Take on Invasives: Japanese Knotweed

This is the fifth in a monthly series on invasive plant species in Northern Michigan, their impact on natural landscapes and what can be done to help control and prevent their spread.

An invasive species is one which has been introduced into a habitat and has a negative impact on its environment. For most invasive plants, this means they crowd out other native species and form dense patches across wide areas, altering the natural landscape until control measures are implemented.

Japanese knotweed doubles down by impacting natural areas and manmade infrastructure. Its root systems can crack driveways, concrete sidewalks and building foundations all the while proving to be very difficult to eradicate or control. 

Japanese knotweed in full flower.

When mowed or weed whacked, knotweed only comes back stronger. Fragments that are scattered can re-root, furthering infestation. It is fast growing and has hollow stems that are similar in appearance to bamboo. Some plants can reach 15 feet in height and form an impenetrable wall.

“Knotweed is my favorite, least-favorite plant,” said Stewardship Team Lead Shelby Bauer, who leads invasive species treatment work at Huron Pines. “It is very good at surviving, and some of the craziest stories I’ve heard are about knotweed growing right into a house. In its native habitat, knotweed is probably super cool but here it’s just fighting you.”

Knotweed was brought to the United States more than a century ago as an ornamental plant. Its yellow-white spike flowers bloom in late summer and fall, making this time of year the best opportunity to identify it and report infestations.

An up-close view of the flower spikes of Japanese knotweed.

Treating knotweed can be done by carefully hand-trimming the shoots, burning the dried shoots on site and repeating when necessary. Composting is a bad idea, since fragments can root in the compost pile or wherever the compost is spread.

Cutting followed by direct application of herbicide is one of the most effective treatment methods but, even so, repeated treatments are often needed to eradicate a patch of knotweed. As such, it is a priority species for Huron Pines.

“Knotweed is a very tough plant to get rid of so it’s best to stay on top of it and report it,” Bauer said.

Reporting can be done through the Midwest Invasive Species Identification Network at misin.msu.edu, an online database Huron Pines uses to prioritize future treatment work.

The Huron Coastal Invasive Species Network is made up of conservation groups, landowners, state and federal agencies that seek to restore native habitat through invasive species education, prevention and management. The Huron Coastal ISN is led by Huron Pines with support from the Michigan Invasive Species Grant Program (Michigan.gov/Invasives).

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