Black Swallowwort is an invasive weed (imported from Europe about 200 years ago) called . It has been found growing in fields, the edges of woods, and in suburban and urban locations. Black Swallowwort is everywhere. It threatens native plant habitats and has been listed on the New England invasive plants list. Many people don’t know the vine’s negative consequences and think it’s just an attractive foliage plant growing on their fence.
Black Swallowwort is a member of the Milkweed family, but unlike Milkweed, it is not a reproductive food host like Milkweed is for the Monarch butterfly. Many Monarchs lay their eggs on Black Swallowwort leaves mistaking them for Milkweed. When the larvae eat the Swallowwort leaves which are toxic, they die. Unfortunately, this vine is encroaching on the Milkweed plants’ habitat.
The vine’s stems are narrow and the leaves which are opposite one another on the stem are oval, shiny dark green with pointed ends. In June and July you’ll notice tiny purple flowers on the plant. When the flowers finish blooming, green seed pods form that reminds one of string beans but look more like chili peppers. The pods turn light brown when ripe and then open up to reveal milkweed like seeds (small flat brown seed heads attached to plumes of silky white hair) that are dispersed by the wind.
Getting rid of this noxious vine is very difficult. Just pulling the vine out of the ground doesn’t remove all of its roots. It has a complex root system most likely entwined with other plant roots, and the root crown and even the tiny white roots you’ll find, must be dug completely out of the ground to prevent more plants from coming up. If you notice tiny Black Swallowwort seedlings popping up around the larger vines, dig them up also.
If you can’t successfully remove the vine, use a scissor or your fingers to take off the seed pods before they become ripe. Put the pods into a heavy duty black plastic bag, seal it with a twist tie, and put the bag in the trash. Don’t put pods in your compost pile or in a lawn bag that will be recycled. Other controls include mowing the vine just as the pods are forming to prevent seed production, or applying a triclopyr-based herbicide such as Roundup Pro before pods form. (Caution: Roundup Pro will kill surrounding plants if it comes in contact with them. Use herbicides responsibly and consult a professional if needed.)