| Japanese knotweed (Polygonum cuspidalum) |
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ODA rating: B
USDA Symbol: POCU6
Oregon Japanese knotweed distribution
Click on image to view larger photo | Photo by Rich Old, XID Services | Photo by Glenn Miller, Oregon Department of Agriculture
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Photo by Glenn Miller, Oregon Department of Agriculture | If images are downloaded and used from the ODA web site please be sure to credit the photographer.
Description
Perennial; blooms July to October. Grows four to nine foot tall and has long creeping rhizomes. Stout stems reddish-brown, nodes slightly swollen. Leaves short stalked, trucate, broadly ovate and 2-6" long by 2-4" wide. Flowers greenish-white to cream in large plume-like clusters at the ends of the stems. Hybrids with giant knotweed are common.
Impacts
Japanese knotweed is a native of Eurasia and was introduced to the United States as an ornamental. Plants grow vigorously along roadsides, waste areas, streams and ditch banks and create dense colonies that exclude native vegetation and greatly alter natural tree regeneration. Established populations are extremely persistent and do not respond to mowing/cutting. Large infestations can be eliminated with approved herbicides, but treatments are costly and time consuming. It poses a significant threat in riparian areas, where it disperses during flood events rapidly colonizing scoured shorelines, islands and adjacent forest land.
Distribution in Oregon
The first record of Japanese knotweed in Oregon is 1965 in Lane County.
 
Biological controls
No approved biocontrol agent is currently available.
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