Black swallow-wort is considered toxic to the monarch caterpillars
University of Minnesota Extension is bringing awareness to black swallow-wort, Cynanchum louiseae, an invasive plant. The plant presents a problem for monarch butterflies, and it threatens natural areas.
Because of its large seedpods forming now, autumn is a good time to look for the plant and destroy it before pods break open and disperse seeds.
According to Angela Gupta, the extension educator in forestry and natural resources, “It’s in the milkweed family, so monarchs will lay their eggs on it,” she said. “However, when the larvae emerge they can’t survive on black swallow-wort and so they die.”
Master Gardeners have since been sharing information through garden tours, farmers’ markets and community councils, and with their neighbors.
“We are all interested in monarchs,” said Janelle Dahmen, the extension master gardener in Hennepin County. “We plant flowers for pollinators, but we want to make sure people know that, even though monarchs may seem to like this one, it isn’t one to keep.”
Removing and reducing black swallow-wort
“We think we have very little of it in the state, so eradication is possible if it’s found and reported quickly,” said Gupta.
The most effective treatment is spot-treating with a selective herbicide, making sure to always follow herbicide label directions. There is a risk that herbicide treatment could harm other plants nearby. If you opt not to use herbicide, you can hand pull the vines. Unfortunately, the vines break off from the roots when pulled. Digging out the complete roots is ideal but difficult because they break as well.
Repeated vine pulling multiple times during the summer and fall may eventually exhaust the vines. Additionally, It is important to make sure no pods release seed or the problem will be compounded. Now is the time to chemically treat or pull the vines.
Do not put vines in your personal compost bin.