May 20, 2018
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Invasive plant should be controlled

The seeds from Oriental bittersweet produce vines that smother and kill forest trees, changing native plant communities forever. People contribute to seed dispersal by decorating with the fruits.
By Reeser Manley

Driving west through Machias, down the stretch of Route 1 that parallels the Machias River, I glanced to my right just in time to spot a sandwich-board display of seasonal wreaths, including one made from Oriental bittersweet (Celastrus orbiculatus). Two people, a man and a woman, were huddled in the back of the pickup parked next to the display.

I pulled in a few yards from the display, walked over to the couple and asked how much they were asking for the bittersweet wreath. A deal was made and I left Machias with the wreath in the back of my car, hoping that all of the seeds remained attached, that not one would find its way to the ground in Marjorie’s garden.

Once home, I carefully placed the wreath into a heavy-duty plastic garbage bag. I’m still working on how to dispose of the seeds, how to avoid starting a local invasion of the “kudzu of the North.”

Introduced to the U.S. by the ornamental horticulture industry in 1860, Oriental bittersweet (native to Korea, China and Japan) was discovered naturalized in 1913. It is now a serious invasive plant pest throughout New England and as far south as Florida and Louisiana.

This twining, woody vine is easily recognized in October by its yellow fall color, often displayed covering the tops of tall trees, and by its fruits. Initially green, the mature fruits change to bright yellow as they mature. Upon ripening, the yellow outer coating splits open to reveal a fleshy red coat that surrounds the seed. This combination of bright red and yellow is responsible for the popularity of bittersweet wreaths and cuttings as holiday decoration.

A single Oriental bittersweet plant can produce 2,000 seeds that are eventually dispersed by birds, small mammals, water and people. Remaining in the digestive tract of fruit-eating birds for days before being expelled, the seeds find their way into natural areas located miles from the parent plant.

The seeds germinate under a wide range of light conditions. In shady areas, seedlings employ a “sit and wait” strategy, persisting indefinitely in the densely shaded understory of a closed forest. When a disturbance opens the canopy, the seedlings respond with rapid growth.

I encountered the destructive potential of Oriental bittersweet (also called Asian bittersweet and climbing spindleberry) while conducting research on invasive plants in a forested area near Boothbay. The forest had been thinned, allowing seedlings of oak and maple to grow rapidly in the sun. Unfortunately, seedlings of Oriental bittersweet had also been waiting for the sunlight and grew rapidly, wrapping their ropelike stems around the tree seedlings, pulling them down. Other bittersweet plants had climbed into the canopies of larger trees, smothering the tree leaves, blocking out the sun.

Oriental bittersweet changes the composition of native plant communities. Thickets are too densely shaded for most herbaceous species to establish and grow, resulting in an eventual monoculture of the invasive vine covering dead support trees and the ground.

Communities throughout Maine are taking action to stop invasions of Oriental bittersweet, using a combination of nonchemical control strategies. Where feasible, individual vines can be pulled up by the roots and removed from the area, remembering that root fragments and crowns will sprout if left behind.

For larger populations where hand removal is not feasible, vines should be frequently cut or mowed, at least every two weeks, to exhaust the carbohydrate reserves in the roots. Cutting only two or three times per season will actually stimulate more root suckering.

The time has come to stop the growing, selling and harvesting of Oriental bittersweet plants and fruits. Education about controlling invasions will help reduce the spread of this noxious weed, but education is not enough. Maine needs legislation that prohibits commercial trade in Oriental bittersweet and other invasive species. The future of our natural landscape is at stake.

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