Japanese knotweed leaves and flowers

Japanese knotweed (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.

Overview

Regulated areas of Japanese knotweed
Japanese knotweed is Restricted (Orange counties)

Other names for this plant include:

  • Common names: Japanese bamboo, Mexican bamboo, fleece flower
  • Scientific names: Fallopia japonica; Polygonum reynoutria; Reynoutria japonica

Ecological threat:

  • New infestations of Japanese knotweed often occur when soil contaminated with rhizomes is transported or when rhizomes are washed downstream during flooding.
  • Poses a significant threat to riparian areas where it prevents streamside tree regeneration, and increases soil erosion.
  • Root fragments as small as a couple inches can resprout, producing new infestations.
  • Disrupts nutrient cycling in forested riparian areas.
  • Plants contain allelopathic compounds (chemicals toxic to surronding vegetation).

Classification in Wisconsin: Restricted

Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for Japanese knotweed was based upon this literature review developed by the department.

Identification

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” wide; 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, black produced by female plants; rare since colonies seldom have both male and female plants. The seed 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 similar 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.

Distribution

Known county distribution of Japanese knotweed
Counties in WI where Japanese knotweed has been reported (as of July 2011). Both vouchered and unvouchered reports included.

Do you have Japanese knotweed in your county but it isn't shaded on the map? Send us a report.

Control

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 repeated and persistent cutting of the plants. Properly dispose of plant debris; fragments as small as a couple 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 [exit DNR] by University of Wisconsin-Extension.

Photos

View Japanese knotweed pictures in our photo gallery!

Resources

Sources for content:

  • Czarapata, Elizabeth; Invasive Plants of the Upper Midwest: an illustrated guide to their identification and control. 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

Links for More Information

Last revised: Monday September 23 2013