Introduction

I never saw saltcedar, Tamarisk, or any of the other common names that Tamarix species go by until I traveled south. So far, they have not ventured far into Canada and certainly haven’t been venturing there long. However, in the south, they thrive in hot, dry areas, particularly saline soils where few plants will grow, allowing them to take over. There are 50 – 60 species of Tamarix in the world, and not one is native to North America. In North America, we have Tamarix aralensis, Tamarix aphylla, Tamarix africana, Tamarix canariensis, Tamarix chinensis, Tamarix gallica, Tamarix parviflora, Tamarix ramosissima, and Tamarix tetragyna.

All Tamarix species should be considered invasive in North America. They have covered over a hundred million acres in the western US alone, and they frequently hybridize, making identification to the species level challenging. For the purpose of identifying them as an invasive species and for Tamarisk or Saltcedar removal or control, identification to the genus level is more than sufficient.

Description of Tamarix spp

Leaves & Stems of Tamarix spp

Tamarix spp are part of the Tamaricaceae family in the Caryophyllales Order of flowering dicot plants. They are either shrubs or small trees from 1- 10 m tall and are often multi-stemmed. They are a long-lived species living 100 years or more. Most species are deciduous. They are almost all very deep-rooted, with a tap root up to 30 m deep that also produces lateral roots up to 50 m long that can produce adventitious buds, especially when covered by shifting sand. Their deep taproots are meant to reach down into the groundwater, allowing them to survive in hot drought-prone climates as long as they are close to the water table.

Young branches are glabrous and are a variety of colors depending on the species. They may be reddish-brown, brown, blackish-brown, dark purple, grey, or black. Older branches have heavy bark that is frequently shredded and may be grey or brownish and reaches 10 – 15 (-30) cm in diameter.

The usually sessile (without leaf stalks) leaves of Saltcedars are much reduced and appear small and bract-like, somewhat resembling that of coniferous tree needles. Leaves are varying shades of dull, light, or dark green and turn golden-orange in the fall.

Flowers & Fruits of Tamarix spp

Tamarisk or Saltcedars produce a racemose inflorescence that is often paniculately branched. They are borne on the ends of current-year branches or in some species’ previous-year branches. In most species in North America, the flowers appear after the leaves have grown back. The flowers are very small, usually pink, and most have small petals only 1 – 2 mm long.

Fruits are multi-sided capsules with up to thousands of tiny seeds. The capsules usually have a tuft of hair that aids in wind dispersal. They can also be dispersed in water. Seed viability is very short, however, from around 20 to a maximum of 120 days, depending on the weather. However, Saltcedars can flower and produce seeds for extended periods of time, allowing them to reproduce prolifically even with their short seed life.

Similar Species Tamarix spp are Frequently Confused With

Other Genera

There are a few other genera that Tamarix species are occasionally confused with that also grow in similar habitats and ranges and have much-reduced leaves. However, all of those can easily be differentiated by their flowers.

Parkinsonia species are members of the Fabaceae family and have larger yellow, somewhat pea-like flowers and produce small legume fruits.

Prosopis glandulosa has sprays of yellow rather than pink flowers, and it is also a Fabaceae that produces long legume fruits.

Lepidospartum squamatum has a limited range native to the southwestern USA and northwestern Mexico and has small yellow Asteraceae-type disk flowers and typically Asteraceae cypsela fruits. Baccharis neglecta has a similar North American range, but it is a perennial plant, not a shrub, and produces individual whitish flowers more typical of the Asteraceae family it belongs to.

Polygonella robusta is an uncommon plant of the Polygonaceae family that is endemic to Florida and produces similar sprays of small pink flowers, but its leaves, while still small, are long and linear compared to all Tamarix species.

Juniperus virginiana smells strongly of Juniper and has characteristic powdery blue “berries” and no pink flowers.

Casuarina equisetifolia is widespread in Mexico and more limited in the southern USA. It has needle-like leaves, but they are longer, divided into septa, and grouped in fascicles, and it produces a globose cone-like fruit.

Tamarix Genera

Tamarix species can be challenging to identify at the species level. For the most part, genus-level identification in North America is sufficient for treating the plants as invasive species, as none are native to North America. However, with a bit of patience and a small amount of skill, they can be identified to the species level as follows:

• Tamarix aralensis Russian Tamarisk has a limited range in North America. It is similar to T. chinensis and T. ramosissima but has a much more limited range. It can be differentiated by its petals that fall off at the time of seed maturation.
• Tamarix aphylla Athel Tamarisk is more widespread in Mexico than in the southern USA. It is the only evergreen species, and its leaves are sheathing on the stems rather than sessile or amplexicaul like all other Tamarix species in our area. It also produces white instead of pink flowers.
• Tamarix africana African Tamarisk has a very limited range in the southern USA. It has the largest flowers (though still small) with petals that are 2 – 3 mm long and racemes that are 5 – 9 mm wide.
• Tamarix canariensis Canary Island Tamarisk has a limited range in the southern USA. It is similar to T. chinensis and T. ramosissima, but its sepal margins are denticulate, and it has obovate petals that are 1.2 – 1.5 mm long.
• Tamarix chinensis Five-Stamen Tamarisk is widespread in Mexico and the USA. It also is the most similar to T. ramosissima, and the two often hybridize, leading some to believe they should be considered the same species. They have five petals and five stamens. They can only be differentiated by this one’s sepal margins that are entire and that some or all of the stamens’ filaments originate from below the nectar disc. Filament inspection requires patience, a hand lens, and some skill. Furthermore, its petals persist at seed maturation, unlike T. aralensis.
• Tamarix gallica French Tamarisk has a limited west and south range in the USA and northern Mexico. It is also similar to T. chinensis and T. ramosissima, but its sepal margins are entire or almost entire, and it has elliptic to ovate petals that are 1.5 – 2 mm long.
• Tamarix parviflora is very widespread in the USA and is limited to the northern parts of Mexico. Its flowers are produced before it gets its leaves, and it usually only has four petals, unlike all other species in North America, whose flowers are five-petaled and arrive after the appearance of leaves.
• Tamarix ramosissima is fairly widespread in both USA and Mexico. It is the most similar to T. chinensis, and the two often hybridize, leading some to believe they should be considered the same species. They have five petals and five stamens. However, this species’ sepal margins are denticulate rather than entire, and all the stamens’ filaments originate from the edge of the nectar disc rather than below it in T. chinensis. Filament inspection requires patience, a hand lens, and some skill. Furthermore, its petals persist at seed maturation, unlike T. aralensis.
• Tamarix tetragyna Four-Stamen Tamarisk is found only in Georgia. It can be differentiated by its flowers that have only four stamens as opposed to the five that all other species in North America have.

Native Distribution of Tamarix spp

There are approximately 100 species of Tamarix, and all are native to Eurasia and Africa. None are native to the Americas.

Habitat Types Where Saltcedar is Found

Saltcedar, with its long taproots, is usually found only near groundwater and is, therefore, particularly common in riparian habitats, including floodplains, permanent or ephemeral stream banks, and around lakes and water reservoirs. They can also grow in upland habitats out of contact with groundwater, but they are less common there and grow less vigorously there. They can grow from 0 – 2000 m above sea level.

There is a common misconception that they are saline-loving plants, but in fact, they tolerate a wide range of soils, including those fairly high in salinity. This gives them a significant competitive advantage in saline areas where most other plants cannot survive. In those conditions, they typically rapidly produce monocultures displacing the few native halophytes found there.

Human Uses of Tamarisk

Tamarisk is used as an ornamental, especially in marginal habitats or saline soils. It was also widely planted as a stream and dune stabilizer in some areas. It is being used in China to fight the expansion of deserts.

Tamarisk wood is sometimes used for firewood, carpentry, and the making of bows and other tools.

In Africa and Asia, Tamarix species are used medicinally to treat wounds and infections as well as liver and spleen disorders. It is seldom currently used in North America medicinally.

Distribution of Tamarix spp in North America

The species was first brought to North America in the 1800s as an ornamental species. There are no species of Tamarix native to North America.

In Canada, Tamarix ramosissima has been recorded but ephemeral in British Columbia. It has been seen but not lasted long enough to confirm in Ontario, Manitoba, Quebec, and Nova Scotia.

In the USA, nine Tamarix spp are found. They are distributed as follows:

• Tamarix aralensis Russian Tamarisk is found in California and North Carolina.
• Tamarix aphylla Athel Tamarisk is found in California, Nevada, Utah, Arizona, and Texas.
• Tamarix africana African Tamarisk is found in California, Arizona, Texas, Louisiana, and South Carolina.
• Tamarix canariensis Canary Island Tamarisk is found in Arizona, Louisiana, Georgia, South Carolina, and North Carolina.
• Tamarix chinensis Five-Stamen Tamarisk is more widespread and is found in Washington, California, Nevada, Utah, Arizona, Montana, Wyoming, Colorado, New Mexico, Oklahoma, Texas, Arkansas, Ohio, and North Carolina.
• Tamarix gallica French Tamarisk is found in Washington, California, New Mexico, Texas, Louisiana, Georgia, South Carolina, and North Carolina.
• Tamarix parviflora is the most widespread species in the USA, being found in Washington, Oregon, California, Idaho, Nevada, Utah, Arizona, Montana, Colorado, New Mexico, Kansas, Oklahoma, Texas, Missouri, Louisiana, Illinois, Mississippi, Kentucky, Tennessee, Florida, North Carolina, Virginia, Michigan, Pennsylvania, Delaware, New Jersey, Connecticut, and Massachusetts.
• Tamarix ramosissima is also fairly widespread, being found in California, Nevada, Utah, Arizona, Colorado, New Mexico, North Dakota, South Dakota, Nebraska, Kansas, Oklahoma, Texas, Arkansas, Louisiana, Mississippi, Georgia, South Carolina, North Carolina and Virginia.
• Tamarix tetragyna Four-Stamen Tamarisk is found in Georgia.

In Mexico, there are five species of Tamarix found as follows:

• Tamarix aphylla is the most widespread found in Baja California, Baja California Sur, Sonora, Sinaloa, Chihuahua, Durango, Jalisco, Michoacan, Coahuila, Nuevo León, Tamaulipas, San Luis Potosi, Guanajuato, and Mexico City.
• Tamarix chinensis is found in northern Mexico in Baja California, Baja California Sur, Sonora, Sinaloa, Chihuahua, Coahuila and Nuevo León. It is possibly found in Mexico City, Mexico State, and Tamaulipas, but this is unconfirmed.
• Tamarix gallica is much less widespread and also only found in the northern states of Baja California, Coahuila, and Nuevo León. There is also an unconfirmed location in Chihuahua.
• Tamarix parviflora is the least widespread in Mexico and is only found on the northern border of Baja California. It is not far from the border in New Mexico, so it may likely spread to Chihuahua from there soon.
• Tamarix ramosissima is the second most widespread in Mexico, with populations found in Baja California, Baja California Sur, Sonora, Sinaloa, Chihuahua, Coahuila, Nuevo León, Guanajuato, Queretaro, Mexico State, Mexico City, and Guerrero.

How Tamarisk Spreads

It is primarily spread long-distance by deliberate human introductions into gardens or as a landscape plant.

Short-distance dispersal occurs through the abundant tiny seeds spread by wind, water, humans or animals. Seeds have a very short viability period; however, if they are not germinated during the summer that they are dispersed, almost none germinate the following year. However, Saltcedar produces seeds over a very long period enabling its rapid spread. Mature tamarisk plants also spread vegetatively by adventitious roots producing colonies of clones. 

Habitats at Risk of Invasion in North America

All riparian areas are at risk of invasion, particularly in the semi-arid and arid southern areas of our continent. More northern areas are less at risk, but we have seen invasion into North and South Dakota, Washington State, and New York, as well as the so-far ephemeral invasion into southern Canada. Tamarisk or Saltcedar has expanded its riparian dominance significantly since the 1800s and now covers huge geographical areas. Its rapid spread does not seem to be slowing, and its range continually grows.

In California desert areas, Saltcedars have now even become established in remote mountain springs, streams, and washes. These areas show no sign of human disturbance and are long distances away from any source of infestation. This means that wild riparian areas far from humans are not safe from Saltcedar invasion.

Impacts of Invasion

Tamarisk thrives in riparian habitats where it quickly displaces native vegetation due to rapid growth, tolerance of a wide range of environmental conditions, and long periods of abundant seed production. Tamarisk also accumulates salt in its leaf glands and excretes it onto the leaf surface. These salts accumulate in the soil when the plants drop their salty leaves. As surface soils become more saline over time, they further exclude native vegetation, producing monocultures. Native herbivores’ preference for native plants gives Tamarix species yet another competitive advantage.

Significant loss of biodiversity occurs in the stands of Saltcedar. Plant diversity is dramatically reduced, resulting in a loss of riparian bird, insect, invertebrate, aquatic and animal diversity.

Potential Benefits of Invasion

Saltcedar provides a habitat for a number of bird species. However, many of these are introduced from the Old World, where Tamarix evolved; though some of our native birds do use it, they still prefer native plants.

Methods to Remove Tamarix spp

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 Saltcedar. Do not plant it in your yard. There are so many Native Plant Species that would make suitable garden specimens that are noninvasive and provide additional wildlife and biodiversity values.

If you see Tamarisks 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.

Physical Control of Saltcedar Tamarisk

Once already established, however, physical control is a labor-intensive but effective means to remove Saltcedar. Physical control is time-consuming and costly, but it usually causes the least amount of environmental damage and leaves the area the most biodiverse afterward.

The best time to remove Saltcedar or Tamarisk is before it has gone to seed. Because it flowers and seeds for an extended period of time, this must be done in the late fall, winter, or early spring. If the plants are in seed upon removal, they can be burned or solarized as the seeds are sensitive and should die easily.

Physical methods to remove Saltcedar generally involve pulling young plants by hand, if very small, or with a weed puller or digging them out with a hoe or shovel.

Mature specimens are generally cut down, but the rootstock must be either dug out with a machine or chemically treated for a period of time to prevent re-sprouting. Isolated larger specimens can be solarized to prevent re-sprouting, but the tarp must remain for 2 growing seasons. Another method is to simply keep returning to the area a few times each growing season for several years in a row to keep cutting any new sprouts. This will eventually starve out the roots and kill the plant.

Along controlled rivers scheduling 5 – 10 year floods which wash out the Saltcedars removing them and leaching salts from the soil. This allows the native vegetation adapted to periodic flooding to grow back and displace the Saltcedar. The seedlings can be easily killed by keeping an area flooded for one month. The flooding method can also be used in low-lying areas that are isolated. Cut down all mature plants to the ground and keep the area flooded for an extended period of time. It will kill or prevent any new seedlings or sprouting and will eventually start rotting out and starving the roots. After the water is allowed to drain, the short seed viability period makes it unlikely that many new seedlings will emerge. Ongoing monitoring and hand-pulling of the few that do grow can be effective at preventing the population from becoming re-established.

Methods Not Recommend

Neither mowing nor burning will effectively control mature Saltcedar. Plants will quickly regrow from their basal stem buds and be supported by their extensive and deep root systems. Seedlings can be controlled by physical removal as mowing is generally ineffective on them as well due to their roots.

Disposal of the Shrubs Once Removed

If you have plants that have seeds on them, they must either be burned or solarized. Though mature plants are fire-adapted and will regrow, their sensitive seeds are vulnerable to fire, so the burning of shrubs is desirable in areas where burning is allowed.

If burning is not allowed, you should solarize the shrubs under a thick black tarp or in thick black garbage bags and leave them in the full sun for 6-8 weeks to be sure that all seeds are no longer viable. Since seed viability is poor, solarizing is also an effective method to destroy seeds.

Chemical Control of Saltcedar Tamarisk

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 increase.

Saltcedar is somewhat resistant to chemical control methods, with up to 30% of plants resprouting up to three years after treatment. This means multiple applications will be needed. Furthermore, there are no chemical control methods that effectively target only the control of Saltcedar. Finally, due to their preference for riparian habitats, the use of herbicides is often not allowed due to proximity to water.

However, professionals and government officials may determine where and when chemical applications can be safely carried out to deal with vast areas of infestation. Chemical control by individuals is generally not recommended.

Biological Control of Saltcedar

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 Saltcedar on their own. Results this high have never been achieved in the field. However, using biological control in conjunction with physical control and ongoing monitoring can be very effective.

So far, in North America, only one biological control agent has been released to control Tamarisk. Diorhabda elongata deserticola is a leaf beetle from western China and eastern Kazakhstan that was released in the USA in 1986. The beetles established well in the more northern areas and, after the third season, were seen to defoliate over 95% of the plants in the area. The populations have been spreading and are successfully helping to control Saltcedar in larger and larger areas. The introduced leaf beetle has also shown to be highly selective for Tamarisk and so far has not been observed eating any native vegetation. This makes it highly suitable for the biological control of Saltcedar.

The leaf beetles, however, did not establish in the southern areas due to increased predation and climatic differences. Additional subspecies of D. elongata have now been obtained from lower latitudes in Europe and Asia to be tested in more southern locations. It is too early for conclusive results.

Goats and cattle will graze on Saltcedar, but cattle, in particular, will selectively graze on native vegetation where it is present. Goats are less selective and will generally eat anything. Grazing would need to be done on a continual basis for at least 4 – 5 years so that they eat all sprouts that try to grow back up from the rootstock. Eventually, this would starve out the rootstock, and the plants would die.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form, this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring. Integrated management is required because the area needs to be monitored for returning sprouts or seedlings; otherwise, all the hard work done in removal could be wasted if the invasive species is allowed to regrow.

Replanting With Native Species is Crucial

In all cases of removal, the site should be replanted once the area has been cleared of Saltcedar and the soil has been leached of excess salts. Leaching the soil involves manually flooding the area with fresh water if there is insufficient rain in the region. Otherwise, waiting until after the rainy season may be sufficient as long as there are no more Saltcedars there.

Bare soil invites more infestations, and high salinity prevents biodiversity from improving. A replanting program should be planned and ready to implement the following year after the removal of Saltcedar. Then ongoing monitoring will be necessary to ensure no sprouts survive and no new seeds are accidentally brought into the area.

Ongoing Monitoring is Essential

In all cases of invasive Saltcedar removal, ongoing monitoring is absolutely essential. Yearly monitoring programs should be put in place to ensure that any surviving sprouting individuals are removed so that the population is not able to recover. Due to the short viability of seeds, new seedlings will stop emerging after a single season, provided no new seeds are accidentally transported to the site. Monitoring is required whether the area is replanted or not. Saltcedar has very deep and widespread roots that will easily regrow and start a new infestation if left unchecked. Yearly monitoring will prevent this and will allow the site to recover so that native plants will return.

References and Resources

CABI on Tamarix ramosissima https://www.cabi.org/isc/datasheet/52503

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

Fire Effects Information System on Tamarix spp https://www.fs.fed.us/database/feis/plants/tree/tamspp/all.html

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

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

Wikipedia Tamarix https://en.wikipedia.org/wiki/Tamarix

Willis, Lyrae (Unpublished).  Plant Families of North America.

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

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

Introduction

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

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

Description of Pyrus calleryana

Leaves & Stems

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

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

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

Flowers & Fruits

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

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

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

Similar Species Frequently Confused With

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

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

Native Distribution of Pyrus calleryana

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

Habitat Types Where Bradford Pear is Found

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

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

Human Uses of Bradford Pear

Callery Pear Pyrus calleryana as Ornamental or Tools

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

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

Distribution of Pyrus calleryana in North America

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

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

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

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

How Callery Pear Spreads

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

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

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

Habitats at Risk of Invasion in North America

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

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

Impacts of Invasion by Pyrus calleryana

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

Potential Benefits of Invasion

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

Methods to Remove Bradford or Callery Pear

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

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

Physical Control of Bradford or Callery Pear

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

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

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

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

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

Disposal of the Trees Once Removed

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

Chemical Control of Bradford or Callery Pear

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

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

Biological Control of Callery Pear

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

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

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

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

Integrated Pest Management & Ongoing Monitoring

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

Replanting With Native Species is Crucial

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

Ongoing Monitoring is Essential

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

References and Resources

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

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

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

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

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

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

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

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

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

Introduction

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

Description of Pueraria montana

Leaves & Stems

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

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

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

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

Flowers & Fruits

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

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

Similar Species Frequently Confused With

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

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

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

Native Distribution of Pueraria montana

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

Habitat Types Where Kudzu is Found

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

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

Human Uses of Kudzu

Kudzu as Food

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

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

Kudzu as Medicine

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

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

Kudzu as Ornamental

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

Distribution of Pueraria spp in North America

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

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

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

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

Kudzu has now been introduced on every continent except Antarctica.

How Kudzu Spreads

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

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

Habitats at Risk of Invasion in North America

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

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

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

Impacts of Invasion

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

Potential Benefits of Invasion

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

Methods to Remove Kudzu

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

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

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

Physical Control of Kudzu

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

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

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

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

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

Disposal of the Shrubs Once Removed

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

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

Chemical Control of Kudzu

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

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

Chemical control is not recommended.

Biological Control of Kudzu

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

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

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

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

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

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

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

Integrated Pest Management & Ongoing Monitoring

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

Replanting With Native Species is Crucial

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

Ongoing Monitoring is Essential

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

References and Resources

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

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

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

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

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

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

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

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

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

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

Introduction

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

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

Description of False Bamboo Reynoutria japonica

Leaves & Stems of Reynoutria japonica

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

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

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

Flowers & Fruits of Reynoutria japonica

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

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

Similar Species Frequently Confused With

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

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

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

Native Distribution of Reynoutria japonica

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

Habitat Types Where Japanese Knotweed is Found

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

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

Human Uses of Japanese Knotweed or False Bamboo

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

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

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

Medicinal Uses of Reynoutria japonica

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

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

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

Distribution of Reynoutria japonica in North America

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

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

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

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

Japanese Knotweed has been introduced to every continent except Antarctica.

How Japanese Knotweed Spreads

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

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

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

Habitats at Risk of Invasion in North America

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

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

Impacts of Invasion by False Bamboo

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

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

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

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

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

Potential Benefits of Invasion

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

Methods to Remove Japanese Knotweed

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

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

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

Physical Control of Japanese Knotweed

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

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

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

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

Disposal of the Shrubs Once Removed

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

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

Chemical Control of Japanese Knotweed

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

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

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

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

Biological Control of Japanese Knotweed False Bamboo

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

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

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

Integrated Pest Management & Ongoing Monitoring

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

Replanting With Native Species is Crucial

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

Ongoing Monitoring is Essential

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

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

References and Resources

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

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

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

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

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

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

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

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

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

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

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

Introduction

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

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

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

Description of Silybum marianum

Leaves & Stems

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

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

Flowers & Fruits

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

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

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

Toxicity

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

Similar Species Frequently Confused With

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

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

Native Distribution of Silybum marianum

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

Habitat Types Where Milk Thistle is Found

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

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

Human Uses of Milk Thistle Silybum marianum

Milk Thistle as Food

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

Milk Thistle as Medicine

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

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

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

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

Ethical Wildcrafting of Silybum marianum and Making of Medicines

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

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

Making Milk Thistle Tinctures

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

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

Distribution of Silybum marianum in North America

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

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

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

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

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

How Blessed Milk Thistle Spreads

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

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

Habitats at Risk of Invasion in North America

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

Impacts of Invasion by Silybum marianum

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

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

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

Potential Benefits of Invasion

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

Methods to Remove Milk Thistle

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

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

Physical Control of Milk Thistle

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

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

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

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

Physical Removal Methods of Milk Thistle That Are Not Recommend

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

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

Disposal of Milk Thistle Removed

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

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

Chemical Control of Milk Thistle

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

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

Biological Control of Milk Thistle

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

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

Biological Control Organisms Used in North America

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

Integrated Pest Management & Ongoing Monitoring

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

Replanting is Often Crucial

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

Ongoing Monitoring is Essential

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

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

References and Resources

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

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

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

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

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

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

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

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

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

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

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!