Non-Native Invasive Species

Oriental bittersweet

Celastrus orbiculatus

It won't be long before the Oriental bittersweet berries ripen. That's when we'll know how much trouble we're in - or, rather, about half the trouble we're in. Bittersweet berries are produced only on female plants, so we can guess that there are at least double the number of apparent vines along our roads, in abandoned fields, and in our woods.

Oriental bittersweet

Oriental bittersweet
Photo: Chris Evans, River to River CWMA, Bugwood.org

But don't we look forward to the ripening of bittersweet in New England? Isn't collecting the red-orange and yellow fruits an autumnal tradition? It certainly is, but not this bittersweet. Today the most common bittersweet in our landscapes, by far, is the hugely aggressive Oriental bittersweet (Celastrus orbiculatus), native to Japan, Korea, and northern China. First brought to North America in the 1800s, it has now nearly supplanted our native American bittersweet (Celastrus scandens), a smaller, slower-growing species. Some botanists worry that the two may hybridize, and, because the Asian species is so much more successful a plant, that its genes may one day "swamp" those of our native species. (This is one of several ways in which organisms can go extinct).

Oriental bittersweet is a strangler. It will climb fifty feet or more in a tree, twining its way up the stem and around the branches, eventually choking off the flow of nutrients and water, or simply bringing the tree down by the force of its weight on high. It can move from tree to tree, like a monkey swinging from branch to branch, clambering through the canopy and shading out plants growing below.

It is enormously robust and resilient. It grows in sun or shade. When cut, it rootsuckers vigorously. It produces huge numbers of fruits, which birds love and which birds move about. And in one study, Oriental bittersweet fruits had a germination rate of seventy percent - the same rate guaranteed by garden suppliers like Johnny's Selected Seeds on most of its offerings!

Oriental bittersweet

Oriental bittersweet
Photo: Leslie J. Merhoff, University of Connecticut, Bugwood.org

It is the fruits that allow us to distinguish Oriental bittersweet from our native species. The invasive species has fruits in clusters at the joints, where leaves meet the stem. American bittersweet has fruits only at the ends of twigs and stems. If it is your custom to go bittersweet-ing on beautiful September days, look for these features. Do not bring home Oriental bittersweet fruits unless you plan to destroy them!

As for destroying plants, start by cutting the stems at their bases. It will prevent vines already in the trees from producing fruit. To prevent resprouting, at present our only proven option is an herbicide known as triclopyr, the active ingredient in the Ortho product Brush-B-Gone™. (There are no known biological controls for this species.) Either brush the cut stump with the solution, or spray the new sprouts early in the next growing season. Triclopyr does not kill grasses and related plants.