Link to the WSNWCB Home Page link to the About Us page Link to the Weed Laws page Link to the Noxious Weed List Link to the Weed Information page link to the Frequently Asked Questions page Link to the Education page Connect to other Links page Link to Contact Us page Link to employment opportunities page

Class B Noxious Weed

Himalayan Knotweed
Polygonum polystachyum
 

Himalayan knotweed flower

Other common names: Bell-Shaped Knotweed
Plant family:
Polyganaceae
Year listed: 2003
Native to: Asia

Why is it a noxious weed?

Himalayan Knotweed grows vigorously and creates dense colonies that exclude native vegetation and greatly alter natural ecosystems. Established populations are extremely persistent and colonies are extremely difficult to eradicate. It poses a significant threat to riparian areas, where it can survive severe floods and is able to rapidly colonize shores and islands, even very small fragments can form new plants.

How would I identify it?

General Description: Clumping perennial with large leaves, hollow stems, and long creeping rhizomes. Not usually confused with other knotweeds, it is recognized by its long slender leaves. Can grow to about 6 feet in height.
 

Leaves:

  • leathery
  • alternate
  • 5 - 12 inches in length
  • sharply pointed tips
  • slightly heart shaped to tapered bases

Flowers:

  • showy
  • pinkish, clusters borne at tips of branches
  • fragrant

Stems:

  • smooth
  • erect with twigs zigzagging slightly from node to node
  • reddish brown at maturity

Line drawings are taken from 1,000 Weeds of North America, copyright of XID Service, Inc and used with permission. Permission granted by the author.

How does it reproduce?

Himalayan Knotweed reproduces vegetatively from rhizomes and by seed. Each node on the plant stock is
able to produce roots and new plants. New plants can sprout from fragments as small as 1 inch.

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

For more information, download our Written Findings or the updated IPM Plant Profile for knotweed.

View the Garden Wise Eastern Washington and Western Washington Guides for better noninvasive plant recommendations for your yard or garden.

More Photos-Click photo to enlarge
Feel free to download, or contact us to receive high-resolution JPEG or TIFF formats

       
    Himalayan knotweed infestation. Photo courtesy of Thurston County Noxious Weed Control. Himalayan knotweed leaves and flowers. Photo courtesy of Thurston County Noxious Weed Control.

 

   

 Last updated August 26, 2008