Bohemian Knotweed

Polygonum x bohemicum

Bohemian Knotweed

Family: Polygonaceae


Weed class: B
Year Listed: 2004
Native to: Asia
Is this Weed Toxic?:

not known to be


Legal listings:

WA Quarantine list, WAC 16-750

Why Is It a Noxious Weed?

It forms dense colonies that exclude native vegetation and greatly alter natural ecosystems. Established populations are extremely persistent and colonies are extremely difficult to eradicate. It poses a significant threat to riparian areas.

How would I identify it?

General Description

Clumping and spreading perennial with large leaves, hollow stems and long creeping rhizomes.

Flower Description

Flower clusters commonly made up of male flowers and located at stem and branch tips. Clusters (panicles) are around 8 to 13 ¾ inches long. Sepals are white to pinkish white in color.

Leaf description

Leaves are alternate, leathery, and oval. The leaf tip ranges from being blunt to a tapered point. The base is slightly indented to deeply heart-shaped. Leaf midveins have hairs. Leaf size is intermediate between Japanese and giant knotweed.

Stem description

Main stems upright, often arched near top, simple to minimally branched, grooved, thick, hollow, weakly woody, swollen at nodes, usually reddish-brown at maturity.

May Be Confused With

Knotweed species resemble each other. Bohemian knotweed is a hybrid between Japanese knotweed and giant knotweed. If you need help with plant identification, please contact your county noxious weed coordinator.

Where does it grow?

An escaped ornamental, knotweed is often found in waste places, neglected gardens, roadsides, streambanks and riverbanks. Please click here to see a county level distribution map of Bohemian knotweed in Washington.

How Does it Reproduce?

Bohemian Knotweed reproduces vegetatively from rhizomes and by seed. Each node on the plant stock is able to produce roots and new plants. New plants can sprout from small fragments.

How Do I Control It?

General Control Strategy

Mowing or cutting plant shoots is ineffective alone; however, mowing followed by herbicide treatments will provide some control. Methods must be repeated if infestation is very large. Care must be taken not to produce new plants. All plant material should be removed, dried and burned if possible. New plants can sprout from very small fragments.

Mechanical Control

Grubbing out small clumps when discovered can prevent new colonies from establishing. Rhizomes and fragments left in the ground, or nearby, can regenerate and spread infestations. The entire root system must be removed since re-sprouting can occur from rhizomes.

Biological Control

There are currently no registered biological control agents for use on any of these Polygonum species. Grazing may be an effective strategy to prevent establishment. Any grazing strategy should be carefully controlled to prevent damage in riparian areas.

Herbicide Control

Please refer to the PNW Weed Management Handbook, or contact your county noxious weed coordinator.

For More Information

See our Written Findings for more information about Bohemian knotweed (Polygonum xbohemicum).

WSDA Knotweed Control Program

King County Noxious Weed Control Program: Knotweed Best Management Practices

Report on knotweeds from the book "Weed Control in Natural Areas in the Western United States"

Additional Photos