Aggressive weed challenges MN gardeners: Japanese knotweed

Mary Hockenberry Meyer, Extension Horticulturist
Gail Hudson, Extension Communication Specialist

Japanese knotweed plants produce female flowers and need a pollen source.
However, the plant spreads primarily through rhizomes (underground stems).
Photo: Mn Dept of Agriculture

Is it the buckthorn of weeds? Beware, this weed is so aggressive, it can even damage pavement! 

This large, fast-growing, shrub-like plant is commonly called Japanese knotweed or Mexican bamboo (Polygonum cuspidatum), which can grow from 3 to 9 feet tall with leaves that are six inches long and four inches across.

It is a tough weed to control thanks to its large system of fleshy, underground rhizomes (stems) as big or bigger than your finger, which can extend up to 5 feet from the plant.  Above ground, the hollow, bamboo-like stems can become tough and woody with age.

Learn how to control or reduce its presence by clicking on "Read more"...

Impact of Japanese knotweed

Japanese knotweed forms tall, dense thickets that shade out and displace native vegetation. It takes over residences, community recreation areas and stream banks, degrades habitat for fish and wildlife, can alter waterways, and facilitate erosion and flooding, and to top it off--become a fire danger. Knotweed growth through pavement cracks and along paved surface edges can even result in damaged pavement.

Persistence is key to get ridding of this weed, which the Minnesota Department of Agriculture includes on its specially regulated list of weeds. 

Effective treatment to get rid of this non-native, invasive plant
requires a multi-pronged approach that will need more to be done
more than once. Photo: Mn Dept of Agriculture

How to remove it

For the best chance to eradicate Japanese knotweed, use both of the following methods: 

Mechanical control:  Try to repeatedly cut it or mow it down once a week. You can also try covering up the plant site with black plastic or mulch.  But it can grow through these. 

Dig out as much of this rhizome system as possible. When new shoots reappear, dig those up, too.

Chemical control: You may also try chemical control with glyphosate (Monsanto Roundup) directly on their leaves. But in order for it to work, you’ll need 6-12” of healthy green tissue to be taken up by the knotweed. More than one application may be needed along with repeated cutting to kill this plant.

The bad news...

It can take two or three years of repeatedly using the measures above to ultimately get rid of this very, very persistent plant. If it has grown in and amongst rocks, it can be very difficult to remove.

For more information, see this article on the University of Minnesota Extension site. You can also learn about Japanese knotweed at the Minnesota Department of Agriculture.