Where does it grow?

Himalayan Knotweed can be found on disturbed moist sites, roadsides, riparian and wetland areas.

What is the distribution in Washington State?

Click on map to enlarge.

• Himalayan Knotweed has been documented in Whatcom, Snohomish, Grays Harbor counties. It has also been documented in Thurston, and King counties, although the extent is not known.
• If you think you may have seen this weed, please contact your county weed coordinator or us at noxiousweeds@agr.wa.gov.

• Distribution map is based on 2003 data.

How do I control it?

General control methods: 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, entire plant material should be removed, dried and burned if possible. New plants can sprout from very small fragments.

Cultural: 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
• entire root system must be removed, since resprouting can occur from rhizomes

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

Mechanical: Mowing or cutting plant shoots is ineffective alone:

• mowing may eventually tax the plant of it's resources
• mowing must be repeated throughout the growing season
• all material must be removed to prevent new infestations

Biological: None known.

Are there plants that may be confused with Himalayan Knotweed?

Knotweed species resemble each other, Himalayan Knotweed can be easily distinguished from the others by it's long slender leaves. If you need help with plant identification, please contract  your county noxious weed coordinator.

For more information