F sachalinensis

A much larger plant than Japanese Knotweed, native to Northern Japan and the Sakhalin Islands and also possibly a remote Korean Island.  It is much less common than F. japonica in Europe, and although it may sometimes occupy quite large areas it does not seem to be as persistent as F. japonica.  F. sachalinensis (Giant Knotweed) was introduced to Britain in the late 1860's, first appearing for sale in the 1869-70 catalogue of the horticulturalist William Bull of Chelsea. 

Both hermaphrodite and female individuals are known from Britain, though we know of no location where the two sexes grow together, and there are few places where male F. sachalinensis grows with F. japonica var. japonica.