Description: These vines have shiny dark green leaves that are oval-shaped when mature, with stems that tend to twine around one another. The plant has very small, purple flowers with five petals. Swallow-worts develop milkweed-like pods by August that turn brown and split open in the fall, releasing fluffy seeds that are dispersed by wind. Unlike milkweed, however, they do not produce milky sap.
Impact: Black swallow-worts grow rapidly in open fields and roadsides where there is sun, pushing out native plants. The plant secretes toxins that prevent other plants from growing near them, as well as being toxic to livestock and pets. Of chief concern is its effect on the monarch butterfly, which recognizes the plant as its primary plant host, milkweeds. When monarchs lay their eggs on swallow-worts, their caterpillars eat the plant and die. Swallow-worts grow in habitats shared by milkweeds, outcompete milkweeds and may lead to a decline in monarch butterfly populations.
Management: Control depends entirely on removing swallow-worts before they can reproduce seeds. Cutting down to the stem and painting with herbicide is effective, but cutting or mowing without applying herbicide afterwards will only encourage regrowth. With large infestations, foliar herbicides can be used in early June and again in August. Seed pods should be removed and either burned or bagged, not composted. Management of black swallow-wort may be necessary for multiple summers to deal with younger shoots, but it will be much easier to handle after the initial treatment.