Milkweed's evil twin

I'll be selling gardeners' gift baskets at the farmer's market in West Newton, MA 9:30-12:30 on June 23, June 30, and every other Saturday thereafter until October 6. Stop by and say hello if you're in the neighborhood! The market is on Elm Street between Washington Street and Border Street.

Recognizing nonnative invasive plants can be discouraging. There are so many thriving along sidewalks, in parks, and on conservation land that it’s easy to feel that it’s too late. There’s no way to weed them all out. We have to pick our battles.

Japanese knotweed (Fallopia japonica) taking over a local parkway

    One manageable thing we can do in our own gardens is to keep an eye out for black swallow-wort (Cynanchum louiseae). 

A tangle of black swallow-wort vines

This vine is popping up all over my neighborhood. The reason it’s a problem, other than its swarming over shrubs and shading them out, is that it fools monarch butterflies. 

Monarch on a real common milkweed (Asclepias syriaca)

Females confuse it with native milkweeds, to which it’s related, and lay their eggs on the impostor plant, thinking it will provide food for their larvae. When the caterpillars hatch, they can’t live on black swallow-wort leaves, and they don’t survive.

    The first black swallow-wort vine I noticed in my yard grew up from the base of a particularly thorny rose bush and twined around the canes like a morning glory. I didn’t spot it until it was 3 feet tall. I never was able to uproot that plant. I couldn’t dig deeply without risking killing the rose. 

A hard place to dig

When I grabbed the base of the vine’s stem and tried to yank the roots out, it snapped at soil level. I keep working at it every year, watching for new tendrils as they appear in spring and pulling out as much of the plant as I can. Some year, I hope, it will run out of energy and die.

    Fortunately, that’s good enough to protect monarchs. Here’s a link to a helpful guide published by Newton Conservators, a local environmental group. As they explain, it’s useful to cut the plant to the ground. Later in the season, you can even help by cutting off the seed pods and throwing them in the trash to keep browsing animals from eating the fruits and then excreting the seeds to germinate somewhere else. Like other parts of invasive plants, the pods should go into the trash, not the compost or even the yard waste. They have awe-inspiring reproductive powers.

Butterfly weed (Asclepias tuberosa) is another milkweed relative and does support monarchs

    Down the block, black swallow-wort has overwhelmed a privet hedge that grows thinly because of the shade of a sugar maple. A net of vines drapes over the low hedge, hiding it completely. That means many seeds will be ready to spread around the neighborhood by the end of the summer.

    With that kind of population pressure, I can’t expect to keep black-swallow-wort out of my yard. I’ll just have to watch for it as I deadhead, prune, and weed. If I catch the sprouts when they’re small, I have a chance to keep them from taking over.

    Newton Conservators and other groups regularly mobilize teams to pull invasives on conservation land. Will this work have to be repeated every year to keep the culprits from growing back? Let’s hope volunteers replace the unwanted plants with native species vigorous enough to hold the ground.

Swamp milkweed (Asclepias incarnata), left,
or common milkweed (Asclepias syriaca), 
right, could be good replacements for black
swallow-wort