Japanese Knotweed
Polygonum cuspidatum (also known as Fallopia japonica)
(Buckwheat Family)

General Description

Japanese knotweed is often confused with its hybrid Bohemian knotweed and the closely related giant knotweed. It is a robust, bamboo-like perennial that spreads by long creeping rhizomes to form dense thickets. Originally imported as an ornamental screen or hedge plant, Japanese knotweed is native to Asia. In North America, this plant is not held in check by natural enemies and is capable of thriving and spreading in a wide range of conditions, especially riverbanks, roadsides and other moist, disturbed areas. Containment and control of all the invasive knotweeds is highly challenging but very important in order to protect uninfested areas from the damage caused by this group of plants.

A class B noxious weed on the WA State noxious weed list, control of Japanese knotweed is highly recommended but not required in King County. Although prohibited for sale in the State of Washington, this plant is sometimes sold in other states and on the internet under the name Fallopia japonica (the Latin name applied to this species in the Flora of North America and in Europe). Introducing this plant to landscapes in King County is highly discouraged due to the high risk of spread by root and stem, its impact on rivers and the difficulty of control once established. Non-invasive alternatives to Japanese knotweed can be found in the Garden Wise booklet available for download or by contacting our office.

Japanese Knotweed photos - click a thumbnail for a larger image

Photos from MissouriPlants.com (external link)

Identification

Stems are stout, cane-like, hollow between the nodes, somewhat reddish-brown, 5 to 8 feet tall, and profusely branched. The plants die back above ground at the end of the growing season. However, the dead reddish brown canes often persist throughout the winter. The stem nodes are swollen and surrounded by thin papery sheaths.

Leaves are thick and tough in texture, with short petioles, 2 to 7 inches long and about two-thirds as wide, spade-shaped with a truncate base and an abruptly narrowed leaf tip. An identifying character is the lack of hairs on the leaf undersides. Instead of hairs, there are low, bump-like structures (scabers) visible on the veins with a hand lens.

The flowers are small, creamy white to greenish white, and grow in showy plume-like, branched clusters from leaf axils near the ends of the stems. Flower clusters are generally longer than the subtending leaf, unlike the shorter flower clusters found on giant knotweed and the mid-size clusters found on the hybrid Bohemian knotweed. Leaf and flower characters are most reliable when looking near the middle of a branch. The fruit is 3-sided, black and shiny.

Japanese knotweed can tolerate partial shade and is most competitive in moist, rich soil. It is often shorter on dry exposed sites. There is also a compact form occasionally used in landscaping that is typically shorter. In King County, Japanese knotweed is commonly found along trails, roadsides and on rivers and streams as well as residential properties and associated greenspaces.

For More Information

See our invasive knotweed page for more information on this group of highly invasive, difficult to control species.

Read the Invasive Knotweed Weed Alert for general information on identification and control (1.27 MB Acrobat file) or for comprehensive biology and control information, download the Invasive Knotweed Best Management Practices (281 KB Acrobat file).

For more detailed information on biology and control of knotweed, download a slide show on knotweed biology and control (3.12 MB, Power Point) or see our text-based handout on the subject, Knotweed Biology and Control (44 KB Acrobat file).

Please see the written findings of the WA State Noxious Weed Control Board for more information on Japanese and Bohemian knotweed.

Information on knotweed identification and distribution is based in large part on the findings reported in PF Zika and A Jacobson's article "An Overlooked Hybrid Japanese Knotweed (Polygonum cuspidatum X sachalinense: Polygonaceae) in North America", published in Rhodora, Vol 105, No 922, pp. 143-152, 2003.