plants, and invasive species found throughout North America, please donate using the GoFundMe link below. Thank you!

Introduction

Milk Thistle or Silybum marianum is an interesting plant. It has gorgeous variegated leaves and will grow in just about any disturbed environment as long as it has sufficient sunlight. It is also a widely used medicinal herb and many people today are still planting it in their garden for this reason. Once planted however, they quickly realize that now they have to remove the Milk Thistle from their yard before it takes over.

About 20 years ago I once planted it in my garden in West Sechelt, British Columbia, Canada for its medicinal properties. I was thrilled by how productive my crop was the first year. However, the second year I was horrified at the number of seedlings popping up where I had not planted them. I had not heard yet of its invasiveness at that time, clearly, or I would never have planted it. So I spent the following two years trying to remove every Milk Thistle seedling I could find. I moved away after that and have often wondered if I managed to stop it from becoming invasive there, or did I start an infestation that got out of control? It literally haunts me a little still to this day.

Until something becomes known as invasive, how are we supposed to know? My suggestion is to do your research before you plant anything new. This is particularly true of medicinal herbs which are often opportunistic species that may invade given the chance. With all of the information available today at our fingertips, it is not necessary to take chances anymore. If it is invasive and you want some raw herb, purchase it online from a reputable source instead.

Description of Silybum marianum

Leaves & Stems

Silybum marianum is part of the Asteraceae (Sunflower) family in the Asterales order of flowering dicot plants. Its stems are either glabrous or slightly tomentose (rough-hairy), lightly grooved, not winged, and grow from 0.3 – 2 m in height. It grows from a basal rosette of leaves up to 1.6 m wide in diameter. Stems are usually branched but not repeatedly and are usually armed with spines. The oldest stems are often hollow.

Its leaves appear strongly variegated in shiny green with milk-white veins. The basal leaves are oblong to lanceolate, coarsely lobed and armed with marginal spines. They are 15 – 65 cm long and are attached to their stems on winged petioles. As the leaves approach the flowers they become smaller and more clasping with an auriculate (ear-shaped) base.

Flowers & Fruits

Silybum marianum flowers anywhere from February to November, depending on the location. The phyllaries’ (floral bracts) appendages are ovate, spreading, and 1 – 4 cm long including their long spiny yellowish tip. The flower heads are from 4 – 12 cm long and 3 – 6 cm wide; they contain disk florets only. The disk florets are 26 – 35 mm long in magenta or purple. The tubes are 13 – 25 mm long, throats are campanulate (bell shaped) and 2 – 3 mm long, and they have 5 – 9 mm long lobes.

Note: disk florets and phyllaries are unique features of Asteraceae family flowers. If you want to learn more about Asteraceae flowers and their unique physical characteristics I suggest you check out the diagrams here https://cronodon.com/BioTech/asteraceae.html . You can also read the Wiki page on Asteraceae, it is very informative. However, the above link has much better diagrams showing the different physical features of the composite flowers.

The fruit is a brown cypsela (a typical angular seed of the sunflower family) with black spots. Although, it is often incorrectly referred to as an achene. Both are simple, dry, indehiscent fruits. However, achenes come from a superior ovary whereas a cypsela comes from an inferior ovary. The distinction is subtle. The cypsela is accompanied by a pappus of hairs or scales that remain attached to the apex that aid in wind dispersal. The pappus is a fluffy white attachment 1.5 – 2 cm long, that is surrounded basally by a yellow ring.

Toxicity

Milk Thistle in some instances can be toxic to ruminant livestock due to nitrogen accumulation. It is not, however, toxic to human beings and is a common and highly popular medicinal herb.

Similar Species Frequently Confused With

There are several native and nonnative thistles in North America that Milk Thistle is frequently confused with. In almost all cases they can easily be distinguished by the unique milky variegation on the leaves of the milk thistle. Sometimes, however, this variegation is more subtle which can lead to occasional misidentification. Similar looking species can be distinguished as follows:

• Cynara cardunculus the Globe Artichoke – popular with gardeners, it has tall stems that are not winged and large, spiny flower heads like Milk Thistle. However, it can be differentiated by its grey-green leaves that are not spiny.
• Cirsium vulgare the Spear Thistle – this has winged stems and its leaves are greenish and not variegated. It has moderate-sized flowers with small spines. 
• Cirsium arvense the Creeping Thistle – has wingless stems but its greenish colored leaves are not variegated and it has smaller and much more slender flower heads.
• Carduus nutans the Musk Thistle – has winged stems but its greenish leaves are not variegated. Its flower heads have small spines and they droop with age.
• Onopordum spp – multiple species in North America that look somewhat similar but all have winged stems and their leaves are either bluish-grey or whitish rather than variegated. They also have smaller flower heads and smaller spines.

Native Distribution of Silybum marianum

Silybum marianum is native to throughout the Mediterranean countries as well as all of Asia from India north to Siberia.

Habitat Types Where Milk Thistle is Found

Milk Thistle grows in open fields, roadsides, agricultural areas, waste areas, any disturbed habitat. It can be found from sea level to 1300 m above sea level.

Silybum marianum requires habitats with full sun and prefers areas that are regularly disturbed. It tolerates drought, full sun, and nitrified soils. While it prefers nutrient-rich soil it can tolerate a wide range of soil types, quality, and moisture levels. It does not seem to prefer rocky soils.

Human Uses of Milk Thistle Silybum marianum

Milk Thistle as Food

Milk thistle has a long history of use in Europe as a food. Leaves with their spines removed were used in salads or cooked and eaten like cabbage. Stalks, roots and flowers were often cooked on their own or added to stews as a vegetable. Seeds were sometimes used as a coffee substitute. While humans use it for a food source it is considered somewhat toxic for livestock to feed on.

Milk Thistle as Medicine

Silybum marianum has been used as a herbal medicine for over 2000 years. It was widely used as a milk stimulant, for liver, kidney and spleen problems, jaundice, gallstones, menstrual problems, and neurological problems.

Currently, Milk Thistle is widely used as a herbal remedy for hepatitis (especially Hepatitis C), cirrhosis, jaundice, diabetes, indigestion, and as part of a gentle cleanse. It is also occasionally used to help treat poisonings from toxic mushrooms, industrial solvents, excess acetaminophen or other over-the-counter or prescription drugs.

Studies have been being done that show it can help as a neuroprotective to aid in treating age-related decline in brain function. Studies are also being done to see if it can help aid and boost the effectiveness of cancer treatments.

The active ingredients in Milk Thistle cypselae are a group of phytochemicals called silymarin. Occasionally it can cause allergic reactions, usually in those with allergies to other plants in the sunflower family. It also may lower blood sugar levels in people with type 2 diabetes so they should be cautious when using it.

Ethical Wildcrafting of Silybum marianum and Making of Medicines

When it comes to truly invasive species with no or little wildlife values there is no need to “ethically” wildcraft. Milk Thistle is a wonderful medicinal herb. If you see it growing in your area and want to harvest pick to your heart’s content. Choose an area free of major sources of pollutants though if you intend to ingest it medicinally.

The achenes are ripe when there is a lot of fluffy white pappus visible. They readily split open at this stage upon picking so keep your bag or basket under the flower head. This will prevent any seeds from spilling on the ground as you harvest. Pick all of the mature seed heads that you find whether you need them all or not. Allow them to continue drying in the bag or basket for a few more weeks until they easily split apart. Process any seeds you want to use. Any excess seeds should be ground before you burn or otherwise dispose of them. Do not throw them directly in your compost, garbage, or anywhere else before grinding them.

Making Milk Thistle Tinctures

To use Silybum marianum as medicine you must remove the fluffy pappus from the achene (seed). Dry the seed thoroughly on a rack or in a paper bag. Then grind it and make it into a tincture using alcohol. You can use vodka or white rum (or other alcohol, they just impart more flavor to the tincture). If you can get your hands on 95% ethyl alcohol this will make the best tincture. Everclear is a common brand but it is not legal for sale in some states and provinces.

To make a tincture put the ground seeds in a jar and cover them with your alcohol of choice. Be sure to add enough alcohol to thoroughly cover the seeds and a little extra to account for any evaporation. When done your tincture should be bright yellow from the resins that contain the active ingredient silymarin. Note that the active ingredient silymarin is almost completely insoluble in water so teas and other water-based extracts will not be effective.

Distribution of Silybum marianum in North America

The species was first brought to North America by the early colonists as a medicinal herb and as a food source. It was intentionally planted and spread with the colonists.

In Canada, Silybum marianum has been recorded in British Columbia, Alberta, Ontario, Quebec, New Brunswick, and Nova Scotia. In Saskatchewan it is reported as ephemeral. Interestingly, it is not reported as present in Manitoba yet the Manitoba government’s department of agriculture has a page on their site telling you how to grow it. They even seem to try to persuade farmers to grow it by spouting the economics and marketing of it. Species known to be invasive in any part of any country should ideally not be encouraged to be grown in other parts of that same country.

In the USA, Milk Thistle is found in Washington, Oregon, California, Nevada, Arizona, Colorado, New Mexico, Oklahoma, Texas, Arkansas, Louisiana, Mississippi, Alabama, Georgia, Tennessee, Virginia, Washington DC, West Virginia, Maryland, Michigan, Indiana, Ohio, Pennsylvania, New York, Connecticut, Massachusetts, and New Hampshire. Since there is still so much habitat suitable for Silybum marianum it is expected that it will be much more widespread in the USA in the future unless adequate control of Milk Thistle is enforced.

In Mexico Silybum marianum has been reported in Baja California, Sonora, Nuevo Leon, Hidalgo, Veracruz, Mexico State, Mexico City, and Puebla. It is likely more widespread than currently reported and will continue to spread into more states throughout Mexico.

Milk Thistle has been introduced on every continent except for Antarctica. It is considered an invasive weed in North America, Australia, New Zealand, and parts of South America.

How Blessed Milk Thistle Spreads

Milk Thistle is primarily spread through long-distance by deliberate human introductions by planting them in their garden. Seeds are still widely sold on the internet and often in garden stores as well due to their popularity as a medicinal herb.

One Silybum marianum plant can produce over 6000 seeds that remain viable in the soil seed bank for 9 years. Furthermore, they can germinate through an extraordinary range of temperatures between 0 – 30 C. Seeds can be transported short-distances by wind, clothes, shoes, equipment, pets, etc.

Habitats at Risk of Invasion in North America

Any open, disturbed habitat is at risk of invasion in North America as long as it has adequate sunlight. This makes agricultural areas, roadsides, new development, riparian areas, clearings, urban fallow, and waste areas particularly at risk. This means that anywhere humans develop the area is at risk of invasion. And since most disturbed habitats have abundant sunlight that means that light requirement is not an issue. Since it is not only drought tolerant but will also germinate at temperatures around 0 C it is expected that Silybum marianum will continue to spread throughout North America. This is a particular concern in the central areas of Canada, the USA, and Mexico where it currently is not found or at least not yet in any abundance.

Impacts of Invasion by Silybum marianum

Silybum marianum frequently invades pastures and agricultural lands where it can rapidly take over and exclude forage grasses and herbs, native plants, and agricultural plants. Due to its height and diameter, a single plant can displace a significant amount of forage plants for livestock.

Milk Thistle is known to accumulate potassium nitrate which can make it toxic to ruminant livestock because the bacteria in their stomachs convert it to nitrate ions. However, it is not considered toxic to humans.

Due to their preference for disturbed habitats they do not pose a very serious threat to undisturbed native environments. However, they will rapidly spread in fallow fields and have occasionally invaded nearby wild grasslands and meadows with proximity to agricultural areas.

Potential Benefits of Invasion

The flowers do produce nectar that is used by both native and agricultural bees. However, the plants that it displaces likely would have also provided the same benefit for the bees.

Methods to Remove Milk Thistle

As always prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in most local garden stores. Do not buy or transport any Milk Thistle. Do not plant it in your yard like I mistakenly did all those years ago.

If you see them being sold online or in your local garden stores please inform them of their invasive status and ask them to do their part and cease selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species. And if you are wanting to grow it solely for the medicine, then buy the medicine or the raw seeds online from a reputable source. Just do not plant those seeds in your yard.

Physical Control of Milk Thistle

Once already established, however, physical control is always the most effective means. Physical control is labor-intensive and time-consuming but it usually causes the least amount of environmental damage.

Physical methods for the removal of Milk Thistle involve the cutting down of the plants. Because it is an annual or biennial species removal of the rootstock is not absolutely necessary. The best time to remove Milk Thistle is in the mid-late spring before the flowers have opened or matured. Immature seeds will continue to mature on cut plants, particularly those that remain attached to the stem. So it is important they be cut down before the flowers mature so they cannot go to seed.

Wear gloves to protect your hands from the spines. If the plant already has a stem use hand cutters, machetes, loppers, etc to cut down the above-ground growth. Be sure to then use a shovel to cut it further just below the surface of the soil so that it is not able to re-grow that year. Alternatively, you can then use a weed puller to remove the root. If it is early in the year and you are only dealing with the basal rosette of leaves then weed pullers can be used to remove the root. A shovel can also be used on a rosette to cut the growth off below the soil surface.

Frequent cultivation in agricultural areas is a good method to control Milk Thistle infestations. Cultivation must occur before the plants go to seed. This should be followed with planting of suitable annual forage plants for your livestock. Keep cultivating yearly if necessary until the infestation is gone. Then yearly monitoring can be done to remove any seedlings that may still try to sprout.

Physical Removal Methods of Milk Thistle That Are Not Recommend

Mechanical mowers are never recommended. They help spread the seeds around and they will not actually kill the plant since they do not remove all the green photosynthetic leaves above ground. The remaining plant will simply re-grow and could still set seed that same year.

Controlled fires are not an adequate method to remove Milk Thistle. Milk Thistle seeds are tolerant of low-temperature fires. Furthermore, the release of nutrients the fire creates as well as the disturbance it creates makes a suitable environment for the seedlings to sprout. Fires may actually encourage their spread.

Disposal of Milk Thistle Removed

If you have plants that have any seeds, whether mature or not, they must be solarized. To solarize put the plants under a thick black tarp, or into thick black garbage bags and leave them in the full sun for a good 8 weeks to ensure that all seeds are no longer viable. Many sources recommend shorter time periods for solarization. However, temperature differences throughout the bag, as well as variable sun conditions during the process, can lead to differential survival of some seeds during solarization. Better safe than sorry, so please solarize as long as possible.

Replanting may be necessary, and ongoing monitoring is absolutely essential. See the Integrated Management section below for more information.

Chemical Control of Milk Thistle

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used species riches significantly increases.

Chemical control is generally not recommended. However, since Silybum marianum is more often a weed of roadsides and agricultural areas chemical control is sometimes used. Since there are no chemical control methods that effectively target only Milk Thistle then handheld spray applications are recommended rather than non-selective machine spraying. Herbicide availability and legality vary from area to area so if you have a large infestation and want to use this method then please research herbicides used in your area. If at all possible try physical control methods first as Milk Thistle removal by physical means is relatively easy for an invasive species.

Biological Control of Milk Thistle

Biological control involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800s to help control the rat. To this day there are still rats in Hawaii but the mongoose has helped to decimate many native bird populations.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. This is because the control agent would need to effectively destroy over 99% of seeds to actually control the Milk Thistle on their own. However, using biological control in conjunction with physical control and ongoing monitoring can be very effective when dealing with large infestations over a wider geographical area. These are the biological control methods that have been used in North America in an attempt to help remove Milk Thistle.

Biological Control Organisms Used in North America

• Rhinocyllus conicus is a seed head weevil is reported to do significant damage and is a viable option to control Milk Thistle. It has been released in the USA with some success. However, it also attacks the native thistles so this method is not recommended.
• Alternaria species, a plant pathogen, is currently being studied for possible use in the USA, but to my knowledge has not yet been released.
• Domestic Goats will feed on Silybum marianum without any ill effects. The goats are non-selective, however, and will eat almost anything in their path. Goats should be kept in a fence to prevent them from grazing outside of the infestation area. They should be allowed to graze from spring till fall to allow them to eat any new regrowth. They will need to be returned the following spring to graze continuously the next year to deal with any seedlings that emerge.
• In agricultural areas maintaining vigorous, dense pasture land is the best method to control Milk Thistle. The types of agricultural plants used in the pastures will of course depend on your location so research what is best in your area.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring. No one method alone will ever control an invasive species, or they would not have become invasive in the first place.

Replanting is Often Crucial

In many cases, particularly in agricultural fields where there has been a lot of Milk Thistle removal the area may need to be replanted. Since they are agricultural the type of planting will depend on the agricultural use as well as the location. Where the infestations occur on roadsides there are usually other weeds that will quickly grow up to take their place. In the occasional case where the area is a natural one, or one being converted to a natural area, then replanting should be done with native species. In that case, a replanting program should already be planned and ready to implement immediately upon the removal of Milk Thistle. Use seeds from the local area if possible to replant native areas. If that is not possible then purchase native plants and native seeds from a reputable nursery specializing in native plants in your area.

Ongoing Monitoring is Essential

In all cases of invasive Milk Thistle removal, ongoing monitoring is absolutely essential. Yearly monitoring programs should be put in place to ensure that any surviving individuals are removed so that the population is not able to recover. This is required whether the area is replanted or not. Milk Thistle can be aggressive and may out-compete planted vegetation if yearly monitoring is not put in place to remove young plants before they have a chance to become established.

Simply revisit the site every spring or so over the following years and remove any new rosettes or full plants that emerge before they are given a chance to go to seed. Seeds remain viable for 9 years in the soil so ongoing visits would be required for at least that long.

References and Resources

CABI Fact Sheet on Silybum marianum https://www.cabi.org/isc/datasheet/50304

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

Eflora Plants of North America http://www.efloras.org/browse.aspx?flora_id=1

iNaturalist Plant Search https://www.inaturalist.org/home

Manitoba Department of Agriculture on Milk Thistle https://www.gov.mb.ca/agriculture/crops/crop-management/milk-thistle.html

Milk Thistle Flower Photo from Wikipedia By fir0002flagstaffotos [at] gmail.com Canon 20D + Sigma 150mm f/2.8 – Own work, GFDL 1.2, https://commons.wikimedia.org/w/index.php?curid=1526654

USDA Plants Database https://plants.sc.egov.usda.gov/home

Wikipedia on Silybum marianum https://en.wikipedia.org/wiki/Silybum_marianum

Willis, Lyrae (2022).  Plant Families of North America. Not yet published.

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Introduction

Purple Loosestrife Lythrum salicaria is a well-known noxious invasive species in North America. It has invaded all of southern Canada and almost the entire continental USA. It invades wetland habitats where the soil remains moist most of the year and grows abundantly on the edges of open water wetlands, lakes, rivers, etc. Purple Loosestrife is an aggressive invade known to create monocultures. Its sturdy rhizomes enable its vegetative spread and it produces a huge number of viable seeds each year. It is particularly invasive in the northeastern USA and in eastern Canada. Efforts to control or remove Purple Loosestrife cost $45 million per year in the USA.

When I was first learning about invasive species back when I was still a teenager on the coast of British Columbia, Canada, Purple Loosestrife was the most well-known invasive species at that time. We were lucky in British Columbia at that time in the Purple Loosestrife did not arrive in BC until 1915 and it is only found in a handful of locations in the southern half of the province. It is still a concern, certainly. But, knowledge and awareness go a long way. Now that many people are aware of the Loosestrife problem early detection becomes easier. Early detection is always the best method to control or remove Purple Loosestrife. It is so much easier to remove Purple Loosestrife from an area when there are only 10 or 20 plants to deal with as opposed to hundreds or thousands.

Description of Lythrum salicaria

Leaves & Stems

Purple Loosestrife is a herbaceous perennial producing 1 – 15 (-50) annual stems 60 – 120 (-200) cm tall from a perennial woody rhizome. The stems are angular and usually four-sided but occasionally grow 6-sided. They are stiff and woody and may be finely pubescent or hairless. Stems are green when young but turn reddish, brownish or purplish with age. The stems are slender and sometimes branched.

Leaves are sessile (no leaf stalk), entire (not toothed) and either lance-shaped or somewhat triangular. They may be smooth or finely pubescent. The leaves are usually arranged oppositely on the stem, one on each side of the node. But they may also be whorled with three or more per node. Upper leaves are usually smaller and arranged alternately on the stem. Leaf size varies with location on the stem as well as light levels, but may be anywhere from 3 – 12 cm long. Leaves turn bright red in autumn when the plant dies back for the winter.

Flowers & Fruits

The showy flowers bloom from June to September, depending on the location. They are usually deep pink but can vary to purple, light pink, or rarely white. Flowers are arranged in dense terminal spikes 10 – 100 cm long. Each flower is 1 – 2 cm in diameter with 6 (5-7) petals that are each 7 – 10 mm long and appear somewhat wrinkled.

In the yellow center of each flower is 12 crowded stamens and a single pistil with a long exserted style and capitate (rounded) stigma. The flowers are trystylous, having three different morphologies of styles and stamens coming in short, medium and long varieties. They are pollinated by a variety of insects that aid in the cross-pollination between the different flower morphs.

The fruits are an oblong–ovoid capsule 3 – 4 mm long, contained within a persistent calyx. The capsules open by two valves to release numerous tiny brownish seeds. Each capsule can contain more than a hundred seeds and a single mature plant can produce up to 2.7 million tiny seeds annually.

Similar Species Frequently Confused With Lythrum salicaria

There are a few native species that grow in the same habitat that Lythrum salicaria is sometimes confused with. They can be differentiated as follows:

• Chamerion angustifolium Fireweed – found in much of the same ranges in North America but usually in drier habitats, though they are sometimes found in moist habitats as well. However, the stems are rounded and smooth, not angular or pubescent and the leaves are always alternate and hairless. The flowers are in looser terminal racemes and each flower only has 4 petals.
• Verbena hastata Blue Vervain – also found in much of the same areas in North America, and in wetland habitats, and it also has stems that are angular and rough-haired. However, they can be distinguished by the leaves that have stalks (as opposed to sessile) and the flowers that smaller, blue-purple, and weakly two-lipped to funnel-shaped.
• Lythrum alatum Winged Loosestrife – found in wetland habitats but is endemic to eastern North America only. Their square stems are always hairless and are winged. Also, the leaves are always stalked and the pink-purple flowers are born solitary in the leaf axils instead of in dense terminal spikes.
• Decodon verticillatus Swamp Loosestrife – also found in wetland habitats but is endemic to eastern North America only. It also has 4-6 sided hairy stems but they are taller, up to 2.5 m. Also, its leaves are short-stalked and shorter, only up to 4 cm long. Its flowers are produced in axillary cymes with 5 – 7 triangular sepals.

Native Distribution of Lythrum salicaria

Lythrum salicaria is native in wetland areas throughout Europe, temperate Asia, and northwest Africa. There is some debate on its status in Australia where it was thought to be a recent introduction. However, pre-colonial pollen samples collected in New South Wales suggest that it may either be native or have been introduced at a much earlier time in human history.

Habitat Types Where Purple Loosestrife is Found

Purple Loosestrife is a semi-aquatic moisture-loving species that can be found in wetlands, marshes, wet meadows and along the side of lakes and other riparian areas. It can also be found in wet ditches, wet gravel pits, and other disturbed areas providing there is an adequate amount of moisture. It is generally not found in areas subjected to extended droughts where the soil remains dry for most/all of the year.

Human Uses of Purple Loosestrife

Purple Loosestrife sadly is still occasionally sold online and in garden stores, despite the widespread knowledge of its invasiveness. It was and still is frequently used as a garden plant for wet areas and ponds for its pretty pink flowers. Many supposedly sterile cultivars have been developed in an attempt to deal with the plant’s invasiveness. However, many of those sterile cultivars that were said not to produce seeds have since been observed producing seeds and spreading. They are particularly at risk of spreading when cross-pollinating with wild varieties. Even though the hybrids themselves may be sterile they are still replacing native vegetation because they spread vegetatively through their rhizomes. The true sterility of these cultivars must be thoroughly investigated before they are allowed to be sold in stores. However, so far, this is not the case.

In herbal medicine Lythrum salicaria is used for its tannins and salicarin that act as astringents (drying agents). This makes it useful in teas to help treat diarrhea and dysentery. It is even considered safe for use by children and even babies. It is also occasionally used in teas for menstrual problems and bacterial infections. Sometimes it is applied externally for bleeding gums, hemorrhoids, varicose veins, and eczema. If you are harvesting Purple Loosestrife in North America remember that the rules of Ethical Wildcrafting do not apply when it comes to invasive species. Remove more of the plant than you even need for your own use. Just be sure to harvest before it goes to see so that you do not further spread the plant.

Distribution of Lythrum salicaria in North America

The species was first brought to North America in the early 1800s. The earliest official report was in 1814 on the eastern seaboard of the US. It was at first probably brought there through accidental introduction by way of seeds traveling in ship ballasts or possibly with livestock in their bedding and feed. It was then spread purposely by settlers and is still sometimes planted in aquatic gardens to this day.

In Canada, Lythrum salicaria has been recorded in all the southern provinces including British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, Prince Edward Island and the island of Newfoundland but so far has not been reported in Labrador. It is also not currently found in any of the Northern Territories.

In the USA, Purple Loosestrife has spread to almost every state. The only continental states where it is not currently found are Arizona, Louisiana, Florida, Georgia, and South Carolina. It is surprising that is not found in Louisiana, Florida, Georgia, or South Carolina since there is a lot of habitat there suitable for invasion. It may in part be due to its preference for temperate habitats. However, given its presence in other southern states this would not rule out invasion in these areas. These areas would be good candidates for monitoring as early removal is the best means of dealing with an infestation of Purple Loosestrife. It is also not currently reported to be found in either Alaska or Hawaii.

In Mexico Lythrum salicaria has so far not been confirmed in any state.

Purple Loosestrife has been introduced to much of the temperate world including southern Africa, North America, South America, and New Zealand.

How Purple Loosestrife Spreads

Lythrum salicaria is primarily spread through long distances by deliberate human introductions in garden plantings where their prolific production of small seeds and their vegetative production by way of their rhizomes then helps them spread to nearby wild areas.

Purple Loosestrife is easily spread short distances by its prolific seeds. A single plant can produce 2.7 million tiny seeds annually that can be carried by wind, water, and animals. While its seeds can germinate in water they are much more prolific when the substrate is moist but not flooded. Seed viability is over 90% and the seeds can remain viable in the soil seed bank for many years.

Purple Loosestrife is also able to regenerate from pieces of roots that are broken off and float away in the nearby water to re-establish elsewhere.

Habitats at Risk of Invasion in North America

Purple Loosestrife is a water-loving species so any ecosystem with seasonal or permanently wet soils is at risk of invasion. These include marshes, bogs, fens, lake shores, and river or creek banks where the water is slow-moving. It is also tolerant of brackish water and will even invade tidal marshes. Purple Loosestrife will also invade wet ditches, canals, wet gravel pits, and other disturbed areas providing there is adequate moisture for it. It will tolerate drier conditions as well, but these habitats are more marginal for Purple Loosestrife.

Purple loosestrife prefers full sun to part shade, however, it can even tolerate full shade. It prefers cool temperate habitats but has been found in warm temperate habitats as well.

The only habitats that are not at risk are ecosystems subject to severe droughts including deserts and rocky mountainous terrains, and any other area where there is not adequate moisture.

Currently in North America it is not found in Louisiana, Florida, Georgia, South Carolina, Alaska or Hawaii but it is expected to invade these areas in the future. The Northern Territories of Canada will likely be at risk in the future as Purple Loosestrife has already been seen to adapt to the colder temperatures and shorter growing seasons as it moved northwards out of southern Ontario into central and northern Ontario. In northern Ontario the plants are smaller and go to seed sooner. In conjunction with climate change, it is not believed that the Northern Territories are exempt from potential invasion. It will likely invade Mexico in the future as well, particularly in the interior and mountainous regions with cooler conditions.

Impacts of Invasion by Lythrum salicaria

In rivers, creeks, canals and other waterways Purple Loosestrife infestations can seriously impact the flow of water. This can lead to costly Purple Loosestrife removal programs and loss of water to farmers in their irrigation canals. Its stiff stems trap debris and sediment and they also die back annually leaving plant litter to decompose. The combination of those two factors gradually change shallow water wetlands into more terrestrial wet meadow habitats. It can significantly reduce the recreational value of wetlands and waterways.

Because Lythrum salicaria is very competitive and grows very quickly it rapidly replaces native vegetation leading to the creation of monocultures. The creation of Purple Loosestrife monocultures affects habitat and food for numerous native birds, insects, and wildlife. It reduces nesting sites and shelter for birds and reduces shelter and habitat areas for fish. Significant declines in species richness (biodiversity) are always observed in habitats where Purple Loosestrife invaded. Increased biodiversity and ecosystem complexity are directly associated with ecosystem resilience. Monocultures create ecosystems that are further susceptible to serious declines.

It even commonly replaces cattails, which themselves are sometimes considered invasive even though they are native. Read this article here for more on common cattails and their supposed invasiveness in North America. The loss of cattails in marshes affects the native muskrats and other animals that feed on the rhizomes, young stalks and flowers. It also significantly reduces bird nesting sites as Purple Loosestrife is not suitable for nesting for most birds that would use the cattails.

Purple Loosestrife loses its leaves and decomposes faster than the native species it replaces. This causes a change in the timing of the release of nutrients into the ecosystem leading to eutrophication of the waterways downstream of the wetland habitat. This impacts native tadpole development as well as reduces the species richness and diversity in those ecosystems.

Potential Benefits of Invasion by Lythrum salicaria

Purple loosestrife is widely used by native and introduced bees alike. It is also visited by a number of native butterflies. However, the native vegetation Lythrum salicaria replaces would have also been a good source of nectar for the bees and butterflies and would have been associated with a multitude of other biodiversity and ecosystem values as well.

Methods to Remove Purple Loosestrife

As always prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in most local garden stores. Do not buy or transport any Purple Loosestrife. Do not plant it in your yard.

If you see them being sold online or in your local garden stores please inform them of their invasive status and ask them to do their part and cease selling them. Even if they inform you that they have only sterile cultivars, sometimes these are mislabeled. Even worse, sterile cultivars have many times now been proven not to be sterile and even if they themselves are sterile many have been shown capable of interbreeding with wild populations. Furthermore, their vegetative spread via their rhizomes still puts them at risk of escaping cultivation by vegetative means. Avoid all cultivars.

Instead, asks stores to sell more native species as ecologically friendly garden alternatives to invasive species. And encourage these stores to do so by only purchasing species that are native to your region. We have so many beautiful native aquatic species all over North America. Just do a little research for your region and you will find many lovely alternatives.

Physical Control of Purple Loosestrife

Once already established, however, physical control is almost always the most effective means. Physical control is labor-intensive and time-consuming but it usually causes the least amount of environmental damage and results in the greatest increase in biodiversity compared to chemical control.

Aquatic and semi-aquatic invasive species are notoriously difficult to control. Choose a time of the year when the water is low and the soil is fully exposed as this will make it easier to dig out the rhizomes. Since Lythrum salicaria is simply a water-loving plant and not a fully aquatic one this should be relatively simple. Monitor the area and find the best time. The soil should however still be wet as this loosens the attachment of the roots to the soil making them easier to pull out. Ideally, remove the plants before flowering but certainly before they have gone to seed. Purple Loosestrife removal while in seed will only spread the prolific seeds around. Since they are herbaceous perennials they will need to be removed during the growing season and cannot be done in the winter after the plant has died back.

When removing single isolated Purple Loosestrife plants hand pulling or the use of a narrow shovel or weed puller can be very effective means of control. It is best to dig them out before the patch spreads to prevent them from spreading to become the dominant ground cover. Younger plants are also much easier to dig out as older rhizomes are larger and more difficult to remove. Simply dig out the plants, gently, trying not to break the rhizomes and taking care to remove any broken pieces from the soil. Also, dig around the edge of the patch a little bit to ensure you get all the rhizomes. Any piece left behind can create a new plant so it is important to get as much of the root as possible. However, also be aware that any soil disturbance can encourage the germination of seeds in the soil seed bank. Therefore, try to disturb the soil as little as is physically possible. It is a delicate balance between getting all the roots and not disturbing the soil too much.

Cutting the plants before they go to seed is not an effective control method. While it is less labor-intensive in the short term it is much more labor-intensive in the long term. This is because it requires multiple visits to the site to cut back the regrowth, and visits each year for multiple years to ensure that it does not regrow and produce seeds. This can be an effective method if done diligently. This is because after some time the rhizome in the soil will eventually wither and die. Without above ground growth for photosynthesis the plant is not able to store energy in its rhizome for the following year’s growth.

Mechanical mowing or the use of machines is generally never recommended as they are nonselective and will destroy any native vegetation still in the patch. If it is a dense monoculture of significant size then this should be assessed by a professional who will then decide the best strategy. In almost all cases, however, this course of action is not recommended.

Flooding of less than 30 cm will not prevent germination. Short-term flooding tends to enhance the survival of Purple Loosestrife because it can adapt to changing environments. Short-term or long-term flooding may negatively impact the native vegetation on site so this method is not recommended. On-site burning is also not recommended for the same reason. Burning can negatively affect any native wetland vegetation on site as these plants are seldom adapted to fire.

Disposal of the Lythrum salicaria Plants Once Removed

In all cases of physical removal any tools that are used will need to be cleaned, solarized, sterilized or heat-treated to kill any viable seeds or rhizome parts that may be on them before moving them to another site. The same goes for the boots you wear on-site, they should be washed and sterilized to ensure that you are not spreading them to any new sites you visit.

Upon removal, the plants must either be burned or solarized. In either case, they are best removed from the site in black garbage bags to prevent the spread of any seeds or rhizome pieces. If you are not able to burn them in your area due to fire bans then you will need to place them in thick black garbage bags and leave them in the full sun for 6 – 8 weeks to be sure that all seeds are no longer viable. While some sources recommend a week of solarization I have seen disposal of solarized bags of invasive species still grow if not solarized long enough. This is in part due to differential temperatures within the bag as well as sunlight intensity and duration during solarization. This is why I always recommend a much longer period of solarization to ensure all seeds are no longer viable before they are disposed of. To dispose of them after treating bring them to your local garbage dump but be sure to inform them that they are invasive species so that they are dealt with accordingly.

See Integrated Management below, as all removal methods require additional management and monitoring.

Chemical Control of Purple Loosestrife

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used species riches significantly increases.

Furthermore there are no chemical control methods that effectively target only Purple Loosestrife. Because they grow in wetlands and near major water sources in most cases the use of herbicides is not permitted.

Chemical control is not recommended.

Biological Control of Purple Loosestrife

Biological control involves the use of a predator, herbivore, disease, or some other biological agent to control an invasive species once it is established in the environment. The idea is an attempt to restore the natural balance that likely existed in its native environment where it had natural enemies that are not present in its new environment, The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800s to help control the rat. To this day there are still rats in Hawaii but the mongoose has helped to decimate many native bird populations.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. This is because the control agent would need to effectively destroy over 99% of the target organism to actually control it on its own. However, using biological control in conjunction with an Integrated Management approach can be very effective. Following is a list of biological control methods that have been used in an attempt to help reduce the population densities of Lythrum salicaria.

Two species of leaf beetle have been used for the biological control of Purple Loosestrife. Neogalerucella (Galerucella) calmariensis The Black-Margined Loosestrife Beetle is a brown beetle with a black line down its thorax. It eats the leaves leaving characteristic round holes. Its larvae feed on leaf buds and leaves. A very similar and closely related species Neogalerucella (Galerucella) pusilla the Golden Loosestrife Beetle also feeds on the plant in a similar manner. It looks nearly identical to N. calmariensis but lacks the black line on its thorax. Both beetles have shown to be extremely effective and have defoliated over 90% of the plants in some areas. Both species have been released in North America.

Two other biological control agents have been released in North America. The Loosestrife Root Weevil Hylobius transversovittatus is a large red nocturnal weevil that feeds on leaves and buds while its larvae burrow into the root and feed on it for over a year. The root damage stunts the plants’ growth and hinders seed production. The Flower Weevil Nanophyes marmoratus is a tiny weevil that lays a single egg in each flower. The larvae emerge and feed on the flower’s ovaries preventing seed production. It then hollows out the flower bud and pupates there.

Caterpillars of the Engrailed Moth Ectropis crepuscularia are also known to feed on purple loosestrife, but according to some sources they are also a pest themselves so they are not recommended as a biological control method.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring. The key point of integrated management approaches is that you cannot simply perform one method, once, and hope it works. All methods usually require ongoing monitoring, repeated applications, and in some cases multiple methods. When it comes to invasive species replanting with native species is also often crucial to prevent the re-establishment of invasives after disturbing the environment in the process of removal.

Replanting With Native Species is Crucial

In most cases of Purple Loosestrife removal the newly disturbed site should be replanted immediately because the bare soil will allow the seed bank of Purple Loosestrife seeds in the soil to germinate. They will then quickly re-invade the patch they were removed from. Exceptions include small isolated populations that have plenty of native species in among or around them as these will invade the bare soil given the chance. In this case, simply implement a monitoring program to ensure that any undesirable seedlings are removed before they regain a foothold there.

In the case of larger patches, a replanting program should already be planned and ready to implement immediately upon completion of the removal of the Purple Loosestrife. To decide which native species to plant examine the local environment to see what is already growing nearby. Ideally, if possible, collect seeds the year before from the area and use those to replant upon removal of Purple Loosestrife. Otherwise, purchase native seeds or native plants from a local nursery specializing in producing plants for restoration. Do not simply buy plants from a regular garden nursery as you may be getting cultivars created by humans even if you think you are buying the native species. Finally, be sure to plant a variety of species to help recreate the biodiversity that was lost.

Ongoing Monitoring is the Last But Most Important Step

In all cases of invasive Purple Loosestrife removal, ongoing monitoring is absolutely essential. Yearly monitoring programs should be put in place to ensure that any surviving individuals are removed so that the population is not able to recover. This is required whether the area is replanted or not. Purple Loosestrife is an aggressive and prolific invader. If given the chance it will reestablish and outcompete planted vegetation if yearly monitoring is not put in place. Monitoring is simple. Each year return to the removal site and destroy any young plants before they have a chance to become established and set seed. Ideally, return to the site twice per year for at least five years, and then once a year after that to be sure they do not return. The added benefit, you will get to see the fruits of your labor when the ecosystem is returned to a healthier and more biodiverse state filled with native species.

References and Resources

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

iNaturalist Plant Search https://www.inaturalist.org/home

Michigan Department of Agriculture and Rural Development Weed Risk Assessment for Lythrum salicaria https://www.michigan.gov/documents/invasives/Lythrum_salicaria_WRA_576399_7.pdf

Ontario Invasive Plant Council Best Management Practices Purple Loosestrife http://www.ontarioinvasiveplants.ca/wp-content/uploads/2016/07/Purple-Loosestrife-BMP-April-2016-final.pdf

Purple Loosestrife ‘Naturalized’ in a Marsh photo from Wikipedia By Saffron Blaze. Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15287781

Purple Loosestrife RxList.com https://www.rxlist.com/purple_loosestrife/supplements.htm

USDA Plants Database https://plants.sc.egov.usda.gov/home

Wikipedia Purple Loosestrife

Willis, Lyrae (2022).  Plant Families of North America. Not yet published.

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