Fallopia japonicaJapanese Knotweed
Japanese knotweed

Origin: East Asia

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 (lower photo). 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.

Japanese knotweed

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. 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, it is a very invasive plant, spreading vegetatively and often distributed accidentally in dumped soil containing fragments of rhizomes.

Japanese knotweed 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.
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