Flora of Northern Ireland
  • Fallopia japonica (Houtt.) Ronse Decraene - Japanese Knotweed - Polygonaceae
Fallopia japonica
© Paul Hackney
Fallopia japonica
© Ian Dodkins
Fallopia japonica
(Map updated: March 2008)

This is a conspicuous and invasive alien species with a perennial underground rhizome from which grow, each year, vertical green leafy shoots about 4 - 6 feet tall. These shoots die at the end of the summer but persist as brown withered leafless stems over the winter.

The leaves are large, broadly oval and pointed, usually with a pale stripe down the middle. Flowers are whitish in loose slender panicles arising from the leaf axils. In its native east Asia some plants produce hermaphrodite flowers and some produce female flowers, but only female flowers are produced by plants in the British Isles.

This species is a native of Japan, Korea, Taiwan and China, but it appears that all the plants grown in European gardens derive from a single Dutch import from Japan made in the 1820s and are all of a single clone bearing only female flowers, and are consequently incapable of reproducing by seed. It is not known when the plant was brought to Ireland though it was probably in the middle of the nineteenth century. Its invasive spread has been by the dumping of garden rubbish containing fragments of rhizome.

The plant may be found in old demesnes, parks, old gardens and on waste tips, roadsides and river banks. Although incapable of reproducing by seed because its flowers are female only, these flowers can be fertilized by pollen from the giant knotweed (Fallopia sachalinensis - a related species native to Korea, Japan and Sakhalin,) to give rise to occasional hybrid plants.

The species is a characteristic early coloniser of volcanic soils in Japan.

This account is based on the recent work of A..P. Connolly, M..L. Hollingsworth and J.P. Bailey, including DNA analysis, published in the BSBI's journal Watsonia and elsewhere.

All names: Fallopia japonica (Houtt.) Ronse Decraene; Reynoutria japonica Houtt.; Polygonum cuspidatum Siebold. & Zucc.