Other Common Names: Sakhalin knotweed
Weed class: B
Year Listed: 1999
Native to: Asia
Is this Weed Toxic?:
not known to be
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?
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.
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.
Leaves are alternate, very large and heart shaped. They are often more than a foot long and 2/3 as wide.
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.
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.
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.
For More Information
See our Written Findings for more information about giant knotweed (Polygonum sachalinense)