Japanese Knotweed
Polygonum cuspidatum
Washington State Classification
Class B — Control Required (Designated)
It is a very aggressive escaped ornamental that is capable of forming dense stands, crowding out all other vegetation and degrading wildlife habitat.
Required control in Region 1 (all western Washington counties)
Quick Reference
Identification
Growth Habit
It is a perennial species with spreading rhizomes and numerous reddish-brown, freely branched stems. Plants can reach 4 to 8 feet in height and is often shrubby.
Leaves
Leaves are alternately arranged with petioles (stalks) and are 4 to 6 inches long, ovate and have a truncated base and an abrupt tip.
Flowers
The whitish to whitish-green flowers are in drooping panicles (clusters) from leaf axils. Male and female flowers are on separate plants.
Fruit & Seeds
The fruits are approximately 1/8 inch long, shiny brown and triangular.
Impact
It is a very aggressive escaped ornamental that is capable of forming dense stands, crowding out all other vegetation and degrading wildlife habitat. It can also create a fire hazard in the dormant season. It is difficult to control once established.
Ecology & Spread
Habitat
It is often found in waste places, gardens, roadsides and stream and riverbanks. Please click here to see a county level distribution map of Japanese knotweed in Washington.
Spread Mechanisms
Reproduction
It spreads by seed and by long stout rhizomes. Colonies rarely establish from seed.
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Control Methods
Mechanical
- Grubbing out small clumps when discovered can prevent new colonies from establishing but is time-consuming. 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. Frequent, repeated cutting of plants over many years can eventually kill the plant. Covering plants with a thick landscape fabric for up to 5 years may provide control. Fabric should be installed after cutting plants back to the ground, provide some give in the fabric so new stem growth won't puncture through, monitor fabric and flatten knotweed growth under it, keep fabric cleared of debris, and repair any holes.
Biological Control
- There are currently no registered biological control agents for use on any of of invasive knotweed species. Grazing may provide some reduction in plants but additional control methods are typically needed. Any grazing strategy should be carefully controlled to prevent damage in riparian areas.