Introduction

Convolvulus arvensis, commonly known as Field Bindweed, is a well-known plant that has been recognized as being invasive in North America for more than 100 years already. This is because of its detrimental effects on agriculture, which led to its early recognition as an invasive weed. Once established, Field Bindweed is notoriously difficult to eradicate. Instead, most removal programs should focus on control rather than eradication. It is already a significant part of North American ecology, so we will never rid ourselves of this weed. However, new patches can be eradicated before they become too well-established. Any growth in natural areas, particularly nature reserves and parkland, should be eradicated where possible in order to preserve the native species also growing there. Agricultural lands, on the other hand, can focus on controlling this invasive pest to mitigate the damage they are causing.

Description of Convolvulus arvensis

Leaves & Stems

Convolvulus arvensis is a herbaceous perennial trailing or climbing vine. It grows from a very deep persistent root system that develops shoots from buds on the roots down to depths of 1 m. It has a main tap root that can grow from 0.5 – 3 m long. Other vertical roots from the main root may penetrate from 5 – 9 m deep, depending on the soil and site conditions. The roots also spread laterally in all directions from the main tap root allowing it to spread adventitiously throughout all of its length. Lateral roots are shallower and generally only penetrate down to 30 cm deep.

The stems are slender and grow from 20 cm to 2 m long and are either hairless or finely pubescent. Stems twine anticlockwise allowing them to climb over structures and other plants.

Leaves are alternate, have a petiole (leaf stalk), and vary significantly in shape and size. They may be lanceolate, ovate, or narrow-oblong and range in size from 1 – 10 cm long and 0.3 – 6 cm wide. The tips are acute, and the leaves themselves may be entire or hastate–sagittate (arrow-shaped) at the base and may be hairless or lightly pubescent.

Flowers & Fruits

Flowers of Convolvulus arvensis are axillary and may be solitary or found in cymes of 2-3 flowers on peduncles (flower stalks). They are subtended by bracteoles 2-4 mm long. The flowers have sepals that are free and obtuse in shape and 2.5 – 4.5 mm long. It has a funnel-shaped corolla with 5 radial pubescent bands. The corolla is entire and not divided into individual lobes, 1 – 2.5 cm in diameter and 1 – 2.5 cm long, and may be found in white or pink varieties. It has 5 stamens that are inserted in the corolla tube and a single style with 2 oblong stigmas.

Field bindweed produces a capsular fruit that is globular or ovoid with a persistent style at the base. It breaks open irregularly and releases four seeds that are 3 – 5 mm in diameter, dark brown or black, with a granular testa (outer coating).

Toxicity

Convolvulus arvensis is considered mildly toxic. It contains tropane alkaloids that can have toxic effects on the autonomic nervous system and have been found to cause intestinal fibrosis in horses. The seeds are especially toxic compared to the rest of the plant.

Similar Species Frequently Confused With

Many, many plants are often confused with Convolvulus arvensis. By far, most of them belong to the Convolvulaceae family and have similar bell-shaped flowers. Occasionally people mistake the unrelated Oenothera speciosa or Evening Primrose for Field Bindweed with its pink flowers. However, this can easily be differentiated by the fact that its corolla actually has 5 overlapping petals and is not at all entire like Convolvulus arvensis. The other non-related plant that is sometimes confused with it is Fallopia convolvulus Black-bindweed or Wild Buckwheat of the Polygonaceae with similar leaves but reddish stems and small lobed flowers. The more closely related and similar-looking species can be differentiated as follows:

• Calystegia sepium (sometimes as Convolvulus sepium) Hedge Bindweed of the Convolvulaceae family, has a sub cosmopolitan distribution with several subspecies native throughout North America. It has pale matt green sagittate leaves that are arranged spirally rather than alternately on the stem. When in bud, the flowers are surrounded by green bracts that are tinged with crimson, and its flowers, when open, are much larger, from 3 – 7 cm in diameter. Its fruit is an almost spherical capsule.
• Calystegia subacaulis of the Convolvulaceae family is a narrow-range species endemic to the North and Central California Coast Ranges and the San Francisco Bay Area, where it is found in woodland and chaparral scrub habitat. In its range, it can be differentiated by its hairy stems and leaves and its short stems that grow no more than 20 cm long.
• Calystegia macrostegia Bellflower of the Convolvulaceae family also has a narrow range found along the Pacific coasts of southern California and western Mexico. Its leaves are large and triangular and are often more than 10 cm wide. It also produces longer stems to 9 m in length and larger flowers 2 – 6 cm in diameter.
• Calystegia spithamaea of the Convolvulaceae family is native to eastern North America, primarily the northeastern USA. Its stems are mostly erect rather than trailing or twining, its light green leaves are quite hairy, and its flowers are not entire and instead have 5 shallow lobes.
• Convolvulus equitans the Wounded Bellflower is native to the southern US and northern Mexico. It can be differentiated by its usually lobed rather than entire leaves and its corolla of similar size but with 5 distinct though shallow lobes, each with a very distinctly acute apex. The flower also has a distinct pink or purple throat giving it the common name of Wounded Bellflower.
• Calystegia silvatica Greater Bindweed is native to Europe but has been introduced to the eastern and western US and Canada. It can easily be differentiated by its larger leaves that are arrow-shaped and its very large flowers up to 9 cm in diameter that are always white and never pink.
• Calystegia purpurata Pacific False Bindweed is a narrow endemic found along the Pacific coast of California. It has lobed leaves that are triangular in shape and larger flowers up to 5 cm in diameter that vary in color from white to pink to purple or cream and often have purple stripes.
• Evolvulus sericeus Silver Dwarf Morning Glory of the Convolvulaceae family is native to the southern USA, Mexico, and parts of tropical Americas. It can easily be differentiated by its very small size, only about 30 cm tall, and the fact that it grows erect rather than a vine. It also has linear to narrowly lanceolate light green or gray-green leaves that are generally hairy.
• Calystegia soldanella the Dunebell is native to beaches of western North America, Europe, and East Asia. It is found growing only on beach sand dunes making it easy to differentiate by its location. Its leaves are also very thick, glossy, and fleshy, producing large singular pink flowers.
• Calystegia occidentalis Chaparral False Bindweed is native to California and Oregon, where it grows in foothills and montane habitats. It has small leaves up to 4 cm that are typically hairy, lobed, and arrow-shaped. It produces 1-4 flowers on a single stalk, and each flower is fairly large, from 2 – 5 cm in diameter, and is white, cream, or yellow in color but never pink.
• Bouchetia erecta of the closely related Solanaceae family is native to the gulf coast of the US. It is a much smaller plant, never more than 30 cm tall, and it has lanceolate leaves and lobed corollas containing bright yellow stamens.
• Ipomoea imperati  Beach Snowdrop of the Convolvulaceae family is native to the Americas but is restricted to beach sand dunes. Its white flowers are similar in shape but are usually larger and may be tinged with blue. Its leaves are highly variable in shape but are glossy and fleshy.
• Ipomoea lacunosa  Snowdrop of the Convolvulaceae family is native to eastern North America, mostly in the USA. It has a much smaller taproot, but the vines can grow to similar lengths, 2 m long. Its leaves are always large (8 – 9 c long) and are ovate when young but become cordate (heart-shaped) when mature, and they are always on long petioles (leaf stalks) at least 3 cm long. The similar-sized, usually white but occasionally pink flowers may or may not be shallowly lobed. It has distinctive pinkish or purple anthers on white filaments, and it produces large seed capsules that are spherical and hairy.

Native Distribution of Convolvulus arvensis

Field Bindweed is native throughout much of Europe and Asia in temperate, tropical, and Mediterranean regions.

Habitat Types Where Field Bindweed is Found

Convolvulus arvensis grows successfully in a wide range of temperate, tropical, and Mediterranean climates. It is very problematic for agriculture in the temperate zones between 60°N to 45°S latitude, though it also grows in tropical regions as well. It is a serious pest in wheat, barley, corn, legume, and sugar beet fields as well as vineyards and tree crops.

While it is known mostly as an agricultural weed, it also is frequently found in natural areas, particularly in riparian habitats and open communities such as meadows and grassland. They thrive in areas with cleared and disturbed ground, such as agricultural areas and fallow fields, as well as new construction, land clearing, roadsides, and waste areas.

Human Uses of Field Bindweed

Convolvulus arvensis has a number of medicinal properties. A decoction of the root may act as a diuretic and a laxative. Tea from the flowers and or the leaves has been used to treat fevers and wounds. Caution should be used, however, as taking too much or for too long could potentially cause blood in the urine and toxic effects on the autonomic nervous system. In small doses over short periods of time, it is considered safe.

The stems are sometimes used as twine for tying up plants and other things. While it works well as twine, its usefulness is short-lived, so it should not be used for long-term storage of things, for example. A green dye is sometimes made from the whole plant.

Distribution of Convolvulus arvensis in North America

The species was first brought to the US likely as a contaminant in seed though it was also intentionally planted in baskets. It was first reported in the wild in Virginia in 1739 and, by the 1800s, had already spread throughout the eastern seaboard. By 1838 it had made its way west to California.

In Canada, Convolvulus arvensis has been recorded throughout the southern provinces of British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, and Prince Edward Island. So far, it has not been reported in Newfoundland or the northern territories.

In the USA, Field Bindweed is even more widespread. It has been reported in every state in the continental USA as well as in Hawaii. It is absent only in Alaska.

In Mexico, Convolvulus arvensis so far has been reported in Baja California Norte, Baja California Sur, Sonora, Sinaloa, Chihuahua, Coahuila, Nuevo Leon, Durango, Tamaulipas, San Luis Potosi, Jalisco, Zacatecas, Aguascalientes, Guanajuato, Queretaro, Hidalgo, Michoacan, Mexico State, Mexico City, and Puebla. Given that the climate is suitable for it in many other states on the Caribbean side and in the south, it is likely that its range is still spreading.

Eurasian Bindweed has been introduced on every continent except Antarctica.

How Field Bindweed Spreads

Convolvulus arvensis was first introduced to the Americas by accidental introduction as a contaminant in other seeds. It may have also been intentionally planted in flower baskets. Since it spread as a contaminant early in the days of European settlement, it has already invaded much of the habitable zones in North America suitable to its growth.

Short-distance dispersal occurs through contamination of its seeds in other plant materials, where it grows as a weed in fields with agricultural crops. Short-distance dispersal of seeds can also occur via water, clothing, animals, vehicles, and machinery.

Birds can also disperse the seeds short and long distances because the seeds can remain viable in the digestive tract for up to 144 hours making migratory birds an important source of long-distance dispersal.

Dispersal also occurs with poorly disposed of yard waste debris filled with seeds or simply cut plants and roots that can regenerate via fragments. Solarization of removed plant matter is critical to prevent further spread this way.

Habitats at Risk of Invasion in North America

Most habitats in North America at risk of invasion have already been invaded. These include any disturbed areas such as agricultural fields, pastures, roadsides, and waste areas. Its range in North America will likely continue to spread in parts of Mexico where it has not yet reached its potential or has not yet been accurately reported as already being there. As climate change continues its warming trend in northern latitudes, it seems likely that it will continue to spread northwards in the southern provinces of Canada and may, in some locations, particularly near the coast, even reach the southern parts of the Northern Territories.

Impacts of Invasion

Field Bindweed threatens agricultural crops where it has been found to compete for water and nutrients, resulting in reductions in yields anywhere in the range of 0 – 100%. While only studied in crops, it is logical to assume a similar threat to native plant communities. In addition to competition for water and nutrients, it is an aggressive grower and climbs over other vegetation, physically smothering it and reducing biodiversity in that way as well.

It has been reported to be mildly toxic to some grazing animals, horses in particular. However, sheep, pigs, and goats seem more resistant to the toxic effects.

Potential Benefits of Invasion

It does not supply any significant nutritional food sources for native or domestic species and threatens species richness. There are no potential benefits of its invasion.

Methods to Remove Field Bindweed

As always, prevention of invasive species introductions is the preferred method. However, in the case of Convolvulus arvensis it has already been well established throughout North America, so control becomes the next best option. Control, however, is made particularly challenging due to both the longevity of the seeds in the soil seed bank (20 or more years) and the ability of the plant to regenerate via fragmentation of the stems or roots.

Fortunately, no one appears to be selling Convolvulus arvensis, so deliberate introductions of new plants are not likely to happen. Sadly this is not the case with many invasive species. Perhaps because Field Bindweed is such a well-known pest plant and has been known as such for well over 100 years now, people have actually stopped intentionally planting it.

Physical Control of Convolvulus arvensis

Once already established, 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. Eradication of Field Bindweed once well established is difficult to impossible, but control and mitigation can be achieved.

Physical methods to remove Field Bindweed generally involve removing mature individuals before they go to seed to reduce the amount of seed in the soil. Since seeds are resilient enough to last more than 20 years in the soil, this makes control particularly challenging. Each year ongoing monitoring will be needed to remove any new seedlings as they generate from the soil seed bank.

The other added factors that make the control of this plant particularly challenging are its ability to resprout after being cut down, as well as its ability to generate new plants via fragmentation. When mature individuals are removed, try to pull out as much of their extensive root system as possible. This includes their very long and deep taproot as well as the lateral roots it produces that generally remain in the top 30 cm of the soil. Use a weed puller or a shovel to dig the taproot. For lateral roots, try to trace them from the main taproot along the soil. Heavy or compacted soils make this challenging, but the more root you remove, the better.

Since it can resprout from any remaining fragments, this absolutely must be followed with ongoing monitoring several times throughout the growing season. This can be done via the cutting of new sprouts or the cultivation method. To use the cutting method, simply cut any shoots that are regenerating, being sure to cut about 8 cm below the soil surface and repeat this for several years. Eventually, fewer and fewer sprouts will regenerate until the food reserves in the root system are completely depleted, and they can no longer regenerate. The more frequent the cutting, the less the root system has a chance to restore any energy stores, and the less treatment time is required. At a minimum, 2-3 years of extensive treatments will be required.

Frequent cultivation is a popular method to deal with resprouting mature plants and new seedlings. If the area is cultivated with a hoe or using agricultural equipment every 14 days throughout the growing season, the Field Bindweed can sometimes be controlled in as little as 2 years. It is critical to return to the patch and cultivate every 14 days, however. If left too long, the new green leaves will supply more energy to the extensive root system and prolong the necessary treatment time.

Solarization of large patches is a less labor-intensive method, and it will both deplete the root reserves and destroy any new seedlings as they try to emerge. Cover the area with a large black or other dark tarp and weigh the tarp down, leaving it there for 3 – 5 years. With many invasive plants, 2 years is sufficient, but due to the extensive root systems of Convolvulus arvensis, a minimum of 3 years is absolutely necessary. Prematurely removing the plastic mulch will no doubt result in re-infestation. The area should also still be monitored around the tarp edges throughout the growing season to look for lateral roots that try to sprout new plants outside of the tarped area.

Disposal of Convolvulus arvensis Once Removed

All plant material, whether it has seeds or not, must either be burned or solarized. Even without seeds, Convolvulus arvensis can generate new plants via root and stem fragmentation, so all plant material must be destroyed before disposal to prevent new infestations from carelessly discarded plant waste. Burning is a quick and effective method to dispose of the waste. However, it is not allowed in all areas and certainly not in all seasons. The other option is to solarize the material. To solarize, put the shrubs under a thick black tarp or into thick black garbage bags and leave them in the full sun for a good 8 weeks at least to be sure that all seeds and fragments are no longer viable. Some sources recommend shorter solarization periods, but this varies with latitude, aspect, cloud cover, etc, so leaving as long as possible ensures complete sterilization of the plant debris. Once properly solarized, the material can then be discarded at your local dump but do still inform them of what the material is so that they can properly dispose of it.

Chemical Control of Field Bindweed

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 Field Bindweed. Also, Field Bindweed has been shown to become herbicide-resistant in some cases after repeated application. Due to the physical nature of Field Bindweed, multiple applications are always needed.

Chemical control is not recommended.

Biological Control of Convolvulus arvensis

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.

Since we have so many native Convolvulus species in North America, and some of them narrow endemic species such as those in California in particular, biological control is not an option. Multiple biological control agents have been tested for Convolvulus arvensis, but so far, all of them have also impacted our native species.

Grazing can help control Field Bindweed, but cattle will not eat it, and horses should not eat it. Sheep and goats will eat it but prefer other plants, including native species. Pigs, however, seem to love the entire plant and will eat both the above-ground plant matter and the roots if given the time. If you have an area that needs to be controlled, build a fence around it. Then allow the pigs to graze unchecked for 2 years, and they should destroy all of the Convolvulus arvensis growing there. Using goats or sheep in a similar manner can also be effective. However, since sheep and goats do not eat the roots, they would need to remain fenced in the patch for 3-4 years instead of 2.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest, most effective, 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 large patch removal, the site will need to be replanted immediately because the bare soil will allow the seed bank of Field Bindweed and other invasives in the soil to germinate and reinvade the patch they were removed from. A replanting program should already be planned and ready to implement immediately upon successful removal of Convolvulus arvensis. If you are just removing an isolated individual replanting with native species is generally not necessary as the native species around it will simply fill in the spot left behind.

Ongoing Monitoring is Essential

In all cases of invasive Field Bindweed removal, ongoing monitoring is absolutely essential. Regular monitoring programs should be put in place to ensure that any surviving individuals are removed so that the population is not able to recover. In the first 3 years, monitoring should be done every 2 weeks to ensure that resprouting plants are destroyed to deplete the energy reserves of their extensive root systems. After the first 3 years, monitoring can be cut down to 2-3 times per growing season to remove any seedlings that germinate or any rootstock still attempting to recover.

Monitoring is required whether the area is replanted or not. Do not assume that once replanted, Convolvulus arvensis will not still try to grow. Field Bindweed is aggressive and prolific and will outcompete planted vegetation if yearly monitoring is not put in place to remove young native plants before they have a chance to become established. Once the area is well established with mature native plants, a simple once-a-year monitoring program is sufficient to root out any new individuals that may try to pop up. And you get to view the successful rewards of your hard work!

References and Resources

CABI on Convolvulus arvensis https://www.cabi.org/isc/datasheet/15101 – ** CABI charges for their datasheets

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

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

Fire Effects Information System on Field Bindweed https://www.fs.fed.us/database/feis/plants/vine/conarv/all.html

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

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

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

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Introduction

Wisteria sinensis Chinese Wisteria and Wisteria floribunda Japanese Wisteria are both popular garden ornamentals that have become invasive throughout North America. In Canada, it only grows in the warmest regions near the coast and so far has not become invasive there. However, it is becoming very invasive throughout the eastern USA, where the climate is similar to its native range in China. There you can see it growing over the top of trees and old buildings and smothering native vegetation. Even in areas where it has not yet become invasive, it is well known to be aggressive and difficult to keep in check, requiring routine maintenance in your garden to keep it from growing over your other plants. There are so many lovely native vines in North America that are not aggressive and invasive, do yourself a favor and research what is native to your area and grow those instead. For instance, if you live in the eastern USA, why not grow American Wisteria or Trumpet Creeper that are native to this area?

Many of the populations found to be invasive in the eastern US have recently been discovered to be hybrids of Wisteria sinensis and Wisteria floribunda. While this article focuses on Chinese Wisteria Wisteria sinensis, keep in mind that many invasive plants are hybrids. Furthermore, both of these species appear very similar and, once hybridized, are difficult to distinguish. Finally, both are not native to North America, so for the purposes of control and eradication, identification to simply Asian Wisteria (as opposed to the less aggressive and native American Wisteria Wisteria frutescens) is sufficient in most cases.

Description of Wisteria sinensis

Leaves & Stems

Chinese Wisteria is a climbing, twining, or trailing perennial vine of the Fabaceae (Legume) family. While it also occasionally can be seen growing more as a shrub, it typically is a vine, and its stems can reach 20 m up into the canopy and reach 38 cm in diameter. They twine clockwise, and they occasionally branch alternately along the stem.

The leaves of Wisteria sinensis are compound and about 0.3 m in length, and these, too, alternate along the stem. It has from 7 to 13 leaflets that are themselves attached opposite along the stock of the compound leaf. Leaflets are oblong in shape, have wavy edges and long tapering tops, and are from 2-6 cm long. When young, the leaflets are covered in silky hairs, but they become mostly hairless as they mature.

Flowers & Fruits

The flowers or Wisteria sinensis are borne on showing dangling racemes 10-50 cm long and 7-10 cm wide. Its flowers have the typical pea family morphology, are very fragrant, and are usually vibrant blue to lavender or violet in color though occasional white varieties are seen. The flowers all mature and open around the same time, unlike Japanese Wisteria (see Similar Species below).

Fruits of Chinese Wisteria are typical legume-type pods that are velvety brown and 10-15 cm long. The pods are narrowed toward the base and have constrictions between the seeds in the pods. The pods each contain from 1 to 8 round but flattened brown seeds roughly 1.2 – 2.5 cm in diameter. 

Toxicity

Many sources consider the flowers, leaves, fruits, and seeds of the Chinese Wisteria to be poisonous. This is not uncommon in the Fabaceae family. Accidental or purposeful ingestion may cause symptoms such as nausea, vomiting, and diarrhea. 

Similar Species Frequently Confused With

The Wisteria are part of the Fabaceae or Legume Family in the dicot order. Occasionally people mistake Chinese Wisteria for species such as:

• Paulownia tomentosa with its purple flowers, but it’s completely unrelated, part of the Paulowniaceae family, and its flowers are large and tubular and not legume-like, and it has massive simple leaves rather than compound leaves.
• Dermatophyllum secundiflorum is also of the Fabaceae and is native to the southern US and northern Mexico and has similar legume-like racemes of purple blossoms. But it can easily be differentiated by its evergreen tree habit and its 6-15 cm long compound leaves of oval leaflets 2-6 cm long with rounded or indented tips rather than tapering.
• Apios americana is a vine of the Fabaceae family, also native to the eastern US, with compound leaves with leaflets that have tapering tips. However, its flowers are not at all similar. Instead, they are red-brown to somewhat purple and occur in dense clusters rather than elongated racemes. This plant is also not poisonous and was an important food source for native peoples before the arrival of the Europeans.
• Syringa vulgaris Common Lilac is also occasionally mistaken for Chinese Wisteria, but these are usually shrubs and can easily be differentiated by the distinctive fragrance of its blossoms and its 4 petal flowers that look nothing like a legume flower.

There are two species in the same genus that are more often confused with Chinese Wisteria.

• Wisteria floribunda Japanese Wisteria looks very similar to Chinese Wisteria, and both are closely related. They are also both found throughout the eastern US though Chinese Wisteria does have a broader distribution and is also occasionally found in the West. Chinese and Japanese Wisteria have often been found hybridizing throughout this range at times, making the actual distribution of each problematic to determine. However, given that both are non-native, when found, identification as an Asian and not American Wisteria is sufficient for management purposes. The two Asian wisterias can be differentiated by their leaves and their twining habit. Japanese Wisteria has 13 to 19 leaflets on its compound leaves, while Chinese wisteria has 7 to 13 leaflets. Japanese Wisteria twines counter-clockwise while Chinese Wisteria twines clockwise. Furthermore, the flowers of Japanese Wisteria bloom sequentially from the bottom up, while Chinese Wisteria blooms all at the same time.
• Wisteria frutescens American Wisteria is native to the eastern USA, sharing much of the same range as Chinese Wisteria. It can usually be easily differentiated by its much smaller racemes, only 5-15 cm in length, which is why it is less popular as a garden ornamental despite its native status and non-invasive habit. It also does not grow as tall, it has a shorter bloom time, its blossoms are not fragrant, and the seed pods are smooth instead of velvety when they mature.

Native Distribution of Wisteria sinensis

Wisteria sinensis is native to China in Guangxi, Guizhou, Hebei, Henan, Hubei, Shaanxi, and Yunnan provinces. It has been widely introduced around the world outside of its native range.

Habitat Types Where Chinese Wisteria is Found

Chinese Wisteria tends to escape cultivation vegetatively when left unchecked. From there, they spread into forest edges, disturbed areas, roadsides, ditches, and riparian habitats. They tend to become invasive in warm temperate climates but remain more controllable in cool temperate climates, where they will suffer dieback from harsh winter temperatures. They will not survive in the colder climates in most of Canada, for example.

Wisteria sinensis prefer deep rich soils but have been found growing in a range of sites. They will not tolerate extended periods of drought but can tolerate some seasonal flooding. While they grow best in full sun, they also tolerate full shade and are capable of spreading in these conditions.

Human Uses of Chinese Wisteria

Chinese Wisteria is widely used as a popular garden ornamental for its abundant fragrant blossoms. They are used on porches, gazebos, fences, walls, and in gardens and parks when also pruned as a shrub.

Since it is poisonous, it is not used as a food source or medicinally.

Distribution of Wisteria sinensis or their hybrids in North America

The species was first brought to the Americas in 1816 as a garden ornamental, where it quickly became popular.

In Canada, Wisteria sinensis has not been recorded outside of cultivation. It is cultivated in the southern coastal regions, however, and may one day escape cultivation there, particularly with climate change and the warming of southern Canada.

In the USA, Chinese Wisteria is found outside of cultivation in Texas, Missouri, Arkansas, Illinois, Kentucky, Tennessee, Mississippi, Alabama, Georgia, Florida, North & South Carolina, Virginia, West Virginia, Delaware, Washington DC, Michigan, Pennsylvania, New Jersey, New York, Massachusetts, Connecticut, Vermont, and Hawaii. It has also been reported in the Pacific coastal states, but it is not known if it has truly escaped cultivation there yet or not.

In Mexico, Wisteria sinensis so far has only been reported in Baja California Norte.

Chinese Knotweed is cultivated in temperate climates on every continent except Antarctica and has been reported as naturalized in New Zealand, Argentina, Chile, Brazil, South Africa, and many European countries.

How Chinese & Japanese Wisteria Spreads

It is primarily spread over long-distance by deliberate human introductions as garden ornamentals that then escape from cultivation when left unchecked in a suitable climate.

Short-distance dispersal occurs primarily from unchecked garden specimens that escape cultivation through vegetative spread. Rooting occurs at nodes in the vines and easily produces new plants this way. Another source of short-distance dispersal is carelessly discarded yard waste dumped in waste sites or forest edges that regenerate and produce new populations. Seeds are produced when the conditions are right and short-distance dispersal can occur this way as well. Since nothing eats the seeds, this is limited to locations right next to the parent plant.

Habitats at Risk of Invasion in North America

All warm temperate areas with adequate rainfall in North America will be at risk of invasion. Cool temperate areas are also at risk but less so as the vine is less aggressive in those regions. It invades forests and riparian areas, in particular throughout these regions but is also a threat to forest edges and shrub meadows, providing there is adequate moisture. Given that these similar conditions exist on the west coast, but Wisteria sinensis so far has not been reported as invasive there, this is the region most at current risk of new invasion in North America. People in those areas should not plant Chinese Wisteria in their gardens and should control or remove any existing plants there before they become a problem.

Desert areas (except valley bottoms with permanent rivers) and montane regions about 1000 m in elevation are not at risk of invasion.

Impacts of Invasion

Wisteria sinensis invades forest edges and penetrates the forest reducing all the biodiversity within it. Chinese Wisteria climbs by twining around trees and killing them by girdling or simply smothering them so they are unable to photosynthesize. They are currently being managed in many state and national parks in the eastern USA because of their detrimental effects on native tree populations and biodiversity. Chinese Wisteria also smothers the native understory vegetation by growing over it and reducing the availability of light. In areas where it gains a foothold in the ecosystem, biodiversity is always significantly reduced.

Potential Benefits of Invasion

Other than the fact that bees still visit their flowers, the rest of the plant is poisonous and provides no wildlife value.

Methods to Remove Chinese Wisteria

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 Chinese Wisteria. Do not plant it in your yard. If it is already growing there, and especially if you live on the west coast, where it has not yet gained a foothold but certainly will if left unchecked, then please destroy it and replace it with a native vine.

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.

Physical Control of Chinese and Japanese Wisteria

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

Physical methods to remove populations of Chinese Wisteria generally involve the physical pulling of all above-ground vines from the soil and the trees. Then this is followed by repeated cutting of the vine down to its rootstock. Since they can resprout many times from their rootstock, this should be repeated every 2-3 weeks from spring until early fall. This will exhaust the rootstock, and it will no longer be able to resprout. It will, of course, need to be monitored the following spring for any surviving rootstocks.

Juvenile plants or isolated individuals can be controlled by digging out the entire rootstock, all roots, and runners. This can be done with a weeding tool on juvenile plants or a pulaski (ax with an adze in one tool that is used for both chopping and digging) on more mature isolated plants.

Since seeds are only responsible for a small portion of the spread of Chinese Wisteria, the plant can be removed at any time. However, it is always best to remove any invasive plant early in the spring or when in flower. If cut while in flower, as long as the flowers are not already turning into fruit, they will not go to seed after being cut.

Disposal of the Shrubs Once Removed

Since the vines can resprout from any nodes, they should either be burned or solarized. Burning is fast and effective. However, it is not allowed in all areas and certainly not in all seasons. To solarize, put the vines under a thick black tarp or into thick black garbage bags and leave them in the full sun for a good 8 weeks to be sure that all seeds are no longer viable. Some sources recommend shorter solarization periods, but in my experience, differences in exposure, latitude, cloud cover, etc, can all lead to differential success. Leave it to solarize as long as possible to ensure they are no longer viable, and then they can be disposed of accordingly.

Chemical Control of Chinese Wisteria

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 Chinese Wisteria. Chinese Wisteria is known to resist herbicide treatments, so multiple applications are always needed. While this may eventually kill the Wisteria, the chemical side effects often pave the way for other herbicide-resistant invaders to come in.

Chemical control is not recommended.

Biological Control of Chinese Wisteria

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

Currently, there are no known biological control methods for Chinese Wisteria. No information could be found on even the use of goats which will eat just about anything. Since all parts of the plants are considered poisonous, it is not recommended. Physical control is the preferred 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 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 large patch removal, the site will need to be replanted immediately because the bare soil will allow the seed bank in the soil to germinate and reinvade the patch they were removed from. A replanting program should already be planned and ready to implement immediately upon the removal of the Chinese Wisteria. The area will need to be monitored for any surviving fragments that resprout.

In the case of isolated individuals replating is probably not necessary. Simply remove the individual(s) and monitor over the following 2-5 years to ensure that no rootstock is remaining and resprouting.

Ongoing Monitoring is Essential

In all cases of invasive Chinese Wisteria removal, ongoing monitoring is absolutely essential. In the first year, monitoring should be done every 2-3 weeks to remove the aggressive regrowth from the rootstock. Then in the following 2-5 years, the area should be monitored at least every spring, summer, and early fall 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. Monitoring will prevent the re-establishment of invasive species and prevent all your hard work in removal from being wasted. And the added bonus is that you get to watch the ecosystem recover.

References and Resources

CABI on Wisteria sinensis https://www.cabi.org/isc/datasheet/56852

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

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

Fire Effects Information System on Chinese Wisteria https://www.fs.fed.us/database/feis/plants/vine/wisspp/all.html

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

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

Wikipedia on Wisteria frutescens https://en.wikipedia.org/wiki/Wisteria_frutescens

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!

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Introduction

Common Periwinkle Vinca minor and Bigleaf Periwinkle Vinca major are very popular garden ornamentals widely sold in nurseries and online stores throughout North America. They are part of the Apocynaceae family in the Gentianales order of dicots. They produce attractive foliage, lovely blue, lavender, or sometimes pink or white flowers, and they require little maintenance. Of course, other than trying to control them to keep them from spreading into your garden beds, lawns, forest, or other areas, they were not intended to grow. In some areas, these plants are not very invasive, but in other areas, they are quite invasive.

Since they spread only vegetatively, the primary source of invasion is through deliberate human introductions as a garden ornamental that they plant and forget about and allow them to escape. We have so many lovely native groundcovers, just do some research into native groundcovers in your region and plant those instead. Native species require no maintenance, they tend not to be invasive, and they offer added wildlife and biodiversity values that invasive species lack altogether.

Description of Vinca major & Vinca minor

Leaves & Stems of Vinca major & Vinca minor

Both species of periwinkle are scrambling vines from stolons up to 1 m long from fibrous roots 3 – 8 cm long. Their vertical stems grow up to 30 cm tall and are semi-succulent but become semi-woody at the base (caudex).

The leaves of Vinca major Bigleaf Periwinkle are opposite, semi-evergreen, heart-shaped (cordate) to triangular in shape, and are covered with a waxy coating (cuticle) and tiny hairs. They are 4 – 8 cm long and 2 – 5 cm wide. Sometimes, they have a finely hairy (ciliate) margin; otherwise, they are entire.

The leaves of Vinca minor Common Periwinkle differ from Vinca major in that they are evergreen, glabrous (hairless), more leathery, narrowly elliptic in shape, and are only 2 – 4.5 cm long by 1 – 2.5 cm wide with an entire margin that is not ciliate.

Flowers of Vinca major & Vinca minor

The flowers of both Periwinkle species are violet to blue and each has 5 petals radiating in pinwheel-like angles from the floral tube (they are partially connate).

The edges of the petals are typically slightly fringed and occasionally the petals are even white or pink.

Bigleaf Periwinkle Vinca major has somewhat larger flowers than the Common Periwinkle, but otherwise, they appear very similar.

Fruits of Vinca major & Vinca minor

The fruits of periwinkles are pairs of slender cylindrical follicles to 5 cm long in Vinca major or 2.5 cm long in Vinca minor.

When the follicles dry, they split open to release 3-5 naked seeds that have no tufts (coma), unlike many seeds released from follicles.

Toxicity of Vinca major & Vinca minor

All parts of the plants are considered poisonous due to the presence of toxic alkaloids, which can attach themselves to the microtubules of the cells and impair their ability to divide, causing cell death.

Periwinkles have been known to poison humans, pets, and livestock. Most wild animals seem to know better than to ingest it; even if it is the only forage material around, most still will not eat it.

Similar Species Vinca major & Vinca minor are Frequently Confused With

There are only two species of Vinca that have been introduced to North America so far. Periwinkle is part of the Apocynaceae or Dogbane and Milkweed Family. However, there are a few other genera native and introduced, that can occasionally be confused for Vinca. They can be differentiated as follows:

• Catharanthus roseus – Madagascar Periwinkle is also of the Apocynaceae and appears superficially similar, but it is an upright shrub that does not have a scrambling habit. Also, its leaves are oval to oblong and 2.5-9 cm long, and they are hairless and arranged in opposite pairs. The flowers are larger, up to 5 cm in diameter, with a very long floral tube. The petals are not at right angles to the tube but may still appear somewhat pinwheel-like in shape. The corolla is also typically various shades of pink or white rather than lavender or blue. It is endemic to Africa but has been widely introduced throughout the southern half of the USA and throughout Mexico.
• Phlox divaricata – This Polemoniaceae is native to the eastern USA and Canada and has strikingly similar flowers in the same color and sometimes with a somewhat pinwheel shape to the petals. However, it is not a vine but rather an upright herb 25-50 cm tall. The leaves are opposite but are lanceolate and lack a leaf stalk (petiole).
• Euonymus fortunei – the Fortune Spindle is an evergreen vine of the Celastraceae family that are occasionally mistaken for Periwinkles. They are native to Asia but are widespread in the USA and Mexico. However, the individual vines grow much longer, to 20 m, and their ovate evergreen leaves are much larger than the Common Periwinkle and are not cordate or triangular like Bigleaf Periwinkle. When in flower, they are hard to mistake for Periwinkle because their flowers are much smaller and are white or greenish-white.
• Hedera helix – Eurasian Ivy is part of the Araliaceae Family and is occasionally mistaken for Vinca major. However, its vines grow much, much longer than Vinca major, and its leaves are alternate rather than opposite, and they are typically larger, though occasionally they can have similar shapes. Their flowers are also much different, being numerous tiny greenish-white flowers in an umbel.

Native Distribution of Vinca major & Vinca minor

Vinca major is native to the Mediterranean region of Europe and the Middle East from Spain east to Turkey plus northern Africa.

Vinca minor is native to central and southern Europe from Portugal, France, Holland, and the Baltics, plus eastern Caucasus and Turkey.

Habitat Types Where Periwinkle Can Be Found

In its native environment, Periwinkles are usually associated with moist, fertile, or moderate soils that are moderately acidic to moderately alkaline.

In North America, it is found in those soil types but has also been found in moderately well-drained soil types, poor soils, and even acidic clays. They grow from sea level up to 2300 m in elevation.

Periwinkles are both found in riparian forests, open forests, grasslands, scrub, and roadsides.

Vinca minor is often found in the understory of successional forests, including mature forest types, while Vinca major is more often found in partially shaded open forests and riparian forests.

Both species can grow in full shade, part shade, and even full sun, provided the soil is moist. In arid climates, both Periwinkles are more restricted to riparian forest understory.

Human Uses of Periwinkle

Periwinkle has been long been used as a ground cover plant as a garden ornamental where it forms dense mats. It has also been used for short hedges and for filling around the bases of trees. Though it rarely remains contained to its original purpose.

The dried leaves, aerial parts, and sometimes the whole dried plant are used to extract medicines that are used to treat a number of different cancers as well as to improve blood circulation, brain function, and cardiovascular disorders.

Vinpocetine is a synthetic nootropic (brain-enhancing) drug derived from vincamine, an alkaloid responsible for much of the medicinal activity of the genus. However, unsupervised medicinal use is not recommended due to the toxicity of the plant, which can make you very ill, cause miscarriages, and possibly even death if consumed in sufficient quantities.

Distribution of Vinca major, Vinca minor in North America

Periwinkles were first brought to North America in the 1700s. It was documented in a review of flora as early as the late 1700s, showing that it was already well established in the eastern USA by then.

In Canada, Vinca major has been recorded in British Columbia. Vinca minor has been recorded in British Columbia, Ontario, Quebec, New Brunswick, Nova Scotia, and possibly Newfoundland (excluding Labrador).

In the USA, Vinca major is found in Washington, Oregon, California, Idaho, Utah, Arizona, New Mexico, Texas, Louisiana, Arkansas, Mississippi, Alabama, Georgia, South Carolina, North Carolina, Virginia, Maryland, Illinois, Kentucky, Tennessee, Ohio, Pennsylvania, New York, and Massachusetts.

In the USA, Vina minor is found throughout the entire eastern USA and most of the southern USA. The only states it has not been reported in yet in the continental USA are Idaho, California, Nevada, Wyoming, Colorado, New Mexico, North & South Dakota, and Oklahoma.

In Mexico, Vinca major so far has been reported in Baja California Norte, Sonora, Sinaloa, Durango, Nayarit, Jalisco, Colima, Michoacan, Oaxaca, Chihuahua, Nuevo Leon, Tamaulipas, San Luis Potosi, Guanajuato, Mexico State, Mexico City, Puebla, Morelos, Veracruz, and Chiapas.

In Mexico, Vinca minor has not yet been reported, but it is located on the northern border of the US in Tucson, Arizona, and San Diego, California, so it will likely be recorded there soon.

Periwinkle, particularly Vinca major, has been introduced on every continent except Antarctica.

How Periwinkle Spreads

It is primarily spread through long-distance by deliberate human introductions as a garden ornamental. Despite its invasive status, this is still the primary cause of long-distance spread.

Short-distance dispersal is primarily through vegetative means. Both species spread readily through their stolons which can quickly grow 25 cm or more before producing another rooted node yielding another clone.

Short-distance dispersal also routinely occurs by fragments regenerating out of dumped yard waste. In its native range, ants sometimes spread the seeds, but in North America spreading by seeds appears to be negligible to nonexistent.

Habitats at Risk of Invasion in North America

Riparian forests and canyon bottomlands are most at risk throughout all of North America due to their preference for moist soil types.

States and provinces located next to the coasts are also at high risk due to the presence of moist soils in those areas. In those regions, all land types are at risk, closed forests, open forests, roadsides, waste areas, grasslands, shrublands, and riparian areas as well.

The habitats not at risk of invasion are open deserts and high mountainous areas above 2300 m elevation. Fortunately, due to its inability to spread by seed, this restricts its spread to vegetative spread as long as people stop deliberately planting it in their gardens.

Impacts of Periwinkle Invasion

Both Vinca major and Vinca minor can grow in poor soils and full shade, and both are also allelopathic, giving them a significant competitive advantage.

Allelopathic plants inhibit the germination and growth of plants of other species growing in their vicinity due to how they alter the soil conditions. As a result, both species tend to form dense mats that smother native understory species as well as prevent the germination of new trees and shrubs.

Reductions in species richness, however, varied from location to location. This has resulted in Periwinkle being viewed as a ‘limited invasive’ species that is ‘stable’. This may partly be due to its spread being limited to vegetative growth.

However, I have seen firsthand its invasive nature in coastal British Columbia, Canada, as well as Virginia and Georgia in the eastern US, where it does indeed create monocultures of dense mats that exclude all other vegetation.

In riparian corridors, its ability to reduce native vegetation reduces the available forage for both wild and domestic grazers. This is particularly problematic where it occurs in riparian habitats found in otherwise arid landscapes.

In California, Vinca major is an important year-round host to the bacteria causing Pierce’s disease, which is a serious threat to the vineyards located there.

Potential Benefits of Invasion

Vinca minor and Vinca major provide no wildlife values in North America, and due to their toxicity and tendency to form monocultures, there are no potential benefits of their presence here.

Methods to Remove Vinca major & Vinca minor

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 Periwinkle, and do not plant it in your yard.

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

Do some research into native ground covers for your area. You will be pleasantly surprised by the number of low to no-maintenance, non-invasive native species that will grow in your area.

Physical Control of Vinca major & Vinca minor

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.

Fortunately, Periwinkle seed production and spread are not an issue, so they can be physically removed at any time of year. Removal is easiest, however, when the soil is moist, as the roots cling less tightly, allowing the plants to be pulled more readily. This means that spring and fall are often the best times, but it depends on your location.

Physical methods to remove Periwinkle generally involve raking up the stolons and then pulling the plants out using hand tools.

When dealing with large infestations, rake it first as this pulls the stolons up, then the mowing macerates the vines. Given their ability to regenerate by fragmentation, this requires repeated treatments throughout the entire growing season and is only recommended for large infestations where hand removal is not feasible.

For smaller infestations raking followed by pulling plants is often sufficient, though, like all control methods, this too will have to be repeated.

Try to pull as many of the roots as possible, but you will not get them all. The starchy roots of the Periwinkles allow them to regenerate following any method of control. As a result, ongoing monitoring is essential.

Solarizing Small Patches

Sometimes a small patch could be solarized by covering it with heavy black tarps and leaving it there in the sun for 4-6 months. This is the least labor-intensive method of control. The area should still be monitored over the next couple of years to be sure that none regenerate. Any that are found can simply be pulled by hand.

Disposal of the Plants Once Removed

Due to their ability to regenerate from fragments, all plants removed should be either burned or solarized. Burning is an easy and efficient way to get rid of plants, but if burning is not allowed in your area, then they should be polarized.

To solarize, put the shrubs under a thick black tarp or into thick black garbage bags and leave them in the full sun for at least 8 weeks to be sure that none of the vines are viable anymore.

Chemical Control of Periwinkle

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 Periwinkle. And the starchy roots appear able to regenerate the plants not long after chemical control methods are used so additional control methods will still be required.

Chemical control is not recommended.

Biological Control of Periwinkle

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.

In the case of Periwinkles, no biological control methods are currently being used. Most animals will not even graze on the plant. Due to its toxicity, it is not even recommended to use goats that eat anything, as it could potentially make them very ill.

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 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 immediately because the bare soil will allow the Periwinkle or other potential invaders to claim the empty space left behind. A replanting program should already be planned and ready to implement immediately upon the removal of the Periwinkle. In most cases, these will be native understory plants, such as ferns, small shrubs, native vines, etc, that would have grown there before the Vinca species invaded. Find a local nursery that specializes in native species or ethically wildcraft your own from the local environment.

The only instances where replanting is not required are small isolated individuals that have not yet had a chance to form a clonal colony. In this case, simply remove the plant(s) and monitor them to make sure they do not return. Nearby native species should be able to rapidly recolonize the space they were removed from.

Ongoing Monitoring is Essential

In all cases of invasive Periwinkle 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.

If yearly monitoring is not put in place to remove young plants before they have a chance to become established, then all your hard work done in the removal process will be wasted if the patch is allowed to regrow.

References and Resources

CABI on Vinca major https://www.cabi.org/isc/datasheet/56402 **CABI now charges for their sheets

Canadensys Plant Search https://data.canadensys.net/vascan/sea rch

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 Periwinkle https://www.fs.fed.us/database/feis/plants/vine/vinspp/all.html

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

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

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!

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Introduction

Heracleum mantegazzianum or Giant Hogweed is an enormous invasive species in North America that is still in the early stages of expanding its range. I first encountered it when I grew up in West Sechelt, BC, Canada. It was about 30 years ago. I had no idea what it was at the time, but it just seemed out of place somehow. Not long after, I started getting interested in plants, and I looked into them and discovered that, yes, in fact, it was out of place in my environment. Fortunately for me, at the time, I had not yet touched the plant to experience the phototoxic effects. I did, many years later, accidentally touch a smaller one that was among many other plants. I quickly realized what it was and stepped back immediately, but it still left some permanent discoloration on my hand. That was just from a quick accidental touch. Many others have not been so fortunate, as it causes massive blisters, scarring, and even potential blindness. Keep your children away from it, as many have suffered from contact with it. Unfortunately, it looks very similar to a native species known as Cow Parsnip; see Similar Species Frequently Confused With below.

Description of Heracleum mantegazzianum

Heracleum mantegazzianum Leaves & Stems

Heracleum mantegazzianum is a huge monocarpic herbaceous perennial from a branched root system that goes 40-60 cm deep and 15 cm across. It reaches heights of 2-4(5) m with hollow stems 3-10 cm in diameter. The stems are covered in blister-like bumps with erect bristly hairs on them. The stems also usually have purplish splotches on them, helping to differentiate them from Cow Parsnip (see similar species below).

It has very large alternate leaves from a basal rosette. As it grows in height, the leaves get smaller up the stem. The lowermost leaves can reach up to 3 m long and 1.7 m wide when fully mature. Leaves are pinnately or ternately lobed, often deeply so, and are coarsely toothed. The above-ground growth dies back and returns after each winter until it flowers, and then it dies.

Heracleum mantegazzianum Flowers

Being monocarpic, Giant Hogweed flowers between 3-5 years of age before it sets seed and then dies. Its inflorescence is a terminal compound umbel 80(100) cm across made of 50-150 rays of bisexual flowers. Surrounding the large main umbel are up to 8 other smaller satellite umbels, which often grow taller than the main one. These satellite umbels may be all male flowers only or could be bisexual. The flowers themselves are on 10-20 mm long pedicels with white to pinkish petals up to 12 mm long each.

Heracleum mantegazzianum Fruit

Heracleum mantegazzianum fruits are flattened elliptical schizocarps 6-18 mm long and 4-10 mm wide. The fruits are narrowly winged and split into 2 mericarps, each with 3-5 elongated oil ducts.

Heracleum mantegazzianum Toxicity

Heracleum mantegazzianum is extremely phototoxic, particularly when in flower. Upon touching the plant, if then exposed to the sun, it can cause severe hives, blistering, and permanent discoloration of the skin. If the sap from the plant gets in the eyes, it can cause temporary or possibly even permanent blindness.

Similar Species Heracleum mantegazzianum is Confused With

Heracleum mantegazzianum is part of the Apiaceae or Carrot Family, which you may recognize by its large umbels of small white flowers. There are a few other genera in the Apiaceae that Heracleum mantegazzianum can be confused with within North America:

• Angelica atropurpurea is native to northeast North America, but it has almost entirely purple stems and grows to 1.8 m tall. Its inflorescence is much more rounded rather than flat-topped and only up to 20 cm wide. Its leaves are also much smaller, softer, and divided into 3 parts, each with its own petiole.
• Daucus carota wild carrot is widely introduced in North America and appears similar superficially, but it also grows to much smaller sizes, has smaller flower heads, and smells like a carrot.
• Poison Hemlock Conium maculatum is sometimes confused with Giant Hogweed. However, it has tripinnate leaves that look fern-like and are very soft to the touch. It also gives off a foul odor not found in Hogweed.
• Pastinaca sativa Parsnip is occasionally confused with Giant Hogweed, but its flowers are yellow instead of white and its large leaves have long petioles, and the plant smells of parsnip.

There are also similar species in the same genus, which may be more difficult to tell apart. They can be differentiated as follows:

• Heracleum maximum (Heracleum lanatum or Heracleum sphondylium subsp. montanum, there is some debate as to its proper taxonomy) is referred to as Cow Parsnip and is native to North America. While it does look strikingly similar, it grows to only 2 m but never to the 4 m that Giant Hogweed can reach. Its leaves, while still lobed, are not as deeply cut as Giant Hogweed, and it lacks the purplish spots on its stems that are commonly found on the latter. Its flower head produces much fewer rays, only 15-50 per umbel, and the main umbel is usually no more than 30 cm across. Furthermore, Giant Hogweed is found only in some states and provinces, while Cow Parsnip is widespread throughout most of North America.
• Heracleum sphondylium German Hogweed has been sporadically introduced throughout the USA and southern Canada. It can be differentiated by its smaller size of 1.2 m tall and its pinnate leaves that are pinnately lobed rather than whole leaves that are deeply divided. The leaves are also not as sharply lobed as that of Giant Hogweed.

Native Distribution of Heracleum mantegazzianum

Heracleum mantegazzianum is native to the southern slopes of the Western Greater Caucasus of southern Russia and Georgia. It has spread much further in Russia and eastern Europe in the last century, and it is believed that at least some of this spread has been of natural origin.

Habitat Types Where Giant Hogweed is Found

In its native environment, Giant Hogweed occupies a wide range of habitats from 50-2000 m in elevation with between 1000-2000 mm of annual rainfall. It thrives in temperate continental climates with hot summers and cold winters. There it is typically found in meadows, forest edges, and clearings.

In areas where it is introduced, it also occupies a wide range of habitats and is often found growing along roadsides, train tracks, rivers, riparian areas, grasslands, meadows, and waste sites.

It grows in a wide range of soil types from heavy to light, seasonally waterlogged to well-drained, and alkaline to neutral, but less often in acidic soils. It does not tolerate wet soils that experience long periods of inundation.

Heracleum mantegazzianum is normally reported in full sun to part shade conditions in North America. However, it has also been found to grow in full shade in some areas.

Human Uses of Heracleum mantegazzianum

The seeds of Heracleum mantegazzianum are used in some Middle Eastern cooking as a spice known as golpar. In its native habitat, beekeepers use the plants for honey production. There were also some reports of people eating the cooked stems, though this is not recommended due to the phototoxicity.

Distribution of Heracleum mantegazzianum in North America

The species was first recorded in New York, USA, in 1917 and had made its way to western North America by the 1930s. It was likely brought as a garden curiosity due to its impressive size. Alternatively, it could have been brought through spice importation since its seeds are sometimes used in Middle Eastern cuisine.

In Canada, Heracleum mantegazzianum has been recorded in British Columbia, Ontario, Quebec, New Brunswick, Nova Scotia, Prince Edward Island, and Newfoundland (excluding Labrador).

In the USA, Giant Hogweed is found in Washington and Oregon in the west and in Illinois, Michigan, Pennsylvania, North Carolina, New York, Connecticut, Massachusetts, and Maine in the eastern States.

So far, in Mexico, Heracleum mantegazzianum has not yet been reported.

Globally Giant Hogweed is also considered an invasive species in much of Europe and New Zealand, and it appears to have recently been introduced to South America.

How Giant Hogweed Spreads

Heracleum mantegazzianum is primarily spread over long-distance by deliberate human introductions as an ornamental as well the importation of the seeds as a condiment. Sometimes the seeds are also found as a contaminant in the international food trade. Its preference for riparian habitats also has enabled its spread by long distances via seeds floating down rivers.

Short-distance dispersal occurs through water, high winds, animals, and sometimes birds. Dispersal can also occur through humans transporting soil and on vehicles or clothing.

Most seeds in the field germinate the following year, but the seeds can remain viable in the soil bank for up to 7 years.

Habitats at Risk of Invasion in North America

Heracleum mantegazzianum spreads mostly through water, humans, and animals and frequently is reported along roadsides as well as riparian areas where it spreads by the movement of water. In multiple locations in North America, where it has spread via waterways along the banks, it has since moved into the adjacent woodlands, grasslands, and fields. Due to its tolerance to a wide range of elevations (50-2000 m), soil types, and sun, most habitats are at risk of invasion.

However, it will not grow in permanently wet soils or arid climates without access to water. It also generally will not penetrate deeply into dense forests, but it will grow readily in open forests and forest edges. Its range is still in the early stages of expansion in North America and will continue if left unchecked.

Impacts of Invasion

In addition to the risks of phototoxicity and potential blindness, there are other impacts of invasion. Due to its large size and phototoxic effects, it can impede access to water, trails, and recreational areas.

Giant Hogweed also negatively impacts soil dynamics because some studies have shown that it decomposes more slowly than native species resulting in a slower turnover of organic matter. The annual litter that it produces can also smother nearby native species preventing their germination or regrowth.

Studies by fisheries in the USA have shown that Heracleum mantegazzianum can shade native species in riparian areas and eventually replace them. This has been shown to increase erosion due to the herbaceous nature of the plant and the timing of flooding, leaving bare soil prone to increased erosion. This can negatively impact flooding as well as habitat for native fish.

Because Giant Hogweed grows to such massive sizes, it can shade out native vegetation anywhere it is allowed to grow. Actual studies on species richness have been conflicting, however, where it has been shown to increase in grassland habitats and a decrease in woodlands where it has invaded. Data on its impacts on species richness in North America are lacking altogether, but it is safe to assume that it will have a negative impact on most habitats.

Potential Benefits of Invasion

Heracleum mantegazzianum provides virtually no wildlife value as most wildlife does not eat the plant, and birds seldom eat the seeds. Bees do visit the flowers, however, including native bees. But as with all invasive species, if they were not present, the native bees would find other native flowers instead.

Methods to Remove Heracleum mantegazzianum

As always, prevention is the preferred method of control. It, like most invasive species, is still widely sold online and sometimes can still be found in your local garden stores. Do not buy or transport any Giant Hogweed, and do not plant it in your yard.

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

Physical Control of Giant Hogweed

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.

Safety PPE to Use During Removal

Physical methods to remove Giant Hogweed are challenging due to the phototoxic side effects of physical contact. Proper PPE will help prevent any temporary or permanent damage to your skin or eyes. Do not attempt to handle the plant in any way without proper PPE. Gloves must always be worn while handling the plant at any stage of growth. Also, wear a thick long sleeve shirt and jeans or other suitable pants made of thick fabric to ensure that they cannot make contact with your skin. Wearing a hat with mosquito netting is also highly recommended to prevent it from coming into contact with your face. Safety glasses are also highly recommended to prevent possible temporary or permanent blindness should any sap get into your eyes. See the Affiliates link at the bottom for suitable PPE.

Dealing With Young Plants

The best time to remove Giant Hogweed is when the plants are young. At this time, they can be hand-pulled wearing gloves or using a weed puller. Try to get the root as much as possible.

Dealing With Mature Plants

If the plant is already large, this makes removal much more challenging. It becomes difficult to remove the above-ground plant growth, and the rootstock also gets quite large and deep. In this case, it is more effective to wait until the flowering year and then remove all flower heads. Because it is monocarpic, it will only flower for one season, and then the plant will die. To remove the flower heads, simply cut them off into a garbage bag while wearing gloves. Do not dispose of the flower heads without solarizing them in the garbage bag first, as the heads will continue to ripen even once cut off from the plant.

You will likely need to bring a footstool or a small ladder with you to reach the top of the larger plants. You will need to return to the site a few times that season as it often will produce new flower heads. If it has gone to seed at any time, be extra careful to cut the heads directly into the garbage bag to prevent the seeds from dropping on the ground. If there are any seeds at all, be sure to return to the site the following year to destroy any seedlings while they are still small enough to easily pull out by hand while wearing gloves.

Another method to deal with mature plants is to sever their taproot or destroy the crown by digging 10-15 cm below the soil and severing it. This method can destroy a mature plant before it flowers. This can still be challenging, however, due to the size of the plant. The above-ground growth will, in most cases, need to first be removed so that you can even get close enough to the taproot or crown to destroy it.

Mechanical Removal

Mechanical removal has been shown to be ineffective, as the plants will return from their hearty rootstock after they have been cut or mowed. Repeated mowings also have proven ineffective. Sometimes repeated mowings have been seen to turn the plant into a perennial that will flower multiple times before dying instead of the monocarpic perennial that naturally only flowers once and dies. Mechanical removal is not recommended. Burning is also ineffective for the same reason that it will likely regrow from its hearty taproot.

Disposal of the Shrubs Once Removed

If you have plants that have seeds on them, they must either be burned or solarized. If you are unable to burn in your area, then you must solarize them. To solarize, put the shrubs under a thick black tarp or into thick black garbage bags and leave them in the full sun for a good 8-10 weeks at least to be sure that all seeds are no longer viable. Some sources recommend shorter solarization periods but differential heat in the bags or under the tarps, cloud cover, and other temperature variances make shorter periods less reliable. That is why it is important they are solarized for as long as possible if using that method.

Chemical Control of Giant Hogweed

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 always significantly increase.

Furthermore, there are no chemical control methods that effectively target only Giant Hogweed. And because it often grows in or near riparian areas, chemical control cannot be used there due to risks to fish and humans.

Chemical control is not recommended.

Biological Control of Heracleum mantegazzianum

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. These control methods are often extremely risky and should only be carried out by professionals after years of rigorous study. 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 usually not native to the environment and could behave differently when released there.

The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. However, using biological control in conjunction with physical control and ongoing monitoring can be very effective.

At this time, the only known biological control method of Heracleum mantegazzianum is the use of intensive grazing by sheep or goats and rooting by pigs. Cattle will selectively avoid eating it, as with most invasive species, so they are not a suitable control method.

Using sheep, goats, and pigs can be challenging, however, because of the phototoxicity issue. Animals with thick, dark pelts are more resistant to toxic side effects. The area of control will need to be penned in, and the sheep or goats be allowed to graze for the first season to destroy the above-ground growth. The following year pigs can be let in to root out the roots and eat them as well as any sprouts as they come up. If pigs are unavailable, then the sheep or goats can be left in there for multiple seasons, and an integrated approach can be used for stubborn plants that keep re-sprouting. In this case, you will need to use physical control by manually destroying the crown or severing the taproot using a shovel.

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 many cases of removal, the site will need to be replanted because the bare soil will allow the seed bank of this and other potentially invasive species to germinate. Single isolated plants can simply be destroyed or dealt with using physical control, and then native species will regrow in their absence. Occasionally, however, Giant Hogweed will produce a large patch. In those cases, a replanting program should be planned and ready to implement upon removal.

Ongoing Monitoring is Essential

In all cases of invasive Giant Hogweed 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. Seeds will also continue to germinate for several years if the plant was fully established in the area and not an isolated case. Seedling removal is by far the easiest method of control. The area should be monitored once a year for several years after the plants were removed to ensure they do not become re-established.

References and Resources

CABI on Heracleum mantegazzianum https://www.cabi.org/isc/datasheet/26911 **CABI now charges for access

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

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

Fire Effects Information System on Giant Hogweed https://www.fs.fed.us/database/feis/plants/forb/herman/all.html

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

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

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!

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Introduction

Ilex aquifolium goes by several common names, including English Holly, European Holly, Common Holly, and Christmas Holly. It is a highly invasive plant in the moist climates of North America, particularly on the west coast, where it is beginning to dominate the understory of some forest ecosystems. Despite this, it is still widely sold in stores throughout this region.

I grew up on the coast of BC, Canada, and remember finding it all the time under the canopy of the forest. I recently returned to the coast and spent some time there and was horrified by the amount it appears to be spreading and replacing our native understory species. In a single forest walk, I managed to pull out about 40 immature plants from the forest canopy. There were several larger ones I could not pull out by hand due to their deep and aggressive root system.

Next time I return to the coast and go for a forest walk, I will be sure to bring a trowel or a small shovel. Do your part on your forest walks and pull out any small holly shrubs you find growing before they get large and too difficult to remove. Just make sure you are not pulling Oregon grape or native hollies if you live on the east coast; see Similar Species below for more information.

Description of Ilex aquifolium

Leaves & Stems of Ilex aquifolium

Common Holly is a slow-growing evergreen perennial shrub or small tree that grows from 5-25 m tall. However, it usually only grows to the shorter 5-10 m shrub height. It may grow from a single trunk, or it may be multi-stemmed. Their trunks are usually less than 40 cm in diameter. Larger trunks can be from 40-80 cm in diameter, but they rarely exceed 100 cm. The bark is green when young but turns grey when it matures. While specimens as old as 500 years have been reported, it seldom lives longer than 100 years.

The leaves of English Holly are arranged alternately on the branches. They are tough, thick, and glossy green and measure 3-12 cm long and 2-6 cm wide. The margins are often characteristically undulate (wavy) and toothed with spiny tipped teeth. Sometimes the leaf margins are smooth, particularly on older specimens.

Flowers & Fruits of Ilex aquifolium

English Holly is a dioecious plant meaning that there are separate male and female plants. The flowers are whitish and have 4 petals and 4 sepals that are both fused near their base. In males, the flowers are usually more yellowish and appear in axillary groups (in clusters in the leaf axils). Female specimens have single flowers or in groups of three, and the flowers or white or somewhat pink. The male and female shrubs are impossible to tell apart until they begin to flower from 4 – 12 years of age.

Fruits are small red or yellow drupes, berry-like in appearance, and only grow on female plants. They are 6-10 mm in diameter. Each drupe contains 3-4 small stony seeds.

Toxicity of Ilex aquifolium

The drupes are toxic to humans and pets, causing vomiting and diarrhea, but they are generally not lethal. Birds typically eat them later in the winter after the frosts have softened them and made them less bitter; they are immune to their toxic effects.

Similar Species Ilex aquifolium is Frequently Confused With

There are a few other plant genera that have similar leaves to Ilex aquifolium. They include:

• Berberis (Oregon Grape) of the Berberidaceae family, has a few native species growing in North America with evergreen, spiny-toothed leaves, but their leaves are opposite rather than alternate, and they have blue-colored berries rather than red or yellow drupes.
• Osmanthus heterophyllus or Holly Olive is a member of the Oleaceae or olive family native to Europe that also has similar leaves and has been introduced in the USA. Holly olive differs from true hollies by its opposite rather than alternate leaves and bark that turns grayish to blackish with age and has a characteristic cracking pattern to it.

Ilex is the only genus in the Aquifoliaceae or Holly Family. Here are a few of its genus in North America that it could be confused with:

• Ilex opaca American Holly – native to the eastern and south-central USA. It has matte yellowish-green leaves that are seldom glossy, as in Ilex aquifolium. Its trunk is grey with wart-like bumps, and its mature branches turn brown rather than grey. It also grows more of a tree than a shrub, with trunk diameters commonly of 50 cm and up to 120 cm in diameter.
• Ilex cornuta Chinese Holly – native to China but widely introduced throughout the eastern and south-central USA. It has more rectangular-shaped leaves with only 5 (4) spines, but they are glossy green like Ilex aquifolium. Chinese Holly never grows to tree size; it rarely exceeds 3 m in height, and its drupes are larger than European Holly. The bark is smooth and grey but becomes flaky with age.
• Ilex vomitoria Yaupon Holly – native to southeastern North America. It is a small tree or shrub with glossy green leaves, but the leaves are much smaller and are not spiny. Instead, the leaves have crenate or coarsely serrated margins.

Native Distribution of Ilex aquifolium

European or Common Holly is native to western and southern Europe, northwest Africa, and southwest Asia.

Habitat Types Where European Holly is Found

European or Common Holly is typically found growing under forest canopies, oaks, and beeches, especially in its native range. It typically requires moist and shady environments, making it particularly suitable as an undergrowth species in any moist forest. Also, it is common on moist and shady slopes, cliffs, and mountain gorges. Sometimes, however, it can also form dense thickets in more open fields and meadows. It is a rugged pioneer species that can tolerate both frosts and droughts.

Human Uses of Common Holly

European or Common Holly is widely used as an ornamental shrub or small tree in gardens. It is also extremely popular in Christmas decorations, where it is put in wreaths, garlands, and various other displays.

In traditional European herbal medicine, the leaves of Ilex aquifolium were used as a diuretic, to treat fevers, and as a laxative. It is seldom used in modern herbal medicine, however, and its berries are considered toxic, so the plant should be used with caution.

Distribution of Ilex aquifolium in North America

In Canada, Ilex aquifolium has been recorded as introduced in BC and possibly in Ontario. It is becoming particularly invasive in coastal BC.

In the USA, English Holly is found in Washington, Oregon, and California in the west and Virginia and New Jersey in the east. It has also been introduced in Hawaii. According to iNaturalist, it is located in many more states, but many may still be in cultivation, and some could be native Ilex spp. that have been misidentified.

In Mexico, Ilex aquifolium has not yet been reported so far.

English Holly has been introduced on every continent. It seems particularly bad in New Zealand, Tasmania, and North America.

How English Holly Spreads

It is primarily spread long-distance by deliberate human introductions as an ornamental species in gardens.

Short-distance dispersal occurs through birds that eat the abundant drupes later in winter after they have softened and their taste improves. They spread the seeds through their feces, where the seeds then germinate in any suitable habitat in both shade and sun.

English Holly also spreads readily vegetatively through suckering. This is how most of the thickets of Common Holly form and how it becomes a dominant understory species of the forests.

Habitats at Risk of Invasion in North America

The way that English Holly spreads through the feces of birds puts all habitats at risk, provided there is enough moisture. They will not grow in permanently wet wetlands, and they also will not grow in arid or semi-arid areas. Any moderately moist to wet climate, however, is at risk of invasion.

Their ability to grow in both sun and shade makes them a common invader of forests in particular but also roadsides, fields, meadows, cliffs, gorges, and anywhere else as long as there is ample moisture.

European Holly has proven to be particularly invasive in the temperate rainforests of northwestern North America, where it easily invades the mixed forest canopies and thrives in the moist environment there.

Impacts of Invasion by Ilex aquifolium

Ilex aquifolium has deep and aggressive roots that tend to out-compete the native species for nutrients and water. There is also evidence that English Holly alters the soil conditions by adding sulfur and large amounts of organic matter to the soil, which makes the conditions less hospitable for native species.

English Holly forms dense thickets that can dominate the understory of the forest and out-shade native shrubs and other native plants, reducing the biodiversity of the forests it invades.

Potential Benefits of Invasion

Thickets of Ilex aquifolium are known to be used as a refuge by small birds and deer. The flowers are attractive to bees, small butterflies, wasps, and flies, and the berries provide an important late-winter food source. However, as with all invasive plants, the native wildlife would have depended on the native plants that grew there before the Common Holly replaced them.

Methods to Remove English Holly

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 European Holly, and never plant it in your yard.

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

If you live in the eastern part of North America, why not grow one of the two native hollies instead, Ilex opaca or Ilex vomitoria? If you live in western North America and want some spiny, glossy green foliage, try growing native Berberis species instead. They have lovely foliage and beautiful blue-colored berries. The native Ilex and Berberis species can even be substituted for Common Holly in Christmas decorations.

Physical Control of European Holly

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

Physical methods to remove European Holly generally involve uprooting the plants. This is best done when the plants are young before their aggressive roots have had a chance to establish themselves. Fortunately, they are a slow-growing species, so on a single walk through the forest, you can single-handedly destroy dozens of young plants with very little effort. Larger plants will need to be dug out with a shovel to get their roots. If the roots are not dug out, they can regrow from the stump.

For larger shrubby plants, use pruners to remove the upper growth before using a shovel to dig out the root mass. For tree forms of Ilex aquifolium, you can try girdling it. This involves removing a strip of bark from all the way around the circumference of the tree. As with all trees and woody shrubs, the only part of the tree’s trunk that is alive and transporting nutrients is the layer directly under the bark. If you cut down to the heartwood and remove the bark in a strip all around the tree, it can no longer transport water and nutrients to the leaves. The tree will then die and can be left to rot or be removed.

The best time to remove European Holly is before it is in berry, in the spring or early summer. If you need to remove them when in berry, be sure to clip off the berries before removing the shrub or tree and place them in the bag for proper disposal.

Disposal of the Shrubs Once Removed

Trees or shrubs without berries on them can be burned, cut into firewood, or left to rot. If you have plants that have mature berries on them, they must either be burned or solarized. If you are unable to burn in your area, then solarizing is a viable option. To solarize, put the shrubs under a thick black tarp or into thick black garbage bags. Then leave them in the full sun for a good 8 weeks at least to be sure that all seeds are no longer viable. The plants can then be disposed of. They can be brought to a garbage dump where you can inform them they are an invasive species so they can deal with them accordingly.

Chemical Control of European Holly

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.

Furthermore, there are no chemical control methods that effectively target only European Holly. If you do use chemical control, it must be stem injected. The tough evergreen leaves readily resist the absorption of pesticides, making foliar applications ineffective.

Chemical control is generally not recommended.

Biological Control of Ilex aquifolium

Biological control involves the use of a predator, herbivore, disease, or some other biological agent to control an invasive species once it is fully 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.

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. However, using biological control in conjunction with physical control and ongoing monitoring can be very effective for some invasive species.

Unfortunately, there are no current methods of biological control for English Holly in North America other than goats. Domestic goats are aggressive and non-selective browsers. They will literally eat anything. This will work best on shrubs and thickets. Often goats will eat the bark off of trees so that they can even kill off the trees if they manage to eat all the way around the tree.

To use goats, the patch should be penned in, and then the goats left there to graze for at least 2 years. They will eventually destroy all the above-ground growth and then eat the new sprouts as they come up. This can then be followed with rootstock removal for any stubborn plants that may remain. Alternatively, simply leave the goats in the area for another year until you see no more sprouts. At this time, they can be moved to another patch, and that patch can be replanted with native vegetation. An ongoing monitoring program should then be implemented to be sure they do not sprout again.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest 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 Sometimes Crucial

Removal of single plants from the understory of the forest will not require replanting with native species. Simply remove and monitor, and other plants will grow in their place. However, in cases of the removal of thickets that dominate the area, the site should be replanted with native species. A replanting program should already be planned and ready to implement once the English Holly has been removed. You will need to research species that are native to your area. Ideally, use ones from your local environment to ensure local ecotypes are represented. Alternatively, you can purchase suitable plants from a nursery in your area that specializes in native species.

Ongoing Monitoring is Essential

In all cases of invasive European Holly 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. Sometimes roots left behind will re-sprout into new plants. Furthermore, birds will keep bringing in new seedlings if there are still plants in the wider surrounding environment. For these reasons, yearly monitoring should be put in place to remove young plants before they have a chance to become established, especially since that is the easiest time to remove them. With ongoing monitoring, we can keep invasive species in check and promote local biodiversity in the process.

References and Resources

Best Management Practices for English Holly in the Metro Vancouver Region http://www.metrovancouver.org/services/regional-planning/PlanningPublications/EnglishHollyBMP.pdf

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

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

Wikipedia on Ilex aquifolium https://en.wikipedia.org/wiki/Ilex_aquifolium

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!

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