Weed: Japanese knotweed (Polygonum cuspidatum Sieb. & Zucc.) aka Japanese Bamboo, Mexican Bamboo, Japanese fleeceflower

Family: Polygonaceae (Buckwheat family)

Images:

Brief Plant Description:  Encycloweedia's Polygonum cuspidatum Page

Current Wyoming Distribution: 

Laramie (Albany County): Found in two flowerbeds and growing through the sidewalk near the University of Wyoming Foundation Building.

 Lander (Fremont County): Found in three flower beds near the downtown area.

How did it get to Wyoming?  It was purposefully introduced as an ornamental (aka Japanese bamboo).

Historical Wyoming Distribution:  It was reported by the current owner of one of the Lander sites to have been introduced in the 1950’s by the previous owner. The history of the Laramie infestations is unknown. The Rocky Mountain Herbarium reports no previous collections from within Wyoming.

Origin:  Native throughout the mountainous regions of Japan, Taiwan, Korea, and Eastern China.

National Distribution:

Reasons for Concern: Japanese knotweed is an aggressive riparian invader that has escaped from ornamental plantings. The most serious problems are occurring in the Pacific Northwest and Northern California.

Legislative Status:  Currently listed as noxious (Class B status) in Washington, Oregon, and California. This plant has not been designated or declared noxious in Wyoming.

Control Methods:  Digging out the root system is effective if you can get the long creeping rhizomes (which may extend 20 feet) in the process. New shoots will emerge from small root pieces and both stems and crowns will re-sprout if left on moist soil. Repeated mowing is not extremely effective due to the large energy reserves in the roots. There are currently no biological control agents for Japanese knotweed. Certain herbicides have been effective for controlling Japanese knotweed. These include:

•         Triclopyr + Glyphosate

–        1% v/v + 2% v/v (high volume)

–        This treatment is best on plants above 100 year flood plain

•         Glyphosate

–        5-8% v/v

–        This treatment is best for plants in 100 year flood plain

•         Glyphosate (Cut stem treatment)

–        (25% v/v)

–        This treatment works best when immediately applied to cut stems

•         Imazapyr

–        3-4 pints/A

–        Broadcast treatment

While it might be better to treat plants in the bud to bloom stage, it may be too difficult to get effective coverage on 3-4 m tall plants. Foliar treatments should be applied when plants are 1-2 m. It should be noted that some re-treatment will likely be necessary with any of these herbicides.

Additional Notes:   While we have not found Japanese knotweed to have escaped into riparian areas in Wyoming, we feel it is important for land managers to become aware of it and monitor any populations growing in residential areas.  We do not want this plant to escape in Wyoming!

USDA NRCS Plants Data-Base - Polygonum cuspidatum Page

Encycloweedia's Polygonum cuspidatum Page

Japanese knotweed Alliance Page