Marginal Way in Ogunquit is one of Maine’s most historic and enjoyable coastal footpaths. It was used by the local tribes before Europeans came to what is now Maine, and is traversed by thousands of people each year.
But Marginal Way is under attack by an invasive plant from Europe that is strangling vegetation along the mile-long trail. Swallowwort has been found up and down the Maine coast.
“This rampant invasive smothers small trees and native toughies like goldenrod, practically swallowing them whole,” according to a bulletin from the Northeastern IPM (Integrated Pest Management) Center.
Black swallowwort is a perennial vine with lance-shaped leaves that is related to milkweed. It grows about 6 feet tall, wrapping around other plants and killing them. Its botanical name is Cynanchum Louiseae, and Cynanchum translates to dog strangler, which leads to its other common name, the dog-strangling vine.
According to a bulletin from the University of Maine Cooperative Extension, “Flowers are purplish-brownish, approximately one-quarter-inch wide, five-petalled, and fragrant. They appear in June and may be found until late summer, particularly on plants growing in the shade. Fruits are long slender green pods (two to three inches), that turn dark brown when ripe. They appear in pairs or sometimes threes, similar to milkweed pods, but longer and narrower.”
A group of people from around Ogunquit are fighting the weed along Marginal Way — and they are looking for help.
“When we got a house abutting the walkway, I started paying more attention, partly out of survival for the garden that I put in,” said Joan Griswold, one of the leaders of the Weed Warriors who go out every Monday in the summer pulling out swallowwort — as well as bittersweet and other invasives — along the trail. “I remember that in the fall it looked like snow on the garden, and I couldn’t figure out what it was. It turned out those were all the little seeds of black swallowwort.”
The Weed Warriors don’t begin pulling along Marginal Way until July, Griswold said.
“If you try to pull it too early, instead of growing 6 to 8 feet high and producing pods, it will make pods 6 to 8 inches off the ground,” she explained. “Like most invasive plants, it uses all its vitality to produce seeds, and they are harder to get down low.”
If the swallowwort is just arriving in a garden and not already prevalent, people should pull it out immediately because they are more likely to get all of the roots at that time.
The Weed Warriors’ big event is Pod Day, scheduled this year on Aug. 10. People are urged to come out in force and pluck off every green swallowwort seed pod, so the plants won’t send the seeds flying to other places that are not yet hit.
Griswold realizes that the effort to remove the swallowwort is probably too late for Marginal Way.
“It’s almost like we should have started all of this stuff 15 years ago,” Griswold said. “It’s almost too late in a lot of cases. I had a professional come and take a look, and he said we are so far behind, the swallowwort is so entrenched, that it just perpetuates itself.”
Black Swallowwort would be bad enough if it just killed all of the native and ornamental plants near where it grows. But it also kills Monarch butterflies.
Monarch butterflies lay their eggs on milkweed, and have to eat milkweed to survive. Because swallowwort and native milkweed are so similar, butterflies will lay their eggs on swallowwort. In an experiment conducted by a student at the University of Rhode Island, all of the eggs laid on swallowwort died.
“They stopped eating after a single bite,” said Dr. Richard Casagrande, the professor overseeing the research.
Mechanically pulling the swallowwort will set it back, but if one node of the root system remains underground, a new plant will sprout. And pulling the root system often disturbs the roots of the plants you want to keep.
Chemical weed killers such as Roundup will kill swallowwort, but it will have to be used several times. Ogunquit has banned the use of synthetic weed killers on town property, so Roundup could not be used on Marginal Way. And Roundup is especially dangerous if it gets into bodies of water, so it would be dangerous if used on swallowwort in the coastal areas where it is most common.
There is hope in controlling swallowwort, but it is a few years off.
The weed is not a problem in Europe, where it originates. That is because Europe has some insects that will eat swallowwort, insects that North America lacks.
Casagrande led some research into such insects, and found two moths that will eat swallowwort and leave other plants alone.
According to an article in the newsletter of URI’s College of the Environment and Life Sciences, the two moths “passed the acid test” and test applications of the moths in two Rhode Island communities.
These test applications take time, but similar biocontrols have been used in Maine, including efforts to control winter moth, which just arrived in Maine last year, and lily leaf beetle, which have been going on for about eight years.
Until then, keep your fingers crossed, pull the vines when you see them and, most importantly, pull those green pods before they send out all of those floating white seeds.
Tom Atwell has been writing the Maine Gardener column since 2004. He is a freelance writer gardening in Cape Elizabeth and can be contacted at 767-2297 or at: