Japanese knotweed (Fallopia japonica var. japonica)
A knotweed native to Japan and introduced, mainly as a garden ornamental, in the 19thC. It has proved to be the most troublesome alien invasive plant species in Britain and is a noxious weed. Japanese knotweed is very vigorous, particularly in full sun. It reaches height 3m in June, dies back to ground level in autumn and sprouts from a dense knot-like mass of rhizomes in spring. F. japonica var. japonica is the tall lowland form of F. japonica (see dwarf Japanese knotweed). It does not set viable seed in Britain and Europe, having been propagated from a single (female) clone. It propagates readily from fragments of rhizome, and from fragments of green stem under favourable conditions, provided a node is present.