(Fallopia japonica or Polygonum cuspidatum)
Japanese knotweed is an herbaceous perennial that forms large colonies of erect, arching stems (resembling bamboo). Stems are round, smooth and hollow with reddish-brown blotches. Plants reach up to 10’ and the dead stalks remain standing through the winter.
Other names for this plant include:
- Common names: Japanese bamboo, Mexican bamboo, fleece flower
- Scientific names: Polygonum reynoutria; Reynoutria japonica
Classification in Wisconsin: Restricted
- Ecological Threat
- New infestations of Japanese knotweed often occur when soil contaminated with rhizomes is transported or when rhizomes are washed downstream during flooding.
- It poses a significant threat to riparian areas where it prevents streamside tree regeneration and increases soil erosion.
- Root fragments as small as a couple of inches can resprout, producing new infestations.
- Disrupts nutrient cycling in forested riparian areas.
- Plants contain allelopathic compounds (chemicals toxic to surrounding vegetation).
Leaves: Simple, alternate, 3-4” wide and 4-6” long. Leaves are spade-shaped and more heart-shaped on young shoots. They have long petioles that are broad at the base and narrow to a fine point. The upper surface is dark green while the lower surface is pale green.
Flowers: Creamy white or greenish; tiny 0.125” broad; borne in plume-like clusters in upper leaf axils near the end of stems. Bloom August through September.
Fruits & seeds: Seeds are small, triangular, shiny, and black and produced by female plants; rare since colonies seldom have male and female plants. The source is enclosed in a winged calyx that contributes to its buoyancy. The seeds have no dormancy requirement and germinate readily.
Roots: Roots are white and present along the rhizome. Plants can also produce adventitious roots on lower stems. Roots extend deeply into the soil creating a dense impenetrable mat.
Similar species: It has hollow stems with distinct raised nodes that give it the appearance of bamboo, though it is not related. Japanese knotweed is identical in appearance to giant knotweed (Polygonum sachalinense; invasive); they are known to hybridize. The best way to tell them apart is by their leaf bases; Japanese knotweed is squared off while giant knotweed is heart-shaped.
See the reported locations of Japanese knotweed in Wisconsin.
Do you know of other populations? Please send us a report.
Mechanical: Hand-pull young plants; dig or till when soil is soft. Plants should be pulled up by the root crown, trying to remove as much of the rhizomes as possible because any rhizomes remaining in the soil will produce new plants at each node. It is possible to eradicate small patches of knotweed with the repeated and persistent cutting of the plants. Properly dispose of plant debris; fragments as small as a couple of inches can resprout, producing new infestations.
Chemical: Plants are more susceptible to herbicides if they are cut when 4-5’ tall and the regrowth treated around 3’ tall. Foliar application of glyphosate with a surfactant, triclopyr formulated for use with water, dicamba, or imazapyr may be effective on large populations. Tests involving large-bore needle injection of glyphosate into the lower nodes of each stem have been successful.
For more information on control techniques, visit the Japanese knotweed factsheet by the University of Wisconsin-Extension.
- Sources for content:
- Czarapata, Elizabeth; Invasive Plants of the Upper Midwest: an illustrated guide to their identification and control. The University of Wisconsin Press. 2005. Pg. 73-75
- Stone, Katharine R. 2010. Polygonum sachalinense, P. cuspidatum, P. × bohemicum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [2013, September 23].
- Invasive Plant Atlas of New England
- University of Wisconsin Extension - Weed Science
- King County Noxious Weeds