News Release

Oriental Bittersweet: A Patient Invader

June 24, 2004


Asheville, NC — USDA Forest Service research on oriental bittersweet confirms suspicions about the plant's destructive invasion of the forests of Southern Appalachia. Studies by Southern Research Station (SRS) scientists at the Bent Creek Experimental Forest near Asheville, North Carolina provide an understanding of the unique “sit and wait” strategy adopted by the destructive vine.

Oriental bittersweet (Celastrus orbiculatus), a woody vine with rounded leaves and small yellowish flowers, was introduced to the United States from Southeast Asia around 1860. The bright orange berries produced in the fall have made oriental bittersweet popular for wreaths and winter flower arrangements, but the pretty vine wreaks havoc on the trees and native plants of the Southern Appalachian forest. The vine can spread by root suckering, but is primarily dispersed by the birds and mammals that eat the berries - and sometimes by people using the vines to decorate. Oriental bittersweet easily proliferates in forest openings created by disturbance.

Oriental bittersweet on mature white pine.  Photo: Z. Hoyle

Oriental bittersweet on mature white pine.
Photo: Z. Hoyle

Asheville, North Carolina, is a hub for oriental bittersweet invasion. The vine is literally moving out along roads and rivers into the public lands that surround the city, and poses a real threat to forest trees and plants. Oriental bittersweet grows fast: the plant can cover tall trees in a season, causing them to collapse from the weight of the vines. Understory plants are smothered by the vines themselves or by lack of light.

The native version of the vine, American bittersweet (Celastrus scandens), looks very similar to oriental bittersweet, except that it flowers and produces berries at the end of stems, while oriental bittersweet produces berries where leaf and stem intersect. Not aggressive or particularly invasive, American bittersweet itself is under threat. Because it hybridizes so easily with oriental bittersweet, the genetic integrity of the native plant may be lost.

Cathryn Greenberg, research ecologist with the Bent Creek unit, has studied the unique strategy that allows oriental bittersweet to spread so rapidly. In 2001, research by Greenberg, Lindsay Smith (University of Tennessee), Douglas Levey (University of Florida), and Evelyn Konopik, a German graduate student working with the National Forests of North Carolina, confirmed the steady spread of oriental bittersweet out of Asheville along the Blue Ridge Parkway .

Using greenhouse experiments, Greenberg, Smith and Levey looked at different factors that might affect how well bittersweet seeds germinate. They found that bare seeds - those with the flesh and pulp removed - had the highest germination rate. The researchers also looked at scarification of the seed covering. The seeds of many plants have a hard seed cover that must bebroken or scratched before germination can begin. Some seeds must literally be etched by the gastrointestinal acids in the stomachs of birds and mammals. “Although birds are thought to be the primary dispersers of oriental bittersweet, no one had looked at whether scarification contributed to germination,” says Greenberg. “We fed seeds to captive birds, and somewhat surprisingly, found no difference in germination rates from seeds that were just defleshed. But the intact fruits with flesh did take longer, suggesting that getting eaten by birds does help the seeds to germinate.”

Experiments manipulating available light showed that light intensity did not affect the proportion of seeds germinating, the time until germination, or seed survival. “We found a high level of germination over a wide range of conditions,” says Greenberg. “Our results confirm that bittersweet seeds are dispersed in large numbers, and that the plant can readily establish and persist in low light under the forest canopy. When a hole in the forest canopy allows light to reach the ground, the plants begin growing rapidly.” Most invasive plants move into disturbed sites with high light and reduced competition from other plants. The unusual “sit and wait” strategy of oriental bittersweet allows the plant to slowly invade an intact forest and wait for a canopy disturbance to spread rapidly.

To learn more about the ecology of oriental bittersweet in forested settings, Henry McNab, researcher forester with the Bent Creek unit and project leader David Loftis analyzed different sites in relation to occurrences of the plant. They found that the presence of bittersweet was associated with moist areas with mature trees and few shrubs. Bittersweet generally did not grow where the forest canopy was dominated by oaks or where there was no bare soil exposed. Oriental bittersweet was also absent from sites with mountain laurel, which tend to be dry.

Unfortunately, very little is known about how to get rid of oriental bittersweet. In the article published about the site research, McNab recommends that managers of lands invaded by oriental bittersweet start by aggressively controlling isolated patches of the vine. Greenberg recommends removing the vines before the fruit forms to minimize the spread of seeds by birds, animals, and people. Vines can be removed fairly easily by hand pulling and clipping, but removing them from highly infested areas is very labor intensive.

During July and August, the Bent Creek unit will start a measured attack on oriental bittersweet. The Forest Service will work with community volunteers trained by the North Carolina Exotic Pest Plant Council and the Asheville Weed Team to clear the invasive vine from sections of the Bent Creek Experimental Forest. Click here for more details.

Last summer, SRS published Nonnative Invasive Plants of the Southern Forests: A Field Guide for Identification and Control, by James H. Miller, Research Ecologist at the Auburn, AL unit. The book provides a comprehensive guide to accurate identification and effective control of 33 nonnative plants and groups of growing concern in the Southern United States. Click here for more information about the guide.

Miller's recommended control procedures for oriental bittersweet:

  • Thoroughly wet all leaves with one of the following herbicides in water with a surfactant (July to October): Garlon 4, Garlon 3A, or a glyphosate herbicide as a 2 percent solution (8 ounces per 3 gallon mix).

  • For stems or vines too tall for foliar sprays, apply Garlon 4 as a 20 percent solution in commercially available basal oil, diesel fuel, or kerosene (2.5 quarts per 3 gallon mix) with a penetrant (check with herbicide distributor) to the lower 16 inches of stems. Or, cut large stems or vines and immediately treat the cut surfaces with one of the following herbicides in water with a surfactant added: Garlon 4 or glyphosate herbicide was a 25 percent solution (32 ounces per 1 gallon mix.).

For more information: James H. Miller at 334-826-8700 or jmiller01@fs.fed.us

# # #

Cathryn Greenberg at 828-667-5261 x 118 or kgreenberg@fs.fed.us

Henry McNab at 828-667-5261 x 119 or hmcnab@fs.fed.us

Ecology and Management of Southern Appalachian Hardwoods unit at Bent Creek: http://www.srs.fs.usda.gov/bentcreek/

Integrated Vegetation Management for Sustaining Southern Forests unit in Auburn, AL: http://www.srs.fs.usda.gov/4105/index.html

References:

Greenberg, Cathryn H.; Smith, Lindsay M.; Levey, Douglas J. 2001. Fruit fate, seed germination and growth of an invasive vine - an experimental test of 'sit and wait' strategy. Biological Invasions 3: 363-372. Full text: http://www.srs.fs.usda.gov/pubs/viewpub.jsp?index=4495

Konopik, Evelyn. 2002. Exotic plants along the Blue Ridge Parkway in North Carolina : the road as migration path. MS thesis, Department of Geography, Friedrich-Alexander-University, Erlangen-Nuremburg , Germany.

McNab, W. Henry; Loftis, David. 2002. Probability of occurrence and habitat features for oriental bittersweet in an oak forest in the Southern Appalachian Mountains, USA . Forestry Ecology and Management 155: 45-54. Full text: http://www.srs.fs.usda.gov/pubs/viewpub.jsp?index=3193

Miller, James H. Nonnative invasive plants of southern forests: a field guide for identification and control. Revised. Gen. Tech. Rep. SRS-62. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station, 93 p. Full text: http://www.srs.fs.usda.gov/pubs/gtr/gtr_srs062/

News Release Contact
Zoë Hoyle
828-257-4388
zhoyle@fs.fed.us