Another invasive plant threatens landscape

Published 9:33 am Wednesday, July 17, 2013

DULUTH — An invasive plant that resembles bamboo is threatening to overwhelm native species in northern Minnesota.

Japanese knotweed is a member of the buckwheat family but its cane-like stems — which can grow taller than 10 feet — make it look and feel like bamboo, so it’s often called that.

It has been found growing in backyards and wooded parks in Duluth. It was imported to the U.S. from Asia in the late 1800s as ornamental shrubbery, spreading from Maine to Minnesota and as far south as Louisiana. It has been reported in 41 states, according to the Minnesota Department of Agriculture.

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There are multiple, isolated infestations in Minnesota, according to the department’s website. An online map from the U.S. Department of Agriculture’s Natural Resources Conservation Service shows populations have been recorded in St. Louis and Lake counties in northeastern Minnesota, and in Ramsey, Washington and Sherburne counties in east-central Minnesota.

The Minnesota Department Natural Resources calls it a “significant threat,” especially near water because it can survive floods and spread before native species recover. The invader tolerates full shade, high temperatures, high salinity and drought.

“I was out along Keene Creek, and it’s just exploding out there up and down the banks,” Judy Gibbs, the city’s invasive species eradication coordinator, said. “The (June 2012) flood may have helped it along. . I think we’re about to get hammered by bamboo here in Duluth if we don’t jump on this.”

City officials are planning two hands-on workshops where participants can find out what it looks like, how to get rid of it and help in the eradication effort.

Japanese knotweed is just the latest in a long line of aquatic and terrestrial invaders from afar threatening native species, often because invaders aren’t susceptible to local diseases and other afflictions. Garlic mustard, spotted knapweed, purple loosestrife, buckthorn and others have been targeted for eradication with varying degrees of success.

Eradicating Japanese knotweed is tough. Fire doesn’t seem to help. If you cut off one stem, another grows faster from the root system. Pulling out the roots with tools can work but takes so much effort that it’s impractical for large patches.

“About the only thing that works is chemical treatment,” Gibbs said. “This stuff grows so fast; people say you can watch it grow.”

The same hardiness that made the plant useful in gardens is what has caused it to spread.

“It’s not just plants it displaces, but insects that need those plants,” Gibbs said. “Those kinds of (streamside) habitats are the only place you’ll find a beautiful little damselfly called the ebony jewelwing. It would be a shame if we lose that little guy because of bamboo.”