Oriental bittersweet has the dubious distinction of being named Minnesota’s weed of the month for September, an “honor” not many plants—or their growers—would appreciate. Up until about 2010, this beautiful woody vine was grown and sold in the state and around the country for its distinctive and eye-catching fruit, which made the plant desirable for use in wreaths and in floral arrangements.
Celastrus orbiculatus was introduced to the U.S. from Asia in the 1860s and quickly gained popularity as an ornamental vine, but like many such well-intentioned introductions, Oriental bittersweet became aggressive and wore out its welcome. It is reported to be invasive from Maine to North Carolina, and west to Minnesota, Missouri and Wisconsin. Often found along forest edges and open woodlands, it can tolerate shade and has thus established a stronghold in many forested areas.
Thick masses of the vigorous vine tend to choke out native trees and shrubs by producing such dense shade that the natives weaken and die, and by girdling other plants. The excessive weight of the vines can uproot and topple their victims.
Photo courtesy of Arthur E. Miller, USDA APHIS PPQ; Bugwood.org
Oriental bittersweet often is confused with its cousin, our native American bittersweet (Celastrus scandens). And it’s no wonder why: Where C. orbiculatus sports bright red berries that emerge from yellow capsules, native C. scandens produces similar berries with orange capsules. Although the differences are subtle, the American variety is better behaved, and the invasive plant tends to create masses of growth that can become dense enough to deter deer from navigating through wooded areas.
Foliar herbicides may be effective, and removal of all plant material—above and below ground—is required to keep this lovely but lethal plant in check. Just don’t be tempted to collect the fruit for decoration.
Photo courtesy of USDA Forest Service – Region 8 – Southern Archive, USDA Forest Service, Bugwood.org
Kudzu moves north
The plant that ate the South is heading north of the Mason-Dixon line as climate change renders northern microclimates more hospitable to the invasive vine. Pueraria montana var. lobata—kudzu—has been spotted making trouble in Illinois and Ohio, and it has even leaped over Lake Erie to find a home in Ontario.
Yet another ill-conceived introduction from Asia, kudzu was introduced to the U.S. in 1876 at the Japanese pavilion during the Centennial Exposition in Philadelphia. For a while, it did a bang-up job of preventing erosion in many areas of the South. But without natural checks and balances, the aggressive vine overtook much of the native vegetation—and the rest is history.
Photo courtesy of Jil Swearingen, USDI National Park Service, Bugwood.org
Until recently, kudzu has been limited to warmer territory. That no longer appears to be the case. “The one thing that’s kept these invasives in check has always been cold weather,” explained Lewis Ziska, a plant physiologist with the USDA’s Agricultural Research Service in Beltsville, Maryland. “As the winters warm as a result of a changing climate, that more or less opens up a Pandora’s box of where these invasives can show up in the future.”