Summer is fading, and water enthusiasts are slowing their trips to the lakes and rivers. Unfortunately, invasive species do not always slow their trek when the weather becomes cooler.

One particular species, Japanese knotweed, is one to be on the watch for during the next couple of weeks along shorelines, ditches, and lawns.

Japanese knotweed, native in eastern Asia, was once a common ornamental sought for its “green screen” and lilac-like flowers. Now it presents a tremendous threat to Wisconsin’s waterways and is listed as one of the world’s 100 worst invasive species.

Japanese knotweed is recognizable by its hollow bamboo-like stem. This time of the year, Japanese knotweed is fairly easy to identify. Besides the bamboo-like stem, the leaves are alternate, egg shaped, dark green and 4 to 8 inches long. Numerous small white flowers will be branching off of the stem until late September.

Winter is also a good time of the year to identify Japanese Knotweed, as the stems are rust-colored.

Being a very frost-susceptible species, it causes this plant to die back, leaving only reddish hollow bamboo-like canes above the ground throughout the winter. Unfortunately, the cold winter weather does not kill the plant.

With the arrival of spring, Japanese Knotweed begins to grow back from its enormous root system and takes full advantage of the new growing season. During peak growing season, it can grow 2 to 4 inches per day, reaching a maximum height of 10 to 20 feet in one growing season.

Its root system can grow 9 feet below ground and expand 60 feet wide. This leads to a decline in native plant growth, displacing wildlife habitat and food. Homeowners can experience problems, as the invasive root system and strong growth can push up through and damage concrete foundations, buildings, roadsides, and retaining walls.

Control of Japanese knotweed is very difficult once established. It is important to do something immediately if a small population is forming on your property. Even small patches of it rarely take less than a year to eradicate, even with multiple attempts.

There are numerous control methods to implement on established sites, including manual or mechanical and herbicide application. Herbicide use in Wisconsin always requires a permit from a regional Wisconsin Department of Natural Resources aquatic plant manager. Combining both methods allows more options and flexibility.

Additional info

If you would like more information or would like to report existing infestations of Japanese Knotweed, contact Lisa Burns at 715.468.4654 or email pictures to for confirmation.

(Copyright © 2019 APG Media)

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