Japanese knotweed (Polygonum cuspidatum (Sieb. & Zucc.)

Japanese knotweed is a member of the Polygonaceae, or buckwheat family, and is native to eastern Asia. First introduced as a garden ornamental from Japan to England in 1825, it was then introduced to North America in the late 1800’s.

In the Pacific Northwest, this species is found from Alaska to California. In Washington State, Japanese knotweed is a Class C noxious weed, with widespread and increasing distribution in riparian areas, moist waste places, roadsides and rights of way, particularly west of the Cascade Range. Wetlands, parks and natural areas are vulnerable to establishment when rhizome fragments are introduced to the site. Fragmentation is caused and spread by natural run-off, and mechanically by roadside machinery used in mowing or grading. Fragments are also spread by contaminated yard clippings - Japanese knotweed is still found as a garden ornamental.

IDENTIFICATION: The reddish, bamboo-like canes of Japanese knotweed are smooth and hollow stems, jointed and swollen at the nodes - a characteristic of the buckwheat family. A papery sheath is present above each stem node and at the base of the leaf stalks, another family characteristic. The overall plant height can reach eight feet, and it often appears shrubby or grows in dense clumps. The stems die back each year at first frost, leaving tall, brown skeletons.

The leaves are alternate, between four and six inches long, and usually ovate (egg-shaped). The small, individual flowers are whitish to greenish, in drooping clusters from the axils of the leaves. In Europe and the United Kingdom, P. cuspidatum is synonymous with Reynoutria japonica.

Similar looking plant: A closely related and similar looking species, giant knotweed (Polygonum sachalinense), also grows in our region, and can share habitat and invasive growth characteristics. The leaf shape and stem are somewhat similar. Giant knotweed is larger, reaching 12 feet tall. The leaf size can be up to 12 inches across and the leaf base is more heart shaped. These two species can hybridize.

Comparison of giant knotweed (L) and Japanese knotweed leaves.

BIOLOGY AND ECOLOGY: Japanese knotweed is a herbaceous perennial with thick, stout rhizomes that can be 30 feet long, or more, and develop into dense mats. Shoots can generate from the rhizomes and from the roots when very near the soil surface or when buried up to 6 feet. Japanese knotweed is dioecious, with male and female flowers on separate plants. Flowers appear in the late summer, usually July through August. The seeds are wind dispersed.

Outside of its native range, the spread of Japanese knotweed is attributed to fragmentation of the rhizomes and roots, rather than seed. Fertile male flowers are rare outside of Japan. In England, fertile males are absent, and viable pollen is from a hybrid cross between P. cuspidatum and P. sachalinense. These hybrids are morphologically similar to P. cuspidatum, and are often mistakenly identified in England.

In its native habitat, Japanese knotweed is commonly found in hills and high mountains in sunny locations. Seedling establishment is common, and this species is a colonizer for both primary and secondary succession of volcanic slopes. The organic content, nutrient content, and the soil types where it grows vary widely. It can tolerate soils with a pH range from 4 to 7.4. Outside its native range, Japanese knotweed prefers moist, open, sunny locations. However, it can grow on dry sites. Growth and spread may be limited by available light.

Early spring emergence is a strategy used to establish and dominate a site. Once established, the rhizomes develop into deep, thick mats, obstructing root removal. Japanese knotweed is also considered a potential flood hazard. Riparian areas mechanically cleared to improve stream flow both cause and spread fragments of Japanese knotweed, and create newly disturbed sites susceptible to infestations.

CONTROL: Prevention of new infestations is necessary. Preventative techniques include eradication of small, newly established sites, monitoring stream corridors for new infestations and long-term follow up of controlled sites.

Mechanical control success of larger infestations varies. Digging is not recommended because of the extensive rhizomes, and fragments will result in new infestations. Cutting the stems will result in new shoot emergence. However, repeated stem cutting during the growing season (before the first frost) eventually reduces the root reserves, and in some cases, effective control is attained. Combining mechanical control with herbicides is effective. Herbicide control alone will reduce large, well established stands of Japanese knotweed.

Please contact your local county noxious weed control program or Washington State Cooperative Extension office for site specific control recommendations and any permits or special requirements needed when controlling weeds in a wet area.

Written by Bridget Simon, Education Specialist for the Washington State Noxious Weed Control Board.

Photos by staff of the Washington State Noxious Weed Control Board, and Bill Wamsley.

Reference material and further information are available from the Washington State Noxious Weed Control Board in Kent, WA.

Published June 1998.