Invasive, Butterfly-Killing Weed Spreading In MA: Black Swallow-Wort

MARLBOROUGH, MA — A highly aggressive and deadly plant is creeping its way across Massachusetts, and local cities and towns are enlisting property owners in stopping it.

The black swallow-wort is a perennial vine native to Europe that resembles native milkweed plants. Black swallow-wort produces pods full of seeds that spread widely in the wind — and those seeds are toxic to other plants, some animals and especially monarch butterflies.

"BSW displaces native plants and habitats, threatens butterflies and songbirds, and is toxic to deer and livestock. It is especially harmful to the monarch butterfly population because it resembles milkweed, where monarchs lay their eggs, and once the monarch larvae hatch, they die from eating the toxic BSW leaves," the city of Somerville warned in a bulletin this month.

Black swallow-wort has spread rapidly in the Boston area, and has been spotted as far west as Framingham — but state invasive plant watchers believe it's likely spread to most parts of Massachusetts. It grows in sun and partial shade in different habitats, including coastal areas and wetlands.

Plant experts advise residents to completely uproot any black swallow-wort plant and put it in a plastic bag to throw away. The plant's root system is fragile, and any root piece left in the ground can easily grow again. The town of Concord has a list of plants property owners can use to replace black swallow-wort.

Here's a guide about how to spot black swallow wort and dispose of it.