Japanese knotweed (Polygonum cuspidatum )

Invader Type: 


Mechanical Control: 

Cut stalks at least once per month throughout the growing season. Use a scythe, loppers or even a lawn mower, depending upon the ground surface you are working on. Repeat cuts for five years. Do not replant until the knotweed is under control and the plants are much smaller and have lost their vigor. Replant with good sized natives.

Check out this video to learn more about Japanese knotweed control

Chemical Control: 

For small infestations: Cut stalks of knotweed in late June. Cut again after August 1 and drip a 18-25% glyphosate herbicide* solution into the stems. An injector gun can also be used for application.

For larger infestations: Cut the plants back in June. In late summer, when other populations are flowering, use a low-volume foliar spray of 3-8% glyphosate. Spray only on nonwindy days and in patches that are absent of native species. Any time you are near water, use aquatic formulations. The following year, spot-treat remaining plants.



Pest Overview and Identification: 

"Japanese knotweed is an upright, shrublike, herbaceous perennial that can grow to over 10 feet in height. As with all members of this family, the base of the stem above each joint is surrounded by a membranous sheath. Stems of Japanese knotweed are smooth, stout and swollen at joints where the leaf meets the stem. Although leaf size may vary, they are normally about 6 inches long by 3 to 4 inches wide, broadly oval to somewhat triangular and pointed at the tip. The minute greenish-white flowers occur in attractive, branched sprays in summer and are followed soon after by small winged fruits. Seeds are triangular, shiny, and very small, about 1/10 inch long." (source: Plant Conservation Alliance Working Group http://www.nps.gov/plants/alien/fact/faja1.htm)

Check out this video to learn more about Japanese Knotweed

Reproductive Strategy / Lifecycle: 

Plants reproduce primarily by vegetative growth but have also been found to reproduce by seed.  Japanese knotweed produces a significant number of seeds (>100,000 seeds/stem if all flowers are pollinated and set seed) but very few seeds germinate and few seedlings survive. Rhizomes, or horizontal plant stems, produce shoots above ground and roots below that can reach 45-60 feet in length and 6 feet in depth.  Extensive rhizomes contribute to the difficulty in controlling Japanese knotweed manually. Until large reservoirs of energy in the root system are depleted, Japanese knotweed will continue to send up new shoots can produce new plants from the nodes.


Ecological Threat: 
  • While bees are attracted to knotweed flowers, the plant is untouched by most native insects. As knotweed populations replace native trees, shrubs, grasses and sedges, native insect populations are reduced. Insect populations are a primary food source for fish, birds and mammals.
  • River shores that are populated by native vegetation are less susceptible to erosion. A combination of native plants has a more complex root structure and can retain soil.