Knotweeds (Polygonum spp.) are invasive perennials, with four species found in British Columbia: Japanese knotweed (Fallopia japonica); Bohemian knotweed (Fallopia x bohemica); Giant knotweed (Fallopia sachalenensis); and Himalayan knotweed (Polygonum polystachyum). Knotweeds thrive in roadside ditches, low-lying areas, irrigation canals, and other water drainage systems. They are also found in riparian areas, along stream banks, and in other areas with high soil moisture. Knotweeds occur in the southwest coastal region, the Shuswap, Kitimat, Stikine, Skeena, Columbia, Okanagan, and Kootenay areas, as well as the Queen Charlotte Islands. Additional plants may exist in many gardens in communities across BC.
Knotweeds have small white-green flowers that grow in showy, plume-like, branched clusters along the stem and leaf joints. Hollow stems stand upright and are bamboo-like with reddish-brown speckles and thin, papery sheaths. Leaves are heart or triangular-shaped on all species except Himalayan, which are elongated and tapered. Stems grow 1-5 metres in height at maturity, with leaves 8-10 centimetres wide and 15 centimetres in length. Giant knotweed leaves are generally twice the size of the other 3 species. A distinguishing feature of Japanese knotweed is the zigzag pattern in which leaves are arranged along the plant’s arching stems.
Knotweeds spread rapidly through root systems that may extend from a parent plant up to 20 metres laterally and up to a depth of 3 metres. They thrive on freshly disturbed soil in moist locations. Knotweeds are dispersed by human activities or by water to downstream areas, and are of particular concern in riparian areas and areas prone to seasonal high water or flooding. Plants emerge in early spring and produce large leaves that can shade out other plant species. Infestations can dominate stream banks and reduce sight lines along roads, fences, and rights-of-way.
Knotweeds threaten biodiversity and disrupt the food chain by reducing available habitat and increasing soil erosion potential. Stream banks are at particular risk as exposed knotweed roots break off and float downstream to form new infestations. Knotweeds can reduce or eliminate access to water bodies for recreation activities including fishing, swimming, boating, canoeing, and kayaking.
Refer to Invasive Knotweeds T.I.P.S. for information on prevention and control methods.
Here is another helpful website (Noxious Weeds by King County, Washington) on knotweeds.