Knotweeds were first introduced to Ireland from Japan as ornamental plants but have now spread extensively around the country. They are highly invasive, causing damage to man-made structures and forming dense stands that exclude other plant species and reduce biodiversity. Knotweeds are identifiable by their bamboo-like, knotted and red-green stems. Below ground, knotweeds have extensive rhizomes that spread aggressively. Human intervention aids dispersal by inadvertent or incorrect disposal of stem or rhizome fragments. Fly-tipping of garden waste has meant that many roadsides are colonized with these species.
Sunny, damp conditions are optimal for the knotweeds but they are tolerant of most environmental conditions. They can often be found on waste ground and by river banks where flooding aids dispersal to new locations. New shoots grow extremely fast in spring, as much as 7-10cm per day. Young knotweed shoots appear similar to reddish asparagus. Mature plants reach heights of 1.5m to 4m. Plants die back in the autumn, with the large rhizome root base storing nutrients for re-growth the following year.