Black Swallowwort


Black swallowwort is an herbaceous perennial in the milkweed family. It is a vine with twining stems up to 6 feet long with dark green, glossy foliage. Flowers are 1/8-inch, purple with a yellow center, and star shaped. Fruit are typical milkweed-like pods, 2- 2 ½ inches in length, 3/8 inch in diameter and smooth skinned. When ripe, the fruits open along a seam and release flattened seeds equipped with downy parachutes that aid in wind dispersal. Black swallow-wort can spread long distances by seed and clones arising from root systems. Thick infestations in full sun can produce 2,000 seeds per square meter. The seeds can give rise to multiple plants per seed which greatly increases the likelihood of seed survival and establishment. Wind dispersal of seed begins in late July to early August in open areas and continues throughout late summer and fall.

A major problem with black swallowwort is that monarch butterflies will confuse it with native milkweed and lay their eggs on the plant. When the caterpillars hatch, they cannot survive, resulting in a 0% survival rate for eggs laid on this plant. This makes removal of this plant at the beginning of the summer an important step in maintaining monarch populations.


The seeds of this plant disperse very easily. If you are working in an area that has reproducing black swallowwort, be sure to check your clothes and tools for seeds. Manual cutting and removal of the plant can be an effective method depending on the size of the affected area, as well as pulling the plant and all root material up and bagging it. Using the ‘cut-and-dab’ method on thicker stems and vines is also effective. Make sure to bag and kill all vascular plant material. Mowing over stems will not kill the vine, but may break the reproductive cycle. As the seeds are so easily spread, removal in spring and early summer before the seeds are fully formed is highly recommended.