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 Callery Pear is Frequently Confused With

There are numerous species, mostly in the Pyrus (pear) and Prunus (plum and cherry) genera that Pyrus calleryana could be confused with as well as some other genera. All genera it can be confused with are in the Rosaceae or Rose 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.

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!

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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 of Kudzu

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 of Kudzu

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 of the Araliaceae family or Poison Ivy Toxicodendron radicans of the Anacardiaceae Family because they have a similar range. However, neither of these are from the Fabaceae 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 ** CABI now charges for its data sheets!

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!

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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 **CABI now charges for access!

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.

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!

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Introduction

Scotch Broom or Cytisus scoparius was a very familiar sight from my childhood growing up on the Sunshine Coast of British Columbia, Canada. Every May to June the power-lines, gravel pits, logging roads, and other disturbed areas were a ‘sea of yellow’ with their yellow blooms. As a child, I did not realize this was a problem. But I learned when I was still a teenager that this was a highly invasive species that did not belong in my environment. I did not understand why it was left to grow unchecked. Why was it simply allowed to take over and replace the native salmon berries, trailing blackberries, and other delicious native species with significant wildlife values? Why was no one trying to remove the Scotch Broom?

Now I understand that things are more complicated than that. For one thing, invasive species were not really a concern many people had when I was still a child. Now there is so much more awareness. I tried to create awareness in my early 20s when I started my own local “Broom Busters” group. For a couple of years each early spring I would go out with a small handful of volunteers and cut down the broom from roadsides. I also contacted the local parks to report broom infestations and assisted with their removal. In other places I would rip out newly established plants by hand before they had a chance to grow.

Description of Cytisus scoparius

Leaves & Stems of Scotch Broom

Cytisus scoparius is a semi-deciduous erect or sprawling shrub of the Fabaceae (legume) family. It lives on average 6 – 15 years and sometimes up to 30 years. They are sometimes evergreen but become deciduous when the winters are colder or during summer droughts. It can grow up to 4 m tall and as much as 6 m across. When young its branches are upright but as it matures they often fall prostrate, particularly in areas with snow. Branches are five-angled and green. When young in particular, it photosynthesizes through their stems throughout the growing season. As they mature the branches become more brownish to grayish in color.

Scotch Broom grows from a deeply forked taproot. Like most members of the Fabaceae family, it has nitrogen-fixing bacteria in its root nodules that allow it to grow and thrive in poor soils.

On young growth the leaves are often solitary and sessile (without a leaf stalk). On mature growth the leaves become three-foliate and are either subsessile or have a petiole (leaf stalk). Leaflets are narrowly elliptic to obovate and are 5 – 20 mm long and 1.5 – 8 mm wide. The upper surface has scattered hairs and the lower surface is usually covered with short hairs.

Flowers & Fruits of Scotch Broom

Flowers are born in the axils of the one-year-old stems. They have pedicels (flower stalks) and are either in pairs or are solitary. The two-lipped calyx is hairless (glabrous) and the upper lip (partially connate sepal) has two teeth while the lower lip has three teeth. The teeth are much shorter than the lip so it makes it difficult to mistake it for a calyx with 5 sepals (lips). The flower itself is golden yellow and 15 – 25 mm long, looking much like a typical legume-type flower.

The fruits are typical legume-type pods except they are quite compressed. They are oblong and green when immature and are 2.5 – 7 cm long and 0.8 – 1.3 cm wide. When they mature they turn black. The pods have brown or white hairs along their margin but otherwise are glabrous. They are explosively dehiscent with a loud snap. In the summer near large patches you can hear the continual snapping as more and more pods split open scattering their seeds.

The seeds are small and black with 3 – 12 seeds per pod and up to 18,000 seeds per plant per year. The seeds can remain viable in the soil seed bank for up to 60 years. They are also resistant to low-temperature fires which may actually help encourage their germination. The seeds are somewhat unique in that have an elaiosome attached to them. An elaiosome is a small appendage that contains an oily substance that makes them attractive to ants. The ants then help to disperse the seeds over short distances.

Toxicity

While Scotch Broom is technically not poisonous it is considered unsafe to consume in any large quantity. This is because of its effect as a cardiac stimulant. It can cause dizziness, heart palpitations, sweating, and dilated pupils. However, no mortalities have been reported.

Similar Species Scotch Broom is Frequently Confused With

There are other species of broom-type plants, all from the same Fabaceae or legume family, that also go by the common name of “broom.” These are all non-native and introduced in North America. They are:

• Striated Broom Cytisus striatus – often misidentified as Cytisus scoparius but has a much more restricted range in North America. C. striatus however, is more of a grey-green in color than C. scoparius. C .scoparius also has larger and more abundant flowers. The pods of C. striatus are densely white-hairy compared to only having hairs along the margin in C. scoparius.
• French Broom Genista monspessulana – this superficially looks very similar. However the branches are not ridged, or green which allows for year-round differentiation. It also will not grow as tall, only reaching a maximum height of about 2.5 m. Furthermore, the seed pods are very covered with dense, long white hairs.
• Spanish Broom Spartium junceum – this plant also superficially looks similar and grows to similar heights. However, it is summer deciduous, losing its leaves early on while it photosynthesizes through its stems. The stems are semi-succulent, grey-green, and rush-like in appearance. The pods are similar however to Scotch Broom, so use the branches to differentiate the two species.
• Gorse Ulex spp – multiple species, another woody leguminous shrub with yellow flowers and small leaves. However, these are easily distinguished because they are armed with large thorns while Cytisus scoparius is thornless.

Native Distribution

Cytisus scoparius is native throughout central and western Europe east to Poland and Hungary. It is also native to northern Africa. It is very common in England and Scotland, hence the common name Scotch Broom. In Australia where it is also invasive, it is often referred to as English Broom.

Interestingly, Scotch Broom in its native habitat is capable of reaching densities where it is also treated as a weed. This is likely due to its ability to rapidly invade the disturbed sites that are abundant anywhere humanity is abundant. It has in recent years been listed as an invasive species in many European countries. This is of course controversial, as it should be. We should not consider native species as invasive. Instead, we should address the problems that helped them create monocultures in the first place. If you are interested in hearing more about ‘native invasive species’ check out my article Are Common Cattails an Invasive Native Species or Do Human Perceptions Need to Change?

Habitat Types Where Scotch Broom is Found

In its native environment, Scotch Broom is prolific in open areas, grasslands, heathlands, open forests, and disturbed areas. It is found from low to middle elevations and occasionally subalpine locations but is uncommon at high altitudes. This is likely due to its lack of hardiness to extreme cold. Scotch Broom can however tolerate moderate freezing temperatures. It seems to prefer Mediterranean-style and cool temperate climates.

Cytisus scoparius is usually found in full sun to partial shade. Full shade is not a habitat where Scotch Broom will grow. If it is found in open forest it will die out when the canopy closes as it will not grow in full shade.

Scotch Broom prefers drier sandy soils but can tolerate richer soils as long as they are not too wet. It can also tolerate very acidic soil conditions.

Human Uses of Scotch Broom

Scotch Broom was widely used as an ornamental and often still is. Newer horticultural varieties apparently pose no risk of invasiveness. However, improperly educated people still sell the seeds of the invasive type online. Sometimes inadequate labeling also leads to invasive Cytisus scoparius being sold in garden stores resulting in accidental introduction.

In herbal medicine, Cytisus scoparius is infused or made into a decoction. It is commonly used as a diuretic due to the presence of scoparin and is sometimes used to treat dropsy. Scotch Broom is also cathartic and a cardiac stimulant. It is said to be a better cardiac stimulant than digoxin (from foxglove) because it does not accumulate in the body. The flowers can also be made into a salve to treat gout. It was also sometimes used historically to induce abortions.

Cytisus scoparius is sometimes used as a substitute for hops, coffee, and capers. It was also used in the tanning of leather and as a yellow dye. The branches were used for brooms, for thatch roofs, and the bark was sometimes stripped and made into a rope.

In folklore and myth, the Scotch Broom was said to be a sign of plenty when there were lots of flowers. It was also used in weddings in place of rosemary. Its fragrance was said to tame wild dogs and wild horses. In Italy, it was burned to ward off witches.

Distribution of Cytisus scoparius in North America

Scotch Broom was first brought to Virginia for animal fodder in the 1800s. It was introduced to California and British Columbia as an ornamental in the mid-1800s. I was always told that the infestation in coastal BC was the result of 3 seeds planted by a Scottish immigrant on Vancouver Island. While I have no idea if that is true or not, it is a very prolific plant and it certainly could be true that 3 seeds alone caused the invasion. However, I do not blame the immigrant of course. Many invasive species were brought to North America that way. People moving to the new world often wanted to bring something from their homeland. And back then virtually no one understood what an invasive species was anyway so they had no idea of the future impact.

In Canada, Cytisus scoparius has been recorded in British Columbia, Prince Edward Island, and Nova Scotia. I do not know what it looks like in the Atlantic provinces but I have seen firsthand the scope of this plant’s invasiveness on Vancouver Island, the Sunshine Coast, and the Lower Mainland. There are what I call “seas of yellow” in the late spring when the Scotch Broom flowers. It is everywhere. It is also in the Okanagan and the Kootenays just not quite as widespread as it is in the coastal areas.

In the USA, Scotch Broom is found in the western states of Washington, Oregon, California, Idaho, Utah, Montana, as well as Alaska. It is fairly widespread in the eastern states where it is found in Michigan, Ohio, Kentucky, Tennessee, Alabama, Georgia, North Carolina, South Carolina, West Virginia, Virginia, Maryland, Delaware, New York, New Jersey, Washington DC, Pennsylvania, Connecticut, Massachusetts, New Hampshire, and Maine. It is also present in Hawaii. According to iNaturalist it has also been recently discovered and confirmed in Colorado, Kansas, and Texas. It is also suspected of being present in multiple other states so it is definitely still expanding its range.

In Mexico Cytisus scoparius so far has only been reported and confirmed in the Mexico City area. However, it is also present on the northern border on the USA side in San Diego, California so it is likely in northern Baja California, or will be soon. It has also been reported on the USA side of the border in El Paso, Texas. It will likely continue to spread south into northern Mexico as it continues to spread in the southern USA.

Scotch Broom has been introduced on every continent except for Antarctica. It is considered highly invasive in several areas outside of North America.

How Scotch Broom Spreads

It is primarily spread through long-distance by deliberate human introductions in gardens, and accidental introduction through seed transport in clothing, shoes, pets, tools, vehicles, outdoor toys, etc. It can also be easily transported in gravel due to its preference for gravelly habitats.

Ship ballast water is another source of long distance dispersal.

Short-distance dispersal occurs through the ants that are attracted to the elaiosomes (oily appendages) on their seeds. Birds and other wildlife also transport seeds. Short distance dispersal also occurs through humans on clothing and shoes as well as vehicles, pets, etc. It also spreads even more locally due to its explosively dehiscent fruit pods that can send the seeds several meters away from the parent plant.

Habitats at Risk of Invasion in North America

In North America, Scotch Broom seems particularly invasive in western North America. It can sometimes be locally quite common in eastern North America. However, it likely has a harder time competing with the lush and relatively aggressive native and non-native vegetation already present there. Still, it is a common sight along roadsides in particular in eastern North America.

Scotch Broom thrives in disturbed habitats. It grows abundantly on roadsides, clearings, waste areas, power lines, and anywhere the land has been cleared for development or forestry. Roadside patches are problematic because they are an easy vector for accidental long-distance dispersal through their prolific seed production that can get carried on clothes, in shoes, and on pets. Disturbed sites are the most at risk of invasion by Scotch Broom (see the photo above under the power-lines in Sechelt, BC, Canada). This threat could be reduced significantly if replanting programs are immediately implemented after a disturbance.

Cytisus scoparius also invades both disturbed and undisturbed grasslands and replaces native grasses as the dominant vegetation type. It also invades open, mixed, or deciduous forests where there is enough light to gain a foothold.

Dunes are also at risk of invasion because it prefers sandy soils. It was used as a stabilizer for dune slopes and edges in both Canada and the USA before its invasiveness was realized.

Rangelands and agricultural lands are also at serious risk of invasion by Scotch Broom, costing ranchers and farmers in labor and money to remove them. They provide little forage value to livestock and they are invasive on farmlands.

Since it is a light-loving species dense closed-canopy forests are not at risk. Wetland habitats are also not apparently at risk due to their preference for drier soils. It is occasionally however found in riparian habitats though does not appear abundant in those areas. It can tolerate some degree of water-logging, as long as it is temporary.

Impacts of Invasion by Cytisus scoparius

Given a chance Cytisus scoparius grows rapidly and prolifically, crowding out other plants to create near monocultures if left unchecked. It destroys habitat and replaces local forage crops used by wild animals, birds, and insects. There are few native species that use Scotch Broom in any capacity.

In grasslands, it threatens our native grasses and significantly reduces species richness. It currently threatens already threatened native butterflies in these habitats. It is very aggressive and tends to outcompete the other native vegetation in these areas as well. To worsen matters, shade-tolerant non-native species such as Kentucky Bluegrass, St. John’s Wort, Wall Bedstraw, and many others will grow under the shade of Scotch Broom. As a result these shade-tolerant non-native species have a much higher proportion of vegetation cover in areas where Scotch Broom has invaded compared to grasslands without it.

In logging patches sometimes the rapid growth of Cytisus scoparius prevents tree seedlings from getting re-established. This costs the logging companies significant losses in terms of invasive species removal and replanting. In Oregon, USA alone it costs an estimated $47 million in lost timber production.

Potential Benefits of Invasion

There are native wildlife species that are known to browse on the leaves and/or seeds including deer, elk, mice, grouse, and quail. However, Scotch Broom dramatically reduces species richness in habitats that it invades, reducing its potential value. Even though native mice have been found to feed on the broom, studies have shown a reduction in deer mice population in habitats where Scotch Broom has invaded.

Methods to Remove Scotch Broom

As always prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in many local garden stores. Do not buy or transport any Scotch Broom. 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. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species. Cultivars that are supposedly non-invasive have too frequently been found to be mis-labeled or otherwise shown to be invasive when it escapes into the wild.

Physical Control Methods

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 to remove Scotch Broom generally involve cutting them down with hand cutters, loppers, hand saws, axes, machetes, or a small power saw, depending on the size of the shrub. Be sure to cut as close to ground level as possible.

Best Time to Remove Scotch Broom

The best time to remove Scotch Broom is anywhere from late fall to early spring before it has a chance to flower again as the seeds mature rapidly and remain viable in the soil bank for up to 60 years. You should never disturb the soil in a Broom patch from late spring to mid-fall. If you disturb the soil it will allow the prolific seeds to germinate and continue to grow the following year.

It is best to pull Scotch Broom after a rain when the ground is wet as the roots will come out of the ground much easier with less disturbance to any surrounding plants that may be present.

In areas of drought cutting in early spring when in flower or when the pods are still very immature can be a very effective method. Cutting followed by drought gives the Scotch Broom much less chance to survive. Simply cut them, dispose of the shrubs and then implement your monitoring program.

Removal Methods to Prevent Resprouting

In wetter areas however they should be removed completely, if at all possible. Cutting alone in these areas results in a higher proportion of plants re-sprouting from the remaining rootstock. Cutting does work better however on larger plants that are woody and no longer green at the base. If you are unable to remove the root them to help ensure against re-sprouting you can either peel the bark on the stump down to below ground level or chop into the stump several times with a hand ax. Both methods help reduce the rate at which they will re-sprout.

Hand-operated weed pullers work very well to remove the entire plant though they are labor-intensive. You can use loppers to cut off some of the lower branches first before using the weed puller. The largest ones may require a shovel to dig them out completely. Alternatively you can chop the stumps and leave them in the ground and let the yearly monitoring program deal with them.

Low-temperature fires are not an effective means of control because this actually encourages the germination of the seeds in the soil seed bank. Furthermore, sprouts will re-grow from burned stumps left in the ground, particularly in wetter climates. However, prescribed burning can be beneficial after physical removal to stimulate the seed germination followed by physical removal of the seedlings. This is a great way to help reduce the seed bank present in the soil.

Mulching in winter or early spring may help prevent seedling germination from areas where the Scotch Broom removal involves uprooting and disturbs the soil.

Mechanical Methods

Tractor-mounted mowers and other mechanical means are not recommended for large infestations as this typically spreads the seeds around and will destroy any native vegetation that may have still been growing there. Physical removal by manual means is labor-intensive, but remains the most viable method.

Disposal of the Shrubs Once Removed

Once the Scotch Broom has been cut down they should be burned on the spot if possible. Never just dump the shrubs with yard waste, even if they were not in seed when cut down due to the tenacity of the seeds. Yard waste dumping piles are often one of the worst sources of invasive species in our environment.

If you are unable to burn them in your area due to fire bans you can solarize them. To solarize you can put the shrubs under a thick black tarp or into black garbage bags. Then leave them in full sun for 10-12 weeks to ensure they are no longer viable. Some sources recommend shorter times for solarization. However, Scotch Broom seeds are notoriously difficult to destroy so it is important they are solarized for as long as possible. Then they can be disposed of. When bringing them to a local garbage dump make sure you tell them they are invasive species so that they will bury them right away.

Chemical Control Methods

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 Scotch Broom. And due to the nature of Scotch Broom, multiple applications are always needed.

Chemical control is not recommended.

Biological Control Methods

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 problem with biological control is that the agent used must be entirely specific to only the target organism. 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 old 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 Scotch Broom on their own. Results this high are seldom achieved in the field. However, using biological control as part of an integrated management approach can be very effective. Following is a list of biological control methods that have been used in North America to reduce the population densities of Cytisus scoparius.

Insects Used for Biological Control

The broom twig mining moth Leucoptera spartifoliella was first released in California in 1960 where it reportedly did significant damage, though the plant is still highly invasive there today. It was later discovered to be already present in Washington state through accidental introduction back in 1941 and the plant is still highly invasive in that region as well. There have been better results with Leucoptera spartifoliella reported in Australia and New Zealand.

The broom seed beetle Bruchidius villosus has been used with some success in New Zealand and Australia where up to 60% of seeds were seen to be eaten. It was accidentally introduced in eastern North America and then later moved to western North America where it has reportedly caused significant damage to Scotch Broom populations. In North Carolina, USA, where it was introduced some years ago it has been found to reduce seed production by 80%. The broom seed beetle is useful in that it feeds on the seeds of both Scotch Broom and Striated Broom so it would be useful in areas such as California where both are present in abundance.

The Scotch Broom seed weevil Exapion fuscirostre is specific to Scotch Broom. It was released in California in 1964 where it was seen destroying 60-90% of seeds produced on site. In Oregon, it was found to attack 40-60% of pods and of those pods, it destroyed an average of 85% of the seeds contained within.

The broom gall mite Aceria genistae was approved for release in New Zealand in 2001 but its maximum dispersal rate has been shown too slow to be of likely benefit to land uses threatened by Scotch Broom. It was recently found in King County, Washington but was not approved for release there so studies are still ongoing.

The sap-sucking broom psyllid Arytainilla spartiophila was released in New Zealand and Australia in the 1990s with some success. In North America, however, it was accidentally introduced into California and showed little success, likely due to the high mortality observed in the psyllid.

Mycoherbicides Used for Biological Control

Mycoherbicides, the use of fungi to harm or destroy the invasive plants are currently being investigated. Chondrostereum purpureum has been field-tested in Canada. Fusarium tumidum is being developed as a mycoherbicide in New Zealand.

Animals Used for Biological Control

Domestic goats and sheep will eat Scotch Broom. Grazing animals are only a viable option when they are to remain in place for many years. Domestic goats in particular will eat just about anything, but my readers told me that Sheep will eat this too. Grazers are especially useful in young patches rather than large, mature, well-established stands of Scotch Broom. The animals would need to be contained in a fence or on a lead that keeps them only in the Scotch Broom patch so that they cannot eat the native species that might be present there. However, as the stumps will continue to regrow for many years if the animals are removed the patch will very quickly be reestablished. This is why the grazers would either need to remain for many years or be used as part of an Integrated Management approach.

Integrated Pest Management & Ongoing Monitoring

Integrated management is usually 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 Scotch Broom removal, the site should be replanted immediately. This is because the bare soil resulting from the removal of Scotch Broom will allow the seed bank in the soil to rapidly germinate and re-invade the patch. A replanting program should already be planned and ready to implement immediately upon removal of the Scotch Broom. Replanting should be timed appropriately for the region in terms of establishment success (usually early or mid spring). Furthermore, the species planted should be local ecotypes of native species already present in the area if at all possible. As the native species grow and fill in the area it provides less available habitat for Scotch Broom. Replanting must be followed by ongoing monitoring.

Ongoing Monitoring is Essential for Success

In all cases of invasive Scotch Broom removal, ongoing monitoring is absolutely essential. This is regardless of the type of control method used and whether or not replanting was also done. 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. It is also necessary to remove any seedlings that sprout in the newly disturbed soil. Scotch Broom is aggressive and prolific and will quickly outcompete planted or natural vegetation if yearly monitoring is not put in place.

References and Resources

CABI on Cytisus scoparius https://www.cabi.org/isc/datasheet/17610 **CABI now charges for access to these

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

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

Fire Effects Information System on Scotch Broom https://www.fs.fed.us/database/feis/plants/shrub/cytspp/all.html

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

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

Wikipedia French Broom https://en.wikipedia.org/wiki/Genista_monspessulana

Wikipedia Spanish Broom https://en.wikipedia.org/wiki/Spartium

Willis, Lyrae (2021).  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!

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Introduction

Flowering Rush or Butomus umbellatus is an aquatic perennial from continental Eurasia and Africa. Despite its common name, it is not actually a rush which is part of the Juncaceae family in the Poales Order of Monocot plants. Flowering Rush is a member of the Butomaceae family of the Alismatales Order of Monocot plants. It is an invasive species in many parts of the world including North America, and once established removal of Flowering Rush can be challenging to say the least.

Butomus umbellatus has been spread around the world because of its use as an ornamental aquatic plant. It then escapes cultivation and wreaks havoc in native wetlands where it chokes out other vegetation. Please do not buy Flowering Rush for your garden as invasive aquatic plants are notoriously difficult to control once they become established. The best course of action would be to research what aquatic ornamentals are native to your area and plant those instead. There are also other non-native aquatic ornamentals that are non-invasive but again do your research for your specific region. Sometimes plants that are invasive in one area are not invasive in others due to differences in climate and other local factors.

Description of Butomus umbellatus

Leaves & Stems of Butomus umbellatus

Flowering Rush is an emergent aquatic perennial from stout rhizomes with bulbils that spontaneously break off and float away allowing it to reproduce vegetatively. In appearance it superficially looks quite similar to native rushes and sedges. The narrow linear leaves grow to 100 cm long, have no teeth, are parallel-veined, and often appear twisted. It has a round flower stem (scape) that can grow from 100 – 150 cm tall, which, without flowers makes it look similar to native rushes. The leaves are three-angled in cross-section, which can also make them easily confused with native sedges.

However, the umbel of pink flowers that appear on top will help you differentiate it from both native rushes and sedges. If you are not certain of the identification, wait for June to August when they are in flower to identify. Then you can mark it with a colorful flagging tape if you need to return to remove it in a season when it is not flowering.

Flowers & Fruits of Butomus umbellatus

Butomus umbellatus produces large clusters of pink flowers in umbels on top of the 100 – 150 cm long scape. Each bisexual flower is from 2 – 3 cm across and is light pink in color with darker veins. It has tepals rather than petals and sepals. Its outer tepals (sepals) are elliptic in shape and are 6 – 7.5 mm long while the inner tepals (petals) are oblanceolate and 9 – 11.5 mm long.

It has 6 – 9 stamens that have 3 – 4.5 mm long filaments topped with 1 mm long anthers. It has 6-9 partially connate (united) carpels crowned with a persistent style. The fruit is a 1 mm long follicle, though it primarily reproduces vegetatively through spontaneous fragmentation of rhizome buds known as bulblets or bulbils.

It flowers from June to August though it produces little if any seeds.

Native Distribution

Flowering Rush or Butomus umbellatus is native to continental Africa and Eurasia including Afghanistan, Albania, Algeria, Armenia, Austria, Azerbaijan, Belarus, Belgium, Bosnia and Herzegovina, Bulgaria, China, Croatia, Czech Republic, Denmark, Estonia, Finland, France, Georgia, Germany, Greece, Hungary, Iran, Iraq, Ireland, Israel, Italy, Jordan, Kazakhstan, Latvia, Lebanon, Lithuania, Moldova, Mongolia, Morocco, Netherlands, Norway, Poland, Portugal, Romania, Russia, Serbia, Spain, Sweden, Switzerland, Syria, Turkey, Turkmenistan, Ukraine, United Kingdom, and Uzbekistan.

It is considered an endangered species in Israel where it is native to but has been threatened by habitat loss.

Human Uses of Flowering Rush

Flowering rush is often used as an aquatic ornamental plant in ponds and water gardens.

Sometimes it is also used as a food source as the bulbets contain more than 50% starch. They are peeled then steamed and eaten. They can also be dried and ground and used as a thickener for soups or ground with grains into flour for breads and other uses.

It has occasionally been used medicinally as an antimicrobial agent. However, recent studies on its use for this did not show positive results.

Habitat Types Where Flowering Rush is Found

Flowering rush is an emergent aquatic found in marshes, lakes, rivers, riparian zones, and wetlands in water up to 3 m deep. It grows to the deepest range that emergent aquatic plants can be found. It can also inhabit storm-water retention ponds, wet gravel pits, and roadside ditches. Sometimes it also grows as a submerged plant in rivers and lakes. It is an aquatic plant that requires access to water year-round but it has tolerance to variable depths of water, making it a suitable invader for disrupted wetlands. It, however, does not tolerate brackish water or salinity so it will not invade estuaries or inland brackish lakes.

Flowering Rush requires sunlight and will not grow in shade. It also prefers cool to warm temperate climates and will not grow in tropical climates. It grows in most soil types including sandy, loamy, and clay and tolerates acid, neutral, and basic pH levels.

Distribution of Butomus umbellatus in North America

The species was first observed in North America in 1897 in the St. Lawrence River area and has spread throughout eastern North America. It has since then continued its spread westward through long-distance dispersal primarily from boaters and intentional plantings from gardeners. From there, once introduced by humans to a new area they will spread vegetatively. Humans are the primary means of long-distance dispersal. This makes wetlands and lake shores located near human settlements at the highest risk of invasion. The bulbets frequently escape a garden pond during high water or from humans or wildlife transporting them.

In Canada, Butomus umbellatus has been recorded in most of eastern Canada including New Brunswick, Nova Scotia, Prince Edward Island, Quebec, Ontario, and Manitoba. It is particularly problematic throughout the Great Lakes region. In western Canada it is found in both BC and Alberta. So far in BC, however, it has only been found in the Lower Mainland. It currently is not found in Saskatchewan, Newfoundland, or any of the northern territories.

In the USA, Flowering Rush has been found in Maine, Connecticut, Vermont, New York, Pennsylvania, Ohio, Michigan, Wisconsin, Indiana, Illinois, Minnesota, North Dakota, South Dakota, Nebraska, Montana, and Idaho. It is particularly problematic in the Great Lakes Region. It is illegal to buy or sell Butomus umbellatus in several of the eastern states. So far it has not been reported in any of the southern states.

In Mexico, so far, Butomus umbellatus has only been reported in one location. In 2017 it was found in Puebla Nuevo, Durango, Mexico on the eastern edge of the Sierra Madres.

How Flowering Rush Spreads

It is primarily dispersed vegetatively through bulblets that are carried by water currents or wildlife to new locations. The little bulbils float in water and are easily transported this way. Muskrats, waterfowl, and water currents are believed to be the primary method of local dispersal.

Humans are believed to be the primary source of long-distance dispersal. Anthropogenic dispersal is primarily done through gardeners planting them in their water gardens. This is likely how it was been introduced to North America in the first place. Boaters are another human source of dispersal where the small bulbils get carried from one location to another.

Habitats at Risk of Invasion by Butomus umbellatus in North America

The habitats most at risk of invasion in BC are marshes, lake shores, wetlands, and any aquatic habitat including roadside ditches. Since it is hardy from zones 3-10 it has a high potential for invasion throughout most of North America, excluding the most northern parts of Canada, desert areas throughout the southern USA and northern Mexico, and the most southern parts of Mexico where the climate becomes tropical.

Worst case scenario is that the flowering rush will invade lake shores, marshes, rivers, streams, and wetlands throughout the continent. They would displace our native wetland vegetation in the process and significantly reduce biodiversity in the affected areas. The associated economic costs will be enormous if it gets out of control since aquatic weeds are notoriously difficult to control. Since it has been seen to compete with even willows and cattails, it is expected to be highly invasive in any aquatic habitat that it gains a foothold in.

Impacts of Invasion by Butomus umbellatus

Studies in Canada have shown that in 40% of the areas where it was found Flowering Rush made up more than 50% of the total vegetation coverage. This suggests it has a high potential for being invasive and displacing native vegetation reducing biodiversity in the process. Light and nutrient levels are also affected where it is found because of its ability to form dense mats of vegetation. This would further increase competition with native species which would then get out-competed. Flowering Rush has been found to increase sedimentation in our waterways as well as reduce water flow through the dense mats they form. Given the other major threats all our wetlands, waterways, and riparian areas already face (habitat loss, increased nutrient input, altered water regime, pollution, other invasive species, etc) this is all of great concern.

Butomus umbellatus has already been shown to negatively affect wild rice (Zizania aquatica) populations. Flowering Rush reduces wild rice biomass which reduces their potential food source to both wildlife and the indigenous peoples who still use the rice as a food source.

Flowering Rush has the potential to clog irrigation channels reducing water delivery for agriculture. It also cogs other waterways impeding the flow of water, boats, and other industrial uses. Butomus umbellatus also reduces open water which affects recreational uses such as boating, swimming, and fishing.

Flowering Rush has also been shown to provide an ideal habitat for Lymnaea stagnalis, the great pond snail. This snail is an intermediate host for Trichobilharzia ocellata which is the trematode responsible for swimmer’s itch.

Potential Benefits of Invasion

Muskrats and waterfowl have been found to feed on the bulbils, providing food for native animals. However, the native vegetation that it displaces would have also provided that same benefit. It may also provide structure for fish that require vegetation for spawning. However, the increased sedimentation and reduced water flow that result from their invasion would counter that apparent benefit as well.

Removal of Flowering Rush

As always prevention is the preferred method of control. Do not buy or transport any of the bulbils. If you acquire them somehow do not simply throw them in the trash. Instead, dry them thoroughly first or burn them to ensure they do not survive to propagate. 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.

Physical Control Methods

Once already established, however, aquatic invasives are notoriously difficult to control. Physical control is the most effective means. This can be challenging, however, depending on water depth. Choose a time of the year when the water is at its lowest point. If necessary, mark the the patch with flagging tape when in flower so that you can return to it when the water is lower and they are not in flower.

When removing single isolated Flowering Rush plants hand pulling can be a very effective means of control to prevent them from getting a foothold in a new environment. Simply dig out the plants, removing all pieces and taking care to remove any floating pieces of bulbs as well. This is the most effective time to control them once they are found in a new environment before they become a dominant plant in the community.

If you are working on a large patch of Flowering Rush it would be a good idea to use an aquatic containment net around the area you are removing Flowering Rush from to keep all the pieces contained. (See Making an Aquatic Containment Net Below). When removing a large patch you can use your hands or a narrow-headed shovel if necessary, but be careful to remove all the rhizomes in the soil so dig around the edge of the patch a little bit to ensure you get all the rhizomes.

How to Make an Aquatic Containment Net

Purchase an appropriate length of fine-mesh netting. Netting can come in all different size meshes, heights and lengths. You want fine-mesh with holes that are less than 5 mm, large netting more than that will allow pieces to flow through. They also come in different depths which you will need to determine based on the depth of your water. You want it to sit on the soil in the water and reach the surface of the water at least.

To make your containment net you need to weigh down the bottom edge of the net. Every 20-30 cm sew, zap-strap or otherwise attach weights or rocks to the bottom edge of the net only. I have folded over the bottom and sewn rocks into nets in the field before and it worked great. This keeps the bottom of the net in the water and prevents it from floating. Then you can use posts pounded into the soil to hold the net in place, again zap straps work good in a pinch for attaching the nets, just make sure you use the reusable ones for this part. Then strain the water in the enclosed area with a net to be sure you remove all the plant pieces before removing the containment net and moving onto the next patch.

Alternative Physical Control Method

Note that simply cutting the plant below the water surface will not kill the plant. And since they spread almost exclusively by vegetative means preventing them from flowering also will not reduce their population. However, sometimes this method can be effective, particularly if deep water levels make it difficult to dig them out. You would need to return to the area and cut them down repeatedly throughout the growing season. This can reduce the area covered by Butomus umbellatus, allowing room and resources for other plants to grow in their place. This could weaken the plants eventually to the point they will be eradicated if this process is repeated annually. This is because they will not be able to put enough energy into their rhizomes to promote their growth for the following year(s). This is less labor-intensive in the short-term than digging them out but is much more labor-intensive in the long term.

Ongoing Monitoring

In all cases of physical removal ongoing monitoring is absolutely essential. Yearly monitoring programs need to be put in place to ensure that any surviving individuals are removed so that the population is not able to recover.

Chemical Control Methods

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 there are no chemical control methods that currently target only Butomus umbellatus. Since the environment is aquatic it is also exceedingly difficult to keep the chemical control method contained so that it does not affect any native species that are there.

Biological Control Methods

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. This 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 1800’s 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.

There are currently no effective biological control methods in use for Butomus umbellatus. However, CABI is working on test studies with a semi-aquatic weevil, a fly, and a fungal pathogen. All are showing some promise but may need to be further studied before put into wide-scale use.

References and Resources

Butomus umbellatus photo from Wikipedia By Ivar Leidus – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=27558951

CABI Biological Control of Flowering Rush https://www.cabi.org/projects/biological-control-of-flowering-rush/ **CABI now charges for access to these!

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

Great Lakes Aquatic Nonindigenous Species Information Systems by NOAA https://nas.er.usgs.gov/queries/greatlakes/FactSheet.aspx?Species_ID=1100&Potential=N&Type=0

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

US Fish & Wildlife Services Flowering Rush (Butomus umbellatus) Ecological Screening Summary https://www.fws.gov/fisheries/ans/erss/highrisk/ERSS-Butomus-umbellatus-FINAL.pdf

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

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

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