Targeted Conservation

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Giant knotweed

Fallopia sachalinensis

 

Species and Names

Species
Fallopia sachalinensis, formerly known as Polygonum sachalinense

Family
Polygonaceae


Description

Giant knotweed is a hardy, herbaceous, rhizomatous perennial. Plants are dioecious, with some plants functionally female (male-sterile) and some male-fertile. Leaves are 15-40cm long by 10-22cm wide; dock-like, ovate-cordate, glabrous except for scattered, long, wavy trichomes on the paler undersides; tips somewhat acuminate. The stems are stout, hollow, up to 5 metres in height; often reddish-brown, little branched. The flowers are a greenish-white; male-fertile examples with exserted anthers, male-sterile flowers with well-developed stigmas. The panicles are dense, to 10cm and fruits are 4-5mm long.

Identification

 

Fallopia sachalinensis leaf shape and size

Fallopia sachalinensis in a typical Victorian Garden location - Scotland

Huge leaves (compare to aviators) similar to Japanese knotweed, bamboo-like stems much straighter than Japonica

Image courtesy of Jan Samanek, Czech Republic
Image courtesy of Tom Heutte, USDA Forest Service, www.forestryimages.org

General Information

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 Fallopia japonica var. Japonica. 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.