Japanese Knotweed


About Knotweed

Japanese Knotweed is a large, perennial plant classified as an invasive species. Knotweed has hollow stems with distinctly raised joints and is sometimes mistaken for bamboo. The plants can grow up to 13 feet tall and leaves range from 2-5 inches long. Knotweed flowers in late summer and early fall. The small flowers are typically cream or white.

Knotweed plants are replacing native plants; covering guardrails, fire hydrants, roads signs; and are interfering with the stormwater flow in roadside ditches. 

DPW has developed an informational brochure on Knotweed, read more here.  A fact sheet specific to this plant can be found on the Department of Agriculture website: http://agriculture.nh.gov/publications-forms/documents/japanese-knotweed.pdf.


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How to Treat Knotweed


Herbicide application is the most effective means of killing knotweed. Spraying of knotweed should be completed at the end of summer/early fall when the plant is large and flowering. Additional treatments should be done approximately every 6 months (spring and fall). 


To kill knotweed by the covering method, start by cutting the plant at the base.  Let the leaves and stems dry out before disposing of them or the knotweed will spread. After cutting the plant, cover the areas with 7-mil thick plastic. Weigh the plastic down with mulch, rocks, soil, etc. The area should remain sealed and covered for 2-5 years. 


Digging or pulling out knotweed can work is small patches, however, it is important to remove as much of the root system as possible. 

Do Not Mow or Weed-whack Knotweed

Mowing, weed-whacking, cutting, and raking are not an effective means of removing knotweed and can actually cause the plant to spread.