Polygonum cuspidatum (Fallopia japonica)
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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.
Legal status in King County, Washington
Public and private landowners are not generally required to control infestations of Japanese 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. Japanese knotweed is a Class B Noxious Weed in Washington, first listed in 1995. 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) (external link) 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.
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) and may be mistakenly sold in Washington under that name although it is illegal. 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.
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.
Additional information on Japanese knotweed
Information on Bohemian 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.
What to do if you find this plant in King County, Washington
Because Japanese knotweed is so widespread, property owners in King County are not required to control it and we are not generally tracking infestations. We can provide advice on how to control Japanese knotweed, but there is generally no legal requirement to do so.
Japanese knotweed photos - click a thumbnail for a larger image
Photos from MissouriPlants.com (external link)