|ODA rating: B and T
USDA Symbol: POSA4
Oregon giant knotweed distribution
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Images courtesy of Glenn Miller, Oregon Dept. of Agriculture.
If images are downloaded and used from the ODA web site please be sure to credit the photographer.
Perennial; blooms July to October. Grows over 12 feett tall. Closely related and similar to Japanese knotweed. Leaf cordate, or heart shaped; often exceeds one foot long. Inflorescence of creamy white flowers sparse, and the flower size does not increase with maturity. Hybrids with Japanese knotweed are common. Japanese knotweed is smaller with truncate leaves.
Giant knotweed is the largest of the knotweeds, enabling this species to dominate and out compete native or beneficial plants. It poses a significant threat to riparian areas where it prevents streamside tree regeneration. Research indicates that giant knotweed produces allelochemicals from the roots, which aid in its dominance and rapid colonization. Giant knotweed is a viable pollen source for Japanese knotweed, resulting in male fertile hybrids which are often confused with Japanese knotweed.
Distribution in Oregon
The first documented site of this plant in Oregon was 1937 in Coos County.
No approved biological control agent is currently available.
Printable trifold knotweed brochure (pdf)