General Description

This Class B noxious weed is widely established in western Washington. Control is strongly recommended, but not required, in King County.

Giant knotweed is the largest of three closely related invasive knotweeds that are found in this area and are listed as noxious weeds. They all share habitat and can occasionally be found growing together. In North America, these imported knotweeds are 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.

Giant Knotweed Photos

Identification

Giant knotweed is the biggest of the three invasive knotweeds, with stems usually between 6 and 16 feet, but reaching as much as 17 feet tall is some areas. The stems are smooth, hollow and light green, resembling the canes of bamboo, and sparingly branched. The leaves are 6 to 16" long, with a deeply heart-shaped base and a blunt leaf tip. Diagnostic hairs on the leaf underside are long, thin and wavy (hairs are sparse and sometimes fall off late in the season, best seen with a hand lens June through mid-September).

The flowers are small, creamy white to greenish white, and grow in short, branched clusters from leaf axils near the ends of the stems. Flower clusters are generally shorter than the subtending leaf, unlike the longer flower clusters found on Japanese 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.

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 giant 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.