Walla Walla County Extension

Weed of the Month

Japanese Knotweed (Fallopia japonica)

Japanese knotweed (Mexican bamboo, fleece-flower, Japanese bamboo) is a hardy, herbaceous perennial that grows to over 10 feet in height. Stems of Japanese knotweed are reddish-brown or green and smooth with slightly swollen leaf nodes that are surrounded by thin papery sheaths.  Broadly oval leaves are pointed at the tip and grow 2 to 6 inches long and about two-thirds as wide.  Branched sprays of tiny greenish-white flowers bloom during summer at the ends of stems and in leaf axils.  Flowers are followed by small, black and shiny triangular fruits.

Japanese knotweed was introduced from Asia in the late 1800’s as an ornamental but spread quickly to become a weed of low-lying areas, roadsides, waste areas, ditch banks, and pastures.  It prefers sunny, moist areas along streams and rivers but once established, it tolerates adverse conditions including full shade, high temperatures, high salinity, and drought. Japanese knotweed is designated a Class B Noxious Weed in the State of Washington.

Japanese knotweed spreads primarily by long, stout rhizomes.  It is often transported to new sites as a contaminant in fill dirt, can be distributed by water, and may also be carried by the wind. 

MANAGEMENT OPTIONS
When possible, completely remove the entire plant including all roots and runners to prevent resprouting. All plant parts (including mature fruit) should be bagged and disposed of in a trash dumpster to prevent reestablishment. 

Large colonies of Japanese knotweed are best controlled by spraying foliage with glyphosate or triclopyr and a non-ionic surfactant. To reduce the risk of damaging non-target species, cut the plant stem about 2 inches above ground level and immediately apply a 25% solution of glyphosate (e.g., Roundup®, or use Rodeo® if applying in or near wetland areas) or triclopyr (e.g., Garlon) and water to the cross-section of the stem.  A subsequent foliar application of glyphosate may still be needed to control new seedlings and resprouts. BE SURE TO FOLLOW ALL PESTICIDE LABEL INSTRUCTIONS AND PRECAUTIONS.