Last updated on July 21, 2025
Black swallowwort is a highly invasive vine that is toxic to monarch butterflies, deer, and outcompetes native plants — and there are particular ways to stop its spread.
The plant is a prolific fast-growing vine that forms extensive patches, covering and smothering desirable plants. Now is the time to really stop its spread, as seed pods disperse thousands of seeds in late July and early August.
You can identify it by the following:
- Leaves are shiny, dark green, and arranged in pairs along twining vines—often seen wrapping around fences and shrubs.
- Flowers are small, dark purple blooms with five petals and a green center, appearing in early summer.
- Pods are slim, green seed pods begin forming in June. These MUST be removed before they turn brown and release seeds into the wind.
To remove black swallowwort you should pull up the entire plant by the roots, and dispose in a sealed trash bag. Do not compost or place in yard waste bags.
Due to a similarity to milkweed, black swallowwort attracts monarch butterflies which lay their eggs on the leaves. But sadly, the leaves are toxic to monarch caterpillars. There is a belief that it is also toxic to deer and livestock.
Black swallowwort (Cynanchum louisea) is not new to the Massachusetts or the country. It originated in southern Europe and was likely imported as an ornamental, and it first appeared in Massachusetts in 1864 where it was reported to have escaped from cultivation in Essex County, according to the Andover’s town website. It’s now in every county in the state.
