Japanese knotweed: Fallopia japonica

This week we are going to talk about Japanese knotweed. This noxious weed is native to East Asia in Japan, China and Korea. This plant has hollow stems and looks like bamboo at a glance. Japanese knotweed will grow in riparian areas, waste areas, roadsides and other disturbed sites. A very aggressive escaped ornamental, Japanese knotweed is capable of forming dense stands and will crowd out other more desirable vegetation. It is a perennial species and spreads by seeds as well as spreading rhizomes.

The flowers of Japanese knotweed are white or whitish-green and grow in drooping clusters from the leaf axils. Male and female flowers are on separate plants. The leaves of Japanese knotweed are alternately arranged with stalks and are four to six inches long. Fruits are approximately 1/8 inch long and are shiny brown and rectangular.

Japanese knotweed was added to the Washington State Noxious Weed List in 1995. It was added to the Grant County Weed List in 2005 after several locations were discovered in Grant County.

So how do we control this plant? Mechanical control such as cutting or tilling are ineffective, because this will likely spread the infestation further. This is because any piece of the plant left behind in the soil can grow a new plant. The most effective means of controlling this pernicious plant is by applying herbicides close to the flowering stage in late summer or autumn.

One interesting fact about Japanese knotweed is that it contains high levels of resveratrol, replacing grapes in some areas for supplement use due to the hardiness of this plant.

Japanese knotweed grows very quickly. If not controlled, it can get out of hand easily. Bees love this plant, because it produces lots of flowers that they can get nectar from. If Japanese knotweed is cut down and not treated, it will send up new suckers that become new plants. Effectively what this means is that if this plant is cut down, you will end up with an even bigger mess than you started with. Japanese knotweed is also notorious for growing under fences and onto neighboring properties.

Give the office a call at (509) 754-2011, Ext. 4710, or visit our website at www.grantcountyweedboard.org for more information.