Large, showy, purple wisteria flowers are covering trees along roadsides this time of year. While they are pretty, most of what you are seeing is also invasive. Several wisteria species live in South Carolina. American (Wisteria frutescens) and Kentucky wisteria (W. macrostachya) are native species. They are not all that aggressive in terms of their growth. However, it is an entirely different story with non-native invasive Chinese (W. sinensis) and Japanese (W. floribunda) wisteria. These are aggressive plants that will grow up and take over old buildings, trees, or anything the vines can grow on. Telling the wisterias apart can be difficult, but there are some ways to do so. Chinese and Japanese wisteria flower during leaf-out, while the native species flower later in the season, after the vines have leafed out as shown in figure 1. Flowers on the invasive species are also almost twice as long as those on natives as shown in figure 2, and native flowers are unscented. Also, seed pods are smooth on the native species and fuzzy on the invasive species. And, they twine differently around whatever they are growing on. Native and Chinese wisteria twine clockwise, while Japanese wisteria twines counterclockwise.
Wisteria’s large woody vines can strangle smaller trees, and the weight of the vines, foliage, and flowers can cause tree limbs to break. What is worse though, in forest stands with wisteria the vines can tie the tops of trees together as shown in figure 3. This causes a very dangerous situation for anyone trying to harvest those trees- can you imagine a feller buncher trying to remove trees that are all tethered together at the top? To remove wisteria from a forest stand that will be harvested, it is best to cut the vines a few years prior – but do this in winter. If you cut vines in summer (when they are full of water) they can choke out living trees as they dry out and constrict. To kill wisteria, foliage can be treated with triclopyr or glyphosate. Cut stems can be treated with triclopyr. When the wisteria is very tall, a combination of cutting the vines and later spraying the regrowth foliage may be effective.
The article was originally featured in the Spring 2020 Edition of the CU in the Woods newsletter.
Author(s)
Dave Coyle, Cooperative Extension, Forestry and Wildlife Specialist
This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.
Clemson University Cooperative Extension Service offers its programs to people of all ages, regardless of race, color, gender, religion, national origin, disability, political beliefs, sexual orientation, gender identity, marital or family status and is an equal opportunity employer.