Japanese knotweed: a plant with strange super powers

Mar 1, 2012

The photo above shows a knotweed stand getting out of control in the Upper Peninsula/Photo by Vern Stephens.

Vern Stephens and Sue Tangora work for the Michigan Department of Natural Resources. They happen to be married to each other. And they have a common enemy.

“This is on my radar of being a 10 on a scale of one out of 10.”

That thing they hate? It’s a plant. An invasive plant called Japanese knotweed. It’s sometimes also called Mexican bamboo. I met up with Vern and Sue at a busy intersection in East Lansing... on a corner lot where Japanese knotweed is going hog wild.

“It looks like bamboo. It gets up to 10-12 feet tall. It’s like being in a jungle, the canopy is above your head, generally in a lot of the sites, you can’t touch the canopy it’s that high above you.”

Maybe you’re thinking... so what? It’s a plant. In fact, it’s been a popular landscape plant in Michigan for years. People like it because it grows fast, so you can use it as a privacy screen to keep out nosy neighbors.

But this plant is crafty. It’s native to Japan, where it’s one of the first plants that comes up after a volcanic eruption. So it can actually push through volcanic rock. The problem with that is... it can also break through the foundation of your home.

“We know in England, Japanese knotweed has been known to be a problem there and it’s to the point where people have trouble getting insurance for homes, some of their insurance rates are really inflated. You see pictures of it growing up a wall inside someone’s home.”

(One couple in the UK had to demolish their home after a knotweed invasion - you can read that article here)

And actually – the knotweed on this corner lot is already breaking through the sidewalk.

Once the plant has gotten a toehold... it’s very hard to get rid of. Its roots can grow at least 9 feet into the earth. It releases a toxin into the soil so nothing else can grow near it. Vern Stephens says you can’t mow it or cut it... because that’ll just make it grow faster. And what’s worse... those little clippings can start a new population in the growing season.

Vern says the only way to treat it right now is with very specific herbicides. He says if you use the wrong herbicide... you could make knotweed grow even faster. It can take years to get rid of it.

In 2005, the Michigan Legislature made Japanese knotweed a prohibited species. Sue Tangora says you’re not required to treat it if you already have it... but you can’t share it.

“It’s illegal to sell at nurseries or farmers markets or anywhere. People will dig it up and share it with their friends or neighbors. Unless you’re really aware of all the laws in Michigan you may not be aware that’s actually illegal to do.”

The DNR is aware of garden clubs in the state that are still exchanging hybrids of Japanese knotweed... and even the hybrids are illegal to move off site.

It’s even illegal to move soil that has the roots of Japanese knotweed in it... because that could spread the plant. And that has caused problems for the guy who owns this corner lot. Vern Stephens says the landowner wanted to develop the site... but now, he can’t build here until the Japanese knotweed is gone.

“He won’t be able to do anything on this until it’s cleaned up, I’m guessing three years minimum.”

The landowner didn’t want to be recorded for this story.

The DNR’s still trying to figure out how widespread Japanese knotweed is... but it’s been found throughout the state, especially in cities... such as Flint, Detroit, and Petoskey.

Al Hansen is the director of parks and recreation for the City of Petoskey. He says they’ve been struggling with knotweed in city parks for about three years. He says they’re treating knotweed on city land. But they’re having trouble getting some private landowners to see knotweed as a problem.

“They don’t realize the consequences when it escapes the landscape beds themselves and that’s the difficult part, because they were able to buy it at one time, and therefore they don’t view it as being an invasive.”

And that’s maybe the biggest problem for officials. It can be hard to get people fired up about plants... especially when it’s something that everyone thought was good.

The DNR recommends certain treatment options for Japanese knotweed.