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Once considered ornamental, knotweed is one of Maine's most damaging invasive plants

State ecologist Justin Schlawin stands in a colony of knotweed in a forest along the Kennebec River near Augusta.
Patty Wight
/
Maine Public
State ecologist Justin Schlawin stands in a colony of knotweed in a forest along the Kennebec River near Augusta.

At Butternut Park in the small central Maine town of Chelsea, you don't have to walk far to feel like you're in a jungle.

State ecologist Justin Schlawin follows a park trail that runs along the Kennebec River into what initially is a typical floodplain forest. He points out maple and ash trees overhead and admires the diversity of plant species in the understory below.

"Whether it's ferns or wood nettles, it tends to be really verdant," he says.

But before long, the trail ends and the forest gives way to one species that dominates and forms a thick cover that towers over Schlawin, blocking out sunlight.

"So we're standing in a shady, eight-foot-tall layer of Japanese knotweed. It's very dense. And so we sort of had to push our way through to get through this vegetation," Schlawin says.

Sometimes confused with bamboo, knotweed has a hollow stem and flat, shovel-shaped leaves as big as your palm. It's been in the U.S. for centuries — initially brought here as an ornamental species. But it's now considered one of the most invasive plants in the world and one of the most damaging in Maine.

Japanese knotweed can grow several inches a day, is strong enough to penetrate asphalt, and can regenerate from tiny fragments no longer than your thumbnail. Here, a single colony has swallowed half this forest — roughly four acres — transforming what should be a diverse landscape with dozens of species into a homogenous one.

"So we've entered the land of single species occupying this floodplain," says Schlawin, surrounded by knotweed.

A branch of knotweed, which includes a zig-zag shaped stem, can grow in dense stands up to 10 feet tall, and has leaves about six inches long and spikes of small white flowers.
Maine Natural Areas Program
/
Maine Department of Agriculture, Conservation and Forestry
A branch of knotweed, which includes a zig-zag shaped stem, can grow in dense stands up to 10 feet tall, and has leaves about six inches long and spikes of small white flowers.

That makes this area less suitable for wildlife, like the woodcock and northern thrush, as well as the rare wood turtle. It also weakens an important function of this floodplain ecosystem.

"Floodplains, they are also important for flood mitigation," says Schlawin. "They're storing a lot of groundwater, there's a lot of backwater sluice throughout floodplains that hold water during flood events."

Schlawin says knotweed poses a significant threat to all floodplains in Maine. Even just a tiny fragment can start a new colony. Part of his job is to prevent it from establishing. But for sites like this one, the colony is so big there's not much that can be done.

"It's not going to be possible to treat a stand like this," he says.

Getting rid of knotweed isn't always hopeless. It can be done in certain places with the right tools.

"Alright, this is a good patch down here," says Amanda Devine as she walks along a steep hillside near the shore at Woodward Preserve in Brunswick. She's a botanist with the Maine Coast Heritage Trust, and she's here to remove knotweed.

Amanda Devine with the Maine Coast Heritage Trust at Woodward Preserve in Brunswick, using a handheld injection tool to deliver herbicide directly into knotweed stems.
Patty Wight
/
Maine Public
Amanda Devine with the Maine Coast Heritage Trust at Woodward Preserve in Brunswick, using a handheld injection tool to deliver herbicide directly into knotweed stems.

Covered head-to-toe in a jumpsuit, rubber boots and gloves, she uses a handheld injection tool to pierce a long, thick needle through a knotweed stem. It delivers a few drops of herbicide directly into the plant.

"It's surprisingly woody," Devine says. "You definitely don't want to get your finger in between the target and this needle, because it's a heavy duty needle."

One by one, she injects herbicide into each knotweed stem thick enough for the needle.

"I think about it as chemotherapy. And if you're a cancer patient chemotherapy is awful, but it might save your life."

Devine is trying to save this coastal hillside from erosion. Getting rid of the knotweed will allow native plants to reclaim the area as groundcover and keep the soil in place.

"This is a good one! This is about an inch in diameter," she says as she kneels down at the base of the plant. "I'm going to give this one two squirts."

Because knotweed has an extensive root system, digging it out doesn't work. And while using herbicide may not seem palatable, Devine says it's an important tool. She applies it carefully and sparingly, and when possible, combines it with other methods such as smothering the plants. Persistence is the name of the game.

The cut stem of a knotweed plant, which has a hollow center.
Maine Natural Areas Program
/
Maine Department of Agriculture, Conservation and Forestry
The cut stem of a knotweed plant, which has a hollow center.

"And you know what, there may always be a little bit of knotweed here," she says. "But if we stay on top of it, if we don't let it get too big, and we manage it, we don't look for eradication, but we look for management and improvement, then, you know, that's a victory too."

A year ago, Devine says knotweed blanketed this hillside. But after an initial chemical treatment, she says it was reduced by 70%. And those kinds of results help to preserve Maine's diverse landscape.

Support for Deep Dive: Invasives is provided by Maine Audubon, Friends of Acadia and Coastal Maine Botanical Gardens.