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If you’ve got Japanese knotweed in your yard, now is the time to spray

In preparing to sell her house, Mary Austin wanted to weed out any unsightly plants in her yard. When she came across Japanese knotweed, instead of discarding it to the side, she bagged up the weed and brought it to a landfill. "I drove around to...

Deborah Moller has several patches of invasive Japanese knotweed on her Duluth property, including this large clump on a property corner. Steve Kuchera / skuchera@duluthnews.com
Deborah Moller has several patches of invasive Japanese knotweed on her Duluth property, including this large clump on a property corner. Steve Kuchera / skuchera@duluthnews.com

In preparing to sell her house, Mary Austin wanted to weed out any unsightly plants in her yard. When she came across Japanese knotweed, instead of discarding it to the side, she bagged up the weed and brought it to a landfill.

"I drove around to the place to dump it out and the worker looked at me and said 'Oh, knotweed huh? I'm quarantining you,' " said Austin.

Because of how easily the plant can spread, the Western Lake Superior Sanitary District takes the disposal of the weed very seriously.

"I knew ahead of time it was invasive and awful and I couldn't throw it on my own compost pile," said Austin. "I thought, 'Man, if the city is that worried about it, it must be bad.' "

Tackling the invasive plant has been a learning process of evolving remedies. After enough trial and error, the city of Duluth narrowed in on a solution to snuffing out Japanese knotweed: use an herbicide and spray it in the fall.

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"I am not a herbicide person," said Diane Desotelle, the natural resources coordinator with the city. "But this is one that you need to hit in the fall. That's the right time to hit it with a herbicide."

Knotweed can't be eradicated by digging up its roots or breaking the stems. Because of an extensive root system, the state and city rejects using traditional weeding methods for getting rid of the plant.

"The stems and roots can resprout plants if they are moved to new locations," said Mari Hardel, with the Department of Agriculture. "So, if somebody digs up knotweed and lays it on the ground in a different spot, there will likely be a new plant in that location."

Desotelle and Hardel both recommend spraying knotweed leaves with foliar sprays. The herbicide gets absorbed by the plant, and is brought down into the root system, where it kills the majority of the plant.

That's why fall is the best time to spray the herbicide. In the spring, when the plant is growing, it's pushing nutrients up through its roots. As area temperatures cool, the plant starts to hibernate underground, making it the perfect time to strike.

A first-year spray won't kill the plant. But Steve Schoenbauer, the coordinator for the city's Duluth Invaders program, said picking the right time and spray can reduce the plant by 80 to 90 percent in the first year.

"If you do these steps in the fall, next year you're going to see good progress," said Desotelle. "But to be on top of it, it could take five or six years."

What used to be primarily a plant one would find along a creek has become a terrestrial invasion across the city. While city officials don't know why the explosion happened, Schoenbauer believes past flood events may be partly to blame.

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"A lot of people planted knotweed along creek sides to keep soil erosion away," said Schoenbauer. "Then in the 2012 flood, all that stuff got washed downstream. Virtually every creek in Duluth has knotweed growing alongside it."

The Department of Agriculture reports the city of Duluth contains one of the most abundant source of knotweed in the state. Even with the recent explosion, the plant's prevalence in the area is nothing new. Some residents have been dealing with it for decades.

"It's just ridiculous," said Duluthian Deborah Moller. "It grows faster than grass. I honestly feel like I have an imported plant farm."

While the plant had already seeded itself when Moller moved into her house in 1998, it has expanded in recent years. Moller said she's tried several methods, from mowing over the plant to putting blackberry poison near the base after it's been cut. So far, nothing has worked.

"It's a tricky plant," she said. "And it's a catch-22, because it's just so pretty."

And she's not the only who thinks that. Japanese knotweed is not an ugly weed. Its leaves are vibrant green ovals that sprout white flowers in the fall. Its ornate aesthetic makes it an attractive plant to add to a garden. It was originally brought stateside to beautify yards and stabilize creek walls.

But the invasive plant has a dark side. Japanese knotweed is a prolific grower, with expansive roots reaching 10 feet down and 60 feet outward. It has no issue repopulating and its stalks can grow a foot and a half a week. This takes up valuable real estate where native plants would thrive.

"Personally I think it's one of the worst," said Schoenbauer. "The roots will grow through anything. It came out of lava fields in Japan originally so it grows off of almost pure rock and pokes holes in stuff."

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Japanese knotweed can also cause extensive property damage to people's homes.

Schoenbauer said its root system will grow through cement and pavement, cracking property walls and basement foundations.

If homeowners do see the plant on their property and they do cut it back, officials recommend leaving the remnants on the ground, where they deteriorate quickly.

In an effort to combat the weed where it's found on public land, the city runs an initiative called Duluth Invaders, a volunteer program that adopts a patch or area where an invasive plant is found. Efforts like these have are being used to push back against invasive plants before they get worse.

"I don't want to create panic for people," said Cheryl Skafte, the Parks and Recreation provisional assistant manager. "There are examples of other communities where you can't get a mortgage on your house if it has knotweed. We're not there yet. And I don't want people to perceive it like that. We want them to be hopeful and responsible for taking the steps to deal with it."

Tips for getting rid of Knotweed

• Spray the plant in the fall, when it starts to sprout white flowers.

• Use specific foliar sprays like: Polaris (imazapyr), Vanquish (dicamba) or Milestone (aminopyralid).

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• You can also hire a licensed contractor to apply the herbicide.

• If the plant is too tall to spray correctly, break the stems and lay them on the ground. Do not move them around and don't dig up the roots, as this is how the plant can reseed.

The shrub-like Japanese knotweed has greenish-white flowers growing in branched clusters; alternate, broadly oval and pointed leaves; and reddish-brown stems. Steve Kuchera / skuchera@duluthnews.com
The shrub-like Japanese knotweed has greenish-white flowers growing in branched clusters; alternate, broadly oval and pointed leaves; and reddish-brown stems. Steve Kuchera / skuchera@duluthnews.com

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