That’s Invasive: Asiatic bittersweet

What’s yellow, climbs trees and utility poles, and has orange berries? Asiatic bittersweet, seen on a roadside near you every autumn.

Even in early November, its yellow leaves can be seen climbing trees and poles throughout Harpswell. Some people are tempted to use those orange-red berry-like seed capsules for decoration. For many years, those little seed capsules found their way into decorative wreaths and arrangements in homes and on doors. Please leave bittersweet out of your holiday decor! A wreath of bittersweet on your door may encourage bittersweet seed dispersal throughout the neighborhood, which leads to new shoots of bittersweet appearing next spring and summer.

Why worry? What harm can a few bittersweet plants do? As an aggressive, nonnative plant, bittersweet (Celastrus orbiculatus) can crowd out native shrubs and trees. When bittersweet climbs a tree, it “strangles” the tree, eventually causing the tree to die.

Since its introduction to North America from eastern Asia as an ornamental vine around 1870, bittersweet has spread from the intended landscapes and naturalized extensively in the eastern half of North America. Thanks to prolific seed production, rapid growth rate, wide tolerance of soil and light conditions, root-suckering, and its attractiveness to people, this plant is categorized as “severely invasive” by the Maine Natural Areas Program.

Some marketing strategies touted the imported bittersweet as being “pest-free,” meaning it did not suffer damage from insects nibbling on its leaves. According to entomologist Douglas Tallamy, this means that bittersweet is not a food source for native insects. His research shows that native plants support a greater number and variety of insects than the most common invasive plants, including bittersweet.

In recent years, Maine has prohibited the sale and propagation of several dozen invasive species, including bittersweet. This plant is now on Maine’s “banned” list.

The Harpswell Invasive Plant Partnership has been working throughout our town to slow the spread of invasive plants. We have removed bittersweet from several Harpswell Heritage Land Trust preserves. While complete eradication is still a goal, we are seeing a reduction in the numbers and size of bittersweet infestations. Volunteer work days at Johnson Field, Otter Brook and Curtis Farm preserves in September and October continued the battle with this persistent invasive plant. Many of the largest vines have been removed and hope remains that one day we will have this alien plant under control.

In the meantime, what can you do for your holiday decorations? Bright-red winterberries make an excellent addition to arrangements. While this native plant does not twine into a wreath, sprigs of berries brighten any wreath or arrangement. Think native when you put together a natural arrangement or decorate your holiday wreath. Dried flowers and fresh, red winterberries will look delightful in your holiday decor.

For further information about invasive plants, visit HIPP’s website at hippmaine.org. You may also email your questions to hipp.maine@gmail.com.