Bulletin #2523, Maine Invasive Plants: Black Swallowwort, Cynanchum louiseae (Milkweed Family)
Developed by the Maine Natural Areas Program and University of Maine Cooperative Extension
Threats to Native Habitats
Black swallowwort is particularly troublesome in open areas and along edges and banks where it grows over other vegetation, blocking light and creating tangled thickets. It alters and degrades natural habitat by crowding out native plants and is a threat to rare species in some locations in New England.
Black swallowwort is a perennial vine with a single non-branching stem that grows up to six feet in length. The vines typically twine and sprawl over other vegetation and die back to the ground each year. The dark green leaves are opposite on the stem and are lanceolate (shaped like a lance head) to heart-shaped. Flowers are purplish-brownish, approximately one-quarter-inch wide, five-petalled, and fragrant. They appear in June and may be found until late summer, particularly on plants growing in the shade. Fruits are long slender green pods (two to three inches), that turn dark brown when ripe. They appear in pairs or sometimes threes, similar to milkweed pods, but longer and narrower. Seeds are also like common milkweed seeds, rounded and flattened, each with an attached tuft of silky hair. Seeds are dispersed by wind. Reproduction is primarily by seed, although plants may spread by trailing, rooting (stoloniferous) stems, creating large colonies. In winter, stems may be found entangled in small shrubs with remnants of old seedpods still attached.
Dense populations occur in open, disturbed places such as areas along roads and railroad tracks, and also along woodland edges and in open woods. This vine typically grows in soils that are moist but not saturated. Prolific growth has been observed in open to partially shaded areas with alkaline soils; this could indicate an adaptation to certain soil and sun conditions.
Black swallowwort is a native of southwestern Europe and was most likely introduced to North America around 1900. The Josselyn Botanical Society has reported its appearance in four Maine counties: Cumberland, Lincoln, Sagadahoc, and York. Nationally it has been reported from Maine to Michigan to Nebraska as well as California.
Mechanical Control: Digging up the root crowns is effective but relatively destructive and the whole crown must be removed. Pulling the plants by hand generally leads to resprouting but can prevent seed production, especially if repeated during the growing season. A less effective method is pod picking, which also limits seed production, but does little damage to the existing population. Mowing is best for preventing seed production. Mow frequently (one to two visits per season) just as the pods are beginning to form.
Chemical Control: Apply a one-percent solution of a triclopyr-based herbicide at an application rate of 2.25–19 L/ha (one to eight quarts per acre). The use of a triclopyr-based herbicide is especially desirable in grassy areas. Repeated herbicide applications will most likely be necessary for vigorous stands. It is best to spray early in the season before viable seeds are produced (before mid-July). Cut-stem applications are recommended for small infestations or if nearby desirable vegetation will be adversely affected. Burning and grazing do not appear to be effective. Use herbicides responsibly and follow the manufacturer’s directions. Contact the Maine Department of Agriculture for information on restrictions that apply to the use of herbicides. Consult a licensed herbicide applicator before applying herbicides over large areas.
This fact sheet was researched and written by Kyle Fletcher, a student in the Plant and Soil Technology Program at Southern Maine Community College. Additional editing by Don Cameron, Maine Natural Areas Program.
Josselyn Botanical Society. Checklist of the Vascular Plants of Maine, Third Revision. Orono, ME: Maine Agricultural and Forest Experiment Station Bulletin 844, 1995.
Bailey, Liberty Hyde, and Ethel Zoe. Hortus III: A concise dictionary of plants cultivated in the United States and Canada. Revised by the staff of The Liberty Hyde Bailey Hortorium, Cornell University. New York: MacMillan Publishing, 1976.
Morisawa, T.L. “Element Stewardship Abstract for Vincetoxicum nigrum (L.) Moench.” Arlington, VA: Wildland Weeds Management and Research, The Nature Conservancy, 1999.
Newcomb, L. Newcomb’s Wildflower Guide. Boston, Toronto: Little, Brown and Company, 1977.
Peterson, R.T. and M. McKenney. Peterson’s Field Guide, Wildflowers. Boston, New York: Houghton Mifflin Company, 1968.
Uva, N., and DiTomaso. Weeds of the Northeast. Ithaca: Comstock Publishing Associates, Cornell University, 1997.
For more information or for a more extensive list of references on invasive species contact:
Department of Conservation
#93 State House Station
Augusta, ME 04333-0093
University of Maine Cooperative Extension
495 College Avenue
Orono, ME 04469
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