Japanese knotweed, (Polygonum cuspidatum, P. sachalinense, P. polystachyum and hybrids) also known commonly as ‘bamboo’, is a native of Japan that was brought to the US from Britain in the late 1800’s as an ornamental.
Japanese knotweed’s stout, hollow, bamboo-like stems and the large (3 to 6 inches long), broadly ovate, alternate leaves are distinctive. Tiny white or greenish-white flowers develop in late summer and grow in numerous linear clusters that form a mass of white over the plant when in full flower (see picture above). The plant is insect pollinated. Frost-killed stems turn bronze colored and may remain upright through winter.
Japanese knotweed is a shrub-like, herbaceous perennial (but dies back to ground each fall) that can grow to ten feet in height and form dense thickets of erect stems that exclude native vegetation and greatly alter natural ecosystems. It poses a significant threat to riparian areas, where it can survive severe floods and is able to rapidly colonize scoured shores and islands. Once established, populations are extremely persistent and difficult to control.
Japanese knotweed can tolerate a variety of adverse conditions including full shade, high temperatures, high salinity and drought. It is often seen along stream banks (erosion and deposition areas) where it often forms an impenetrable mass of stems; it also occurs in wetlands, roadways and waste places. Knotweed reproduces sexually as well as vegetatively through an extensive network of rhizomes (roots that can sprout new stems) that may spread up to 65 feet from the parent plant.
Knotweed spreads primarily by its long, stout rhizomes. It may also colonize new areas through wind dispersed seed as well as through transported root and stem fragments as small as ½ inch. The seeds have no dormancy requirements and germinate readily. Knotweed is often transported to new sites in floodwaters and as a contaminant in fill (along roads).