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Japanese Knotweed (Polygonum cuspidatum)

Description: Japanese knotweed, also known as Mexican Bamboo, Crimson Beauty, Fleeceflower, and Reynoutria, is a herbaceous perennial in the buckwheat family. The tall (3-9 feet) stems are stout, hollow, smooth, and form dense clumps. The plant reproduces by seeds and by rhizomes, which can reach 45 to 60 feet in length. The leaves are 2-6 inches long and 2-5 inches wide, flattened at the base, and taper to a point at the end. The tiny greenish-white flowers occur in attractive sprays at the end of the stem. Knotweed flowers from July to August and are followed soon after by small, winged fruits.

Distribution: Native to Asia, Japanese knotweed came to the United States as an ornamental from England in the early 1900’s. It has since spread to scattered locations in the United States from coast to coast. This extremely noxious weed is found in approximately 15 locations in Johnson County, and has spread to other County’s around the state as well. Japanese knotweed is found typically in moist, open habitats. It can tolerate a variety of adverse conditions such as high salinity and temperatures, dust, and drought. It can be found near water sources such as streams, ponds, and rivers, waste areas, roadsides, old homesites, and anywhere it has escaped from cultivation. Japanese knotweed does not appear to grow in well-shaded areas.

 photo of Japanese Knotweed

Photo courtesy of King County, WA Noxious Weed Control Program
What’s The Problem?: Japanese knotweed is extremely persistent, and once established, is very difficult, if not impossible, to totally eliminate. The extremely long rhizomes (45-60 feet) spread quickly to uninfested areas and send up new growth. Pieces as small as 1 inch of rhizome left in the soil may resprout whole new plants. Japanese knotweed forms large, dense, mostly pure stands where no other plants can grow underneath. Traditional mowing control methods seem to be spreading the plant, rather than killing or stressing it.

Management Methods: The best tool for control of Japanese knotweed is to prevent establishment. Grubbing (digging up) small initial populations can be of limited effectiveness, so long as all roots and runners are removed. Soil disturbed by digging may promote regrowth of this or other noxious weeds. Small stands may also be controlled by repeated cutting, not mowing, (3 or more times per year), which may need to be supplemented by reseeding of competitive vegetation once growth of knotweed has been reduced. Hand pulling of small plants is possible, however, roots will resprout if not removed. All cutting or plants should be bagged up and removed from the site. Mowing of japanese knotweed is NOT a good alternative, as the small pieces chopped up can be thrown or carried by the mower and regrow in uninfested areas. Mowing should be avoided, unless vegetation can be cleaned off the mower at the disturbed site to prevent movement. Another control possibility is shading. Knotweed requires large amounts of light, and covering with black plastic may reduce growth. This is, however, very labor intensive, and still may not provide sufficient control, as rhizomes may spread past the shading material. Prescribed burning may offer a small degree of usefulness to remove above ground dormant plant material for other types of management. Large, established stands present a much greater degree of management difficulty. Biological control (use of insects, pathogens, etc.) research has not yet produced a viable management alternative. At present, the only method to control large stands of knotweed is with repeated applications of herbicides. Even this has been shown to be of limited effectiveness, and complete eradication may not be possible. Because japanese knotweed tends to grow near water sources, herbicide choices may be limited. Be sure to read and fully understand chemical labels before usage. Monitoring of areas before and after management is critical to prevent establishment, and to evaluate effectiveness of attempted weed control. Knowledge of the growth characteristics of japanese knotweed should be obtained before any type of management activity is undertaken. Consult with weed management professionals to correctly identify and manage japanese knotweed populations.