Japanese Knotweed

Japanese knotweed is a perennial that quickly grows to heights of 5-10 feet in large clones, potentially up to several acres in size. The stems are reddish brown and are hollow and bamboo-like. When they die back, the stems remain upright through winter. Mature leaves are 3-5" wide and 4-9" long and lighter on the lower surface. Young leaves are heart-shaped. Two inch long clusters of tiny white flowers bloom in late summer. Japanese knotweed can reproduce by seed, but spreads primarily by underground rhizomes, which can reach 6 feet deep, 60 feet long, and become strong enough to damage pavement and penetrate building foundations.

Japanese knotweed grows in a variety of habitats, in many soil types and a range of moisture conditions. It forms dense thickets that exclude native vegetation. Of particular concern is its ability to invade valuable wetland areas and line the banks of waterways where it forms an impenetrable wall of stems. This crowds out native vegetation and leaves banks vulnerable to erosion when the plants drops its leaves in winter. It is also found along roads, railroads, and utility pathways. Japanese knotweed is often spread through movement of its rhizomes; plant fragments that easily move in the water of streams, by transportation in fill dirt, on contaminated equipment, or through roadside mowing and plowing.

Trying to remove Japanese knotweed by pulling or digging is generally ineffective due to its deep underground rhizomes. Pulling and digging may further spread the plant if the pieces are not disposed of properly. Herbicide application is effective when the entire clone is treated repeatedly. Applications of herbicides containing glyphosate are typically used after spring leaf out and on resprouts emerging after cutting. Follow-up for a few years is required. For additional information on Japanese knotweed and it's control, visit http://dnr.wi.gov/invasives/fact/japanese_knotweed.htm