A member of the milkweed family is August’s Weed of the Month. Black swallow-wort (Cynanchum louiseae), also called dog-strangling vine, is a perennial, herbaceous vine that can form large patches and crowd out native vegetation. It was introduced to North America from southern Europe in the 1800s as an ornamental plant, and in 1864 was recorded escaping from a botanic garden in Massachusetts. Since its introduction to North America, it has been found invading abandoned farm fields, pastures, and prairies throughout the Eastern and Midwestern United States.
Black swallow-wort has twining stems up to six feet long. It has dark green, glossy foliage and star-shaped, dark purple flowers with a yellow center. The flowers are only 1/8 inch in size and develop into a milkweed pod to disperse its seed by the wind.
The plant poses many ecological threats to the Midwest. It out-competes native plants by forming a large root system that exudes chemicals to prevent other plants, such as the native butterfly milkweed, from growing. Black swallow-wort also threatens monarch butterflies by crowding out native milkweed host plants. In addition, female monarchs will lay their eggs on black swallow-wort but the plant is lethally toxic to the caterpillars after they hatch and begin feeding. It can also thrive in wooded areas to form a monoculture in the forest understory. In Minnesota, black swallow-wort has been found growing in Carver, Chisago, Hennepin, and Ramsey counties.
Black swallow-wort’s characteristics make it a challenge to control. It grows over other vegetation to block light and create tangled masses. As a target weed on Minnesota’s Noxious Weed Eradicate List, it is required by law that all above- and below-ground plant parts be destroyed. Recommended management practices for black swallow-wort include the following:
Small infestations can be hand pulled or dug to remove the root crowns.
Foliar and cut stem herbicide applications can be effective. For specific herbicide recommendations, contact your University of Minnesota Regional Extension Educator.
All management practices for black swallow-wort should include yearly monitoring to ensure the depletion of the seedbank.
To report infestations of black swallow-wort or any other noxious weeds on the eradicate list, please notify the MDA by email at firstname.lastname@example.org or voicemail at 1-888-545-6684 (toll-free).