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Himalayan knotweed (Polygonum polystachyum)
ODA rating: B
USDA Symbol: POPO5
Oregon himalayan knotweed distribution
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Images courtesy of Glenn Miller, Oregon Department of Agriculture

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Perennial; blooms July to October. Grows to 6 ft tall. Closely related to giant and Japanese knotweeds. Leaves lanceolate and 4 to 8 inches long, unlike Japanese and giant knotweeds which are ovate and cordate, respectively.
This species is the least common of the three weedy knotweed species in the Pacific Northwest. Plants grow vigorously and create dense colonies that exclude native vegetation, but they are much shorter than related species and seem less adaptable to dense shade. Established populations are persistent and difficult to eradicate due to their proximety to streams. It poses a significant threat to riparian areas, where it uses flood events for its dispersal by rapidly colonizing scoured shores and islands. It poses a significant threat to riparian areas where flood events can disperse and aid rapid colonization of new populations.
Distribution in Oregon
The first record of this plant is 1934 in Polk County.

Biological controls
No approved biological agent is currently available.