Flowered invasive plant in Michigan a real home-wrecker

A pretty, invasive plant that flowers in late summer and early fall is spreading throughout Michigan. And it's so prolific and tough, it can grow through sidewalks, driveways and building foundations.

Japanese knotweed, native to East Asia, has become such a pervasive invader in Great Britain that those with it on their property can have trouble getting a mortgage or home insurance. It's not to that point in Michigan — and concerned ecologists want to keep it that way.

The law prohibits bringing the plant into the state or moving it around within Michigan.

The shrub-like plant features a hollow, bamboo-like stem and broad leaves in a zigzag pattern up the shoot. It grows up to 10 feet in height. In Michigan, the plant blooms small, creamy white flowers in August and September. Its root network and rhizomes — a stem that grows horizontally under the ground — can grow up to 65 feet away from the weed, shooting up additional plants along its path.

"When you cut it down and try to kill it this year, you haven't killed it; there's the rest underground and it will be coming up again," said Susan Julian, a land specialist with the North Oakland Headwaters Land Conservancy.

Japanese knotweed grows so fast and thick it can crowd out native plant species and destroy natural animal habitat. A YouTube time-lapse video from a British resident shows the plant overwhelming an area of his property in only a few weeks.

"We've seen places the size of a football field, the size of a tennis court, where nothing else grows besides Japanese knotweed. So, it's wreaking havoc on the environment," said Sue Tangora, who leads the Michigan Department of Natural Resources — Forest Resources Division's invasive species efforts.

But Japanese knotweed isn't just prolific. It can literally be a home-wrecker.

"It's very tenacious, very hard to kill," Tangora said.  "It's one of the first things to grow up through volcanic rock. It can grow through your foundation, your sidewalks. I've had calls from people who had their driveway done two years ago, they had soil brought in and, boom, Japanese knotweed grew up through their driveway."

Japanese knotweed first came to North America and Europe as an ornamental plant, valued for its ability to grow quickly into a hedge, its attractive flowers and their ability to attract pollinators such as bees and butterflies, and for the plant's perceived medicinal purposes, Tangora said. Its growth is controlled by competing plants in Asia, but not as well in Europe and North America.

Now, "the horse is out of the barn" for the plant's spread as an invasive species, she said.

"We're finding it in hundreds of sites throughout Michigan," she said. "We're seeing this plant mostly where you have humans and dwellings. It's not really much of a problem in forests. You're going to see a lot of it in southeast Michigan, quite a bit of it in Detroit. Once you get an eye for it, you'll start seeing it everywhere."

There's no better time than the present to fight Japanese knotweed, Tangora said.

"The most susceptible time for this plant is right now, in the fall, before it goes dormant," she said.

Treatment historically has involved cutting down the plant to about 2 to 3 inches above ground and treating the cut stalk with a non-restrictive, glyphosate concentrated herbicide. But Tangora recommended against cutting the plants before the herbicide treatment. "You want as much leaf surface available for that herbicide to translocate to the roots," she said, adding that if cut plant material is improperly disposed, it can spread   the knotweed elsewhere.

And it's not a one-off job, Tangora said. "If you start to treat it, you're up to five years treating it," she said.

For information on treatment options,  contact your local cooperative invasive species management area, listed at www.michiganinvasives.org/managementareas.

Responding to invasive species before they are fully established provides the best hope toward controlling them, Julian noted. Once established — like zebra and quagga mussels in the Great Lakes, or Japanese knotweed in Great Britain — they become much more difficult, and costly, to eradicate, she said.

"We may be able to get the best of this invader," she said. "Many times, we've noticed them too late."

(2016 © Detroit Free Press)


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