It’s leaf-peeping season in New England, a time when visitors and locals alike visit farmstands for pumpkins, gourds and corn stalks, or tramp nearby fields and roadsides for weeds and berries to decorate their doors, porches, and mailboxes.

You may still find fat bunches of oriental bittersweet hanging with the Indian corn and bunches of upside-down strawflowers at farmstands. Or you may collect some from your favorite spot near the railroad trestle where the vines grow lush and thick with yellow-orange berries.

But if you’ve ever spent your summer weekends trying to eradicate this scourge, you probably won’t be tempted to decorate your home with it.

Oriental bittersweet, Celastrus orbiculatus, imported from Asia in the mid-1800’s, was widely planted along new railroad beds to prevent the soil from eroding. Gardeners quickly adapted it, training the vine to climb garden trellises. It quickly became established from Louisiana to Maine. Its bright orange berries and twisting stems make it a natural choice for making wreaths and sprays that decorate New England doors each year.

How could such a charming ornamental plant become such a problem? In its native Asia, bittersweet dominates lowland slopes and thickets. Here in North America, oriental bittersweet is extremely successful in almost every habitat, from floodplain forests to dry, rocky slopes.

It poses a serious threat to other species and to entire habitats because of its ability to twine around and grow over other vegetation. In addition it has a high reproductive rate, long-range seed dispersal, and the ability to produce new plants from its root system.

Bittersweet has an affinity for forest edges, where it has an opportunity to twine around and grow over other plants, while also receiving the light it needs to flourish and set fruit. It often strangles the trees and shrubs it climbs by twining around their trunks and branches, eventually constricting the flow of water and nutrients to their leaves. Trees girdled and weighted down by bittersweet vines growing up into their canopies also become more susceptible to damage by wind, snow and ice.

Birds and other wildlife that eat the bright orange fruit in winter disperse its seeds in their droppings. Unfortunately, we humans also spread this invasive plant by decorating with the beautiful berries, then tossing them into compost or brush piles or other outdoor locations.

To make matters worse, oriental bittersweet readily hybridizes with its native cousin, American bittersweet, Celastrus scandens, which occurs in similar habitats. Hybridization may ultimately destroy the genetic integrity of the native species.

How do you tell the difference? Both varieties produce waxy orange berries, which burst out of pale yellow seedcases as they ripen in the fall. American bittersweet sets its berries in clusters at the end of a branch while Oriental bittersweet distributes its fruits evenly along the stem. The oriental species also has a decidedly more rounded leaf than that of American bittersweet.
           
Because of these characteristics and the threat this plant poses to native plant communities,
in 2004, the N.H. Invasive Species Committee placed oriental bittersweet on a list of 18 invasive land plants now prohibited from sale, transport, distribution, propagation or transplantation in New Hampshire.
           
If you have an infestation of oriental bittersweet, how do you cope? The most effective way is to watch for and remove new small plants. Larger plants will require cutting combined with herbicides. If you have only a few small plants, you may be able to control them mowing or cutting the vines and pulling the roots. Weekly mowing will eventually kill plants, but less frequent mowing (fewer than three times per year) will only stimulate root-suckering. You can treat older, taller bittersweet plants—vines can climb 60 feet or more, with stems growing several inches in diameter—by cutting vines and immediately treating cut stems with the herbicide triclopyr.

For more information on invasive plants in New Hampshire, download the N.H. Department of Agriculture, Markets & Food’s Guide to Invasive Upland Plant species.

By Margaret Hagen, Director of UNH Cooperative Extension Family, Home & Garden Education Center

For more information call the UNH Cooperative Extension's Family, Home & Garden Center's Info-line (toll free) at 1-877-398-4769 or send us an email. Volunteers are available to answer your questions Monday through Friday 9:00am to 2:00 p.m.