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

Occasionally people mistake Chinese Wisteria for non-related species such as:

• Paulownia tomentosa with its purple flowers, but its flowers are large and tubular and not legume-like, and it has massive simple leaves rather than compound leaves.
• Dermatophyllum secundiflorum 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.

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