Invasive Knotweeds
Polygonum x bohemicum, P. cuspidatum, P. sachalinense
(Buckwheat Family)

General Description

In the Pacific Northwest, there are three closely related 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. All three species of knotweed are Class B noxious weeds on the WA State noxious weed list and control of these aggressive species is strongly recommended but not required in King County.

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, these 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. All three 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.

Knotweed photos - click a thumbnail for a larger image

Identification

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 Washignton. Hybrid between giant and Japanese knotweed and shares characters of both parent species. Often confused with Japanese knotweed.
• Japanese knotweed (Polygonum cuspidatum)
  The shortest, most ornamental of the knotweeds. 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.

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 managment 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.

For More Information

For detailed information on invasive knotweed, download a slide show on knotweed biology and control (3.12 MB, Power Point, may take several minutes to complete download) or see our handout covering the same information in text format, Knotweed Biology and Control (44 KB Acrobat file).

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). Also see the written findings of the WA State Noxious Weed Control Board for more information on Japanese and Bohemian knotweed.