Himalayan Knotweed
Persicaria wallichii
Washington State Classification
Class B — Control Required (Designated)
Himalayan knotweed grows vigorously into dense colonies that exclude native vegetation, alter natural ecosystems and are difficult to eradicate.
Required control in Region 1 (all western Washington counties)
Quick Reference
Identification
Growth Habit
Himalayan knotweed is a clumping perennial with large leaves, hollow stems, and long creeping rhizomes. It is not usually confused with other knotweeds—it is recognized by its long slender leaves. It can grow to about 6 feet in height.
Leaves
Leaves are leathery, alternate, 5 to 12 inches in length with sharply pointed tips. They have slightly heart shaped to tapered bases.
Flowers
Flowers are showy, pinkish, and fragrant with clusters borne at tips of branches.
Fruit & Seeds
The small 3-sided achenes (seeds) have the old sepals attached and are smooth.
Impact
Himalayan knotweed grows vigorously into dense colonies that exclude native vegetation, alter natural ecosystems and are difficult to eradicate. It poses a significant threat to riparian areas, where plants can survive severe floods. Small fragments can form new plants.
Ecology & Spread
Habitat
Himalayan knotweed can be found on disturbed moist sites, roadsides, riparian and wetland areas. Please click here to see a county level distribution map of Himalayan knotweed in Washington.
Spread Mechanisms
Reproduction
Himalayan knotweed reproduces vegetatively from rhizomes and by seed. Each node on the plant is able to produce roots and new plants. New plants can sprout from fragments as small as 1 inch.
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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.