Invasive knotweed identification and control
Polygonum x bohemicum, Polygonum cuspidatum, Polygonum sachalinense, Persicaria wallichii, Buckwheat Family
Invasive knotweeds, mostly non-regulated Class B noxious weeds, are perennials found throughout King County, especially on roadways and riverbanks. Growing in large, dense thickets, they reach 4-13 feet tall, with bamboo-like, green-reddish canes and branched clusters of small white to pink flowers. They reproduce mainly vegetatively via extensive roots and rhizomes, a stem fragments. These plants are highly aggressive, clogging waterways, eroding banks, and even growing through building foundations.
In the Pacific Northwest, there are four similar species of invasive knotweed that are difficult to tell apart and share similar habitat, impacts and control methods. They are all large, robust perennials that spread by long creeping rhizomes to form dense thickets. These tall, bamboo-like plants were introduced from Asia as ornamentals beginning in the early 1800's in England and in the United States by 1890.
Due to their widespread use, the lack of natural predators, and their ability to spread by root and stem fragments, invasive knotweed species have spread and become widely established throughout North America and Europe. Knotweed clones can completely clog small waterways and displace streamside vegetation, increasing bank erosion and lowering the quality of riparian habitat for fish and wildlife. Rapid spring growth and deep, extensive roots enable knotweed to outcompete most other plants, even small trees and shrubs. Knotweeds can tolerate partial shade and are most competitive in moist, rich soil. Invasive knotweed species are commonly found along roadsides and on stream banks but also may be present in yards, vacant lots, edges of fields, parks and many other places.
Public and private landowners are not generally required to control infestations of invasive 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. Invasive knotweed species are all Class B Noxious Weeds in Washington. They have not been designated for required control in the county by the Washington State Noxious Weed Control Board, but they have been selected for required control in limited parts of the county by the King County Noxious Weed Control Board. Because control of these species is not generally required in the county, they are 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.
These species are also 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 these species, into or within the state of Washington. For more information see Noxious weed lists and laws.
Stems are stout, cane-like, and reddish-brown. The plants die back at the end of the growing season but their old reddish-brown canes often persist. The stem nodes are swollen and surrounded by thin papery sheaths. Leaves are either heart-shaped or spade-shaped or somewhere in between. 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. The fruit is 3-sided, black and shiny.
- Bohemian knotweed (Polygonum x bohemicum)
The most common invasive knotweed in western Washington. Hybrid between giant and Japanese knotweed and shares characters of both parent species. Often confused with Japanese knotweed.
- Japanese knotweed (Polygonum cuspidatum)
The most ornamental of the knotweeds and shorter than Bohemian or giant. Also called Fallopia japonica (this name is commonly used in Europe and in the nursery trade).
- Giant knotweed (Polygonum sachalinense)
The largest knotweed, up to 16 feet tall or more, also known as elephant ear bamboo because of its very large leaves.
- Himalayan knotweed (Persicaria wallichii)
The most distinctive of these species, it tends to be shorter, denser and more clump-forming and has pinker flowers and stems that are not hollow.
County-led knotweed control projects
Control of knotweed is challenging and often requires a watershed approach to be effective along waterways. The King County Noxious Weed Control Program is engaged in several cooperative weed management projects in the county to tackle this difficult weed along certain waterways with assistance from local landowners and state and federal grants. For information on these projects, please see our Reports page or contact our office and ask to speak with the Knotweed Program Manager.
Invasive Knotweed Brochure (2 Mb)
Himalayan Knotweed Fact Sheet (156 Kb)
Knotweed Best Management Practices (649 Kb)
Knotweed Weed Alert (215 Kb)
Knotweed Biology and Control Slide Show (7.09 Mb)
WA State Noxious Weed Control Board (external link)