Giant Knotweed

Polygonum sachalinense

Giant Knotweed

Family: Polygonaceae

Other Common Names: Sakhalin knotweed
Weed class: B
Year Listed: 1999
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 creates dense colonies that exclude native vegetation and greatly alter natural ecosystems. Established populations are extremely persistent and difficult to eradicate, even very small fragments can form new plants.

How would I identify it?

General Description

Giant knotweed is an herbaceous perennial, strongly rhizomatous, growing over 12 feet tall. It has huge elephant-ear shaped leaves. It is similar to Japanese knotweed but its features are larger in size.

Flower Description

Flowers are white with a greenish tint and are in small clusters (panicles) in the leaf axils mainly on the upper portions of the stems.

Leaf description

Leaves are alternate, very large and  heart shaped. They are often more than a foot long and 2/3 as wide.

Stem description

Stems are hollow, jointed and swollen at the nodes.

Fruit Seed Description

The seed structure is a 3-sided achene that is shiny and brown-black.

May Be Confused With

Knotweed species resemble each other, as the name implies, giant knotweed is much larger than the other knotweeds found in Washington. If you need help with plant identification, please contact your county noxious weed coordinator.

Where does it grow?

Giant Knotweed is found along stream banks, in moist waste places, neglected gardens, roadsides and railroad right-of-ways. Please click here to see a county level distribution map of giant knotweed in Washington.

How Does it Reproduce?

Giant Knotweed reproduces mainly by rhizomes.

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 giant knotweed (Polygonum sachalinense)

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