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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 are 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.
Legal status in King County, Washington
Public and private landowners are not generally required to control infestations of giant knotweed that occur on their property in King County, Washington, except in selected areas on the Green River and its tributaries and on the Cedar River and its tributaries, as described on the King County Weed List. Giant knotweed is a Class B Noxious Weed in Washington, first listed in 1999. It has not been designated for required control in the county by the Washington State Noxious Weed Control Board, but it has been selected for required control in limited parts of the county by the King County Noxious Weed Control Board. Because control is not generally required in the county, it is on the list of Non-Regulated Noxious Weeds for King County. For more information, see Noxious Weed Lists and Laws or visit the website of the Washington State Noxious Weed Control Board.
This species is on the Washington quarantine list (known as the prohibited plants list) and it is prohibited to transport, buy, sell, offer for sale, or to distribute plants or plant parts, seeds in packets, blends or "wildflower mixes" of this species, into or within the state of Washington. For more information, see Noxious Weed Lists and Laws.
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 itadori (Japanese) knotweed and the mid-size clusters found on hybrid knotweed. Leaf and flower characters are most reliable when looking near the middle of a branch. The fruit is 3-sided, black and shiny.
See our invasive knotweed page for more information on this group of highly invasive, difficult to control species.
Information on hybrid 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.
Program offices are located at 201 S. Jackson St., Suite 600, Seattle, WA 98104. To contact staff, see the Noxious Weed Control Program Directory, send an email, or call 206-477-WEED (206-477-9333).
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