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Japanese knotweed, a major noxious weed

Bohemian knotweed is the most common variety in Minnesota.

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

Listed as one of the world’s worst weeds, gardeners in Minnesota should be well aware of this plant. Originally planted for showy flowers and the ability to grow in poor sites, it has been growing in Minnesota for many years, but its invasiveness is increasing. 

It is a tough weed to control due to its large system of fleshy, underground rhizomes (stems), as big as 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.

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 cracks and along paved surface edges can result in damaged pavement and sidewalks.

There are three different species of knotweed found in Minnesota and all are on the Minnesota Department of Agriculture Prohibited Noxious Weeds Control List, which states, “Species on this list must be controlled, meaning efforts must be made to prevent the spread, maturation and dispersal of any propagating parts, thereby reducing established populations and preventing reproduction and spread.”

The Minnesota Department of Natural Resources has recommendations for controlling knotweeds: 

  • For small infestations dig or pull the plant by hand or with shovels. 
  • Mowing may spread plants to new areas and is not recommended. 
  • Do not allow plant parts to enter waterways during control efforts. 
  • Do not spread knotweeds to new locations. 
  • Contact your local county agricultural inspector for disposal options. 
  • Plants can be kept on site for burning or piled and covered with a tarp for decay. Be sure to monitor the site and remove any plants that sprout from the burn or decay site. 
  • Contact your local yard waste or compost site to see if they accept noxious weeds. 
  • You can only transport noxious weeds to a disposal site and the MDA requires the load be protected to prevent the spread of noxious weeds during transport. 
  • It is illegal in Minnesota to dispose of plants in a landfill. 

See the Minnesota Department of Agriculture noxious weed disposal website for additional information.

Because Japanese knotweed can grow from stem fragments and from small pieces of the rhizome, you need to be very careful if you try to dig out the rhizomes to remove the plants. Killing it on site is the best way to proceed. This will reduce the spread of the plants, their seed or any pieces that may fall and contaminate a new area when transporting the plant.

Repeatedly removing the tops of the plants can weaken the plant. If possible you can cover or tarp the area after removing the tops to further weaken the plants by eliminating the light source. The plants will still grow under the tarp and you will need to cut them off and keep the tarp on for several months or a full growing season. 

When using a pesticide like glyphosate (Roundup) directly on the leaves, you’ll need 6-12 inches 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.

It can take two or three years of repeatedly using the measures above to ultimately get rid of this very, very persistent plant. 

See our article on Japanese knotweed for more information.

Mary H. Meyer, Extension horticulturist

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