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Itadori or Japanese knotweed is often confused with hybrid (Bohemian) knotweed and giant knotweed. It is a robust, bamboo-like perennial that spreads by long creeping rhizomes to form dense thickets. Originally imported as an ornamental plant, itadori knotweed is native to Japan and other parts of Asia. Although itadori knotweed is edible and valued where it is native, in North America the species is not held in check by local insects or competitive vegetation. It 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 itadori 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. Itadori 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) 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.

This plant is sometimes sold as Fallopia in other states and on the Internet, but this is not allowed in Washington state. 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 itadori knotweed can be found in the Garden Wise booklet available for download or by contacting our office

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 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.

Itadori 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, itadori knotweed is commonly found along trails, roadsides and on rivers and streams as well as residential properties and associated greenspaces.

Additional information on itadori (Japanese) knotweed
Stop the Spread of Invasive Knotweed - click to download

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.

Other Links

Invasive knotweed control videos video

Knotweed Control Classes - Take one of our classes to learn how to control knotweed and become eligible to borrow stem injectors

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.

Itadori knotweed photos - click a thumbnail for a larger image

Japanese knotweed flower closeup
Japanese knotweed flower clusters
Japanese knotweed flowering stems

Photos from MissouriPlants.com (external link)

 

Related information

Related agencies


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).