Giant Knotweed is native to Japan, the island of Sakhalin to the north of Japan, and the remote island of Ullung-do between Japan and Korea.
It is a striking and huge plant, forming stands of largely unbranched stems, bearing leaves up to 3 times larger than those of Japanese knotweed. These are reminiscent of the foliage of the Broad-leaved Dock, Rumex obtusifolius. Flowering of both male and female plants starts later than Japanese knotweed, and flowering is less prolific.
The role of true Fallopia sachalinensis seedlings in the spread of the plants in the UK has received little discussion, though it is likely to be insignificant. Substantial genetic diversity in the plants has been noted, though this is likely to have arisen through multiple introductions of the species from various parts of the native range, rather than through home-produced seedlings.
Fallopia sachalinensis spreads by the extension of the rhizome system, and is mainly dispersed by the transport of portions of rhizome to fresh sites. It has not shown the destructive competitiveness of Japanese knotweed. Its true importance is as the main pollen parent of the hybrid, Fallopia x bohemica, whose hybrid vigour may render it the greatest threat, so far, of all.