Invasive Knotweed Complex
The invasive knotweed complex consists of four species in the family Polygonaceae. Knotweeds are large, rhizomatous, herbaceous perennial plants that vary in height from 5 to 20 feet (Duncan, 2013; Parkinson & Mangold, 2017). The round, hollow, and thick stems with swollen nodes resemble bamboo. The stems are green to reddish in color, often red-speckled. The multiple high erect stems are often arched near the top (Parkinson & Mangold, 2017; King County BMP, 2015; WA NWCB). The large sized leaves are alternately arranged, bright green with smooth edges. In general leaves are ovate or broadly elliptic with a pointed tip, however leaf shape varies between species (Bobis et al., 2019; King County BMP, 2015). The rhizomatous root system of knotweed forms a deep, dense mat. Underground rhizomes are thick and extensive, storing large quantities of carbohydrates and spread aggressively (WA NWCB; King County BMP, 2015). Rhizomes can grow 23 feet from parent plant, 50 to 65 feet laterally and penetrate 7 feet deep into the soil (King County BMP, 2015; Duncan, 2013). Rhizomes are root-like subterranean stems commonly horizontal in position that usually produce roots below and send shoots up towards the surface. Flowers are small and white to green in color with the exception of Himalayan flowers which are pink to white in color. Flowers grow in showy plume-like clusters and emerge from where leaves meet the stem; flowers have 5 petals (technically tepals), rarely 4, all of which are upright (Parkinson & Mangold, 2017; King County BMP, 2015). In the Pacific Northwest the invasive knotweed complex consists of four species including Bohemian, Giant, Japanese and Himalayan. All four species are Class B state listed noxious weeds in Washington State.