This story was originally published in May 2018.
Just as your mom might have told you, beauty is only skin deep.
That’s the case with a trio of invasive plants that have been identified as problematic in Maine, according to a couple of plant experts here. Yellow iris, ornamental jewelweed and black swallow-wort may be pretty, but they can smother the state’s native flora and provide less-nutritious food for native fauna, according to Tori Jackson, a University of Maine Cooperative Extension associate professor.
“This beauty comes at a steep price,” she wrote in the May edition of Maine Home Garden News, a monthly newsletter published by the University of Maine Cooperative Extension.
She’s not kidding. The three plants were singled out by the Maine Natural Areas Program as priority species in 2018, with the hope that more awareness of them by Mainers will lead to early detection and eradication.
“They’re new enough that a lot of gardeners may not have seen them yet,” Jackson said. “All home gardeners and homeowners and people who care for landscapes are really the first line of defense when it comes to invasive species. Keeping an eye out will be a great public service that home gardeners can do.”
Of the three, perhaps yellow iris is the showiest and most beautiful. It’s the only completely yellow large iris in North America, and was intentionally brought here from its native habitats in Europe, North Africa and Asia as a horticultural plant, according to Nancy Olmstead, invasive plant biologist for the Maine Natural Areas Program. It’s also very invasive, and can spread rapidly, she said. She has spotted it along the coast as far north as Mount Desert Island and in isolated inland locations.
“If the seeds can fall in a waterway or be washed in a waterway with runoff, they can be transported downstream,” Olmstead said, adding that a colleague first noticed abundant populations of the iris along the Spurwink River in Cumberland County. “He also found it not just in the salt marsh areas along the river but also forested wetlands. He documented a doubling of the plants in a year. It can be a very aggressive plant.”
Jewelweed, swallow-wort can take over
Then there’s ornamental jewelweed, which also is known as Himalayan balsam, and is native to India and the western Himalaya. The annual with large pink or purple flowers looks a lot like Maine’s native jewelweed, which has orange flowers, but the pink one “will take over a landscape,” Jackson said. It also may attract bee pollinators away from native plants. Olmstead said that it has started to settle in to pockets of Maine, including the Rockland area.
“The pink jewelweed is very abundant locally in that area,” she said. “Folks there might be surprised to hear that plant is a new threat. But really you don’t find that plant downeast. It’s a great opportunity for folks to become aware and take quick action against it.”
The third plant on her list is black swallow-wort, a member of the milkweed family that is native to southwestern Europe. It’s a perennial vine with small black flowers.
“It is such a pretty plant,” Jackson said. “And so few plants have naturally black flowers.”
But its good looks definitely belie its more negative qualities. Even though it is in the milkweed family and has a seed that looks similar to the common milkweed, it is toxic to monarch butterflies. It also can choke out native plants.
“It has a very smothering growth habit,” Olmstead said. “It climbs other vegetation. It can really block light and reduce the vigor of the other vegetation. It’s a very aggressive vine that is particularly harmful in an open sun environment.”
So far, black swallow-wort is mostly limited to Maine’s southern coast. The Casco Bay area has been hit by it “very badly,” Olmstead said, and it has spread east to areas around Damariscotta and Newcastle. But it is on the move.
“Unfortunately, it’s making its way inland,” she said. “It’s been located in Augusta, and people found it in the Sebago Lake region last summer. Because the seed is so small, it’s easy to transport.”
What to do if you find them
If people notice these plants in Maine, there are different strategies to take, the experts said. If they find them on municipal, state or federal property, they are encouraged to take photos and make a public report on the iMapInvasives website.
“Public reports are super helpful for us to understand where plants are distributed in the United States,” Olmstead said.
If the plants are on their own property, people can take a more aggressive approach. According to Jackson, the best thing to do as soon as a homeowner notices a plant they’ve never seen before is to get it identified. If they can’t do so on their own, they can head to their local University of Maine Cooperative Extension office to consult with the horticulturists there. Then, as soon as the plants have been positively identified as unwanted invasives, people can don their gardening gloves and get to work.
“For the most part, we’re talking about hand pulling and really trying to get out as much of the root system as possible,” Jackson said. “We recommend that people put them in black plastic bags, which will actually kill the plants. We don’t want these things to end up in compost piles, which will never get hot enough to kill the seeds. And then take them to the transfer station.”
According to Olmstead, home gardeners generally need to be both vigilant and tenacious to combat these plants. Jewelweed can be managed by pulling it before it goes to seed. But the other two are different. The yellow iris is a wetland species and is a long-lived plant, she said.
“Repeated cutting can eventually reduce the vigor of the plant. You have to be persistent. It can have quite a vigorous root mass,” she said.
Black swallow-wort also requires some work. A recent study showed that it needs to be cut four times per year in order to prevent flowering and seeds, Olmstead said. And similar to Japanese knotweed, it has a lot of underground root reserves that need to be exhausted.
“A lot of diligence and manual labor is required to treat that plant,” she said.
But the work is worth it, according to the experts.
“Early detection is so important,” Olmstead said. “Just like in cancer, or like in any kind of weed. An ounce of prevention is worth a pound of cure.”
