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Arcata Field Office

Japanese knotweed (Polygonum cuspidatum)

Knotweed family (Polygonaceae)
Japanese Knotweed 
 
Description: Japanese knotweed is a dense growing shrub reaching heights of up to ten feet. The semi-woody stem is hollow with enlarged nodes. It is recognized by its extremely dense growth form, often occurring in large monocultural thickets, and by its large, alternate, dark-green leaves and the panicles of small white flowers.
 
Habitat: Japanese knotweed commonly invades disturbed areas with high light, such as roads sides and stream banks. It is a riparian species that spreads quickly to form dense, tall thickets that shade out other species. In the winter, when knotweed canes die back, bare ground along river banks are exposed which dramatically increases erosion.   The dense patches shades and displaces other plant life and reduces wildlife habitat.   
 
Distribution: Japanese knotweed is native to eastern Asia and was introduced from Japan as an ornamental garden plant in the late 1800's.   It is now widely distributed in much of the eastern U.S., and coastal areas of Oregon and Washington.   In northern California, there are about two dozen known infestations in Del Norte and Humboldt Counties that are being monitored and most with eradication efforts underway.  

Flowering period: Japanese knotweed blooms in the summer months from June to August.   Reproduction occurs both by vegetative cuttings and seeds, making this plant extremely hard to eradicate. 
 
Eradication note: Manually excavate small infestations removing every fragment of root being careful to burn all parts of the plant. If infestation is too large or threatens facilities, consider stem injection of knotweed in August with a glyphosate based herbicide with a special tool called a JK Injection Tool.

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