Invasive Alien Species in Northern Ireland

Fallopia japonica, Japanese Knotweed

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Fallopia japonica
© Paul Hackney
Fallopia japonica
© Ian Dodkins
Fallopia japonica
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Fallopia japonica (Houtt.) Ronse Decraene, Houtt., Siebold. & Zucc.
This species was introduced into Irish gardens probably in the middle of the nineteenth century. It is incapable of spreading by seed, but is nonetheless highly invasive and ecologically damaging.
The plant may be found in old demesnes, parks, old gardens and on waste tips, roadsides and river banks. It is an early coloniser of volcanic soils in Japan.
This is a conspicuous and invasive herbaceous perennial species with an underground rhizome from which grow, each year, vertical green leafy shoots about 1.5 - 2.5m 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 are hermaphrodite and some are female, but only female plants grow in Ireland.
Country of origin
A native of Japan, Korea, Taiwan and China, but it appears that all the plants grown in our gardens derive from Japan.
Location in Ireland
It is widespread and common throughout Ireland, on river banks, in parks and demesnes, or on waste ground.
Life cycle
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 out of gardens has been brought about by the dumping of soil or garden rubbish containing fragments of rhizome.
Wildlife and habitat impacts
The plant forms extensive monocultures which virtually obliterate other plant species, except for the early flowering species such as bluebell (Hyacinthoides non-scripta).
Key vectors
It is spread entirely by the movement or dumping of soil containing fragments of rhizomes.
What you can do as an individual
The Wildlife (Northern Ireland) Order 1985 makes it an offence to cause this species to grow in the wild. Waste containing Japanese knotweed should be disposed of in accordance with this Order.
Management measures
Attempting to get rid of stands of Japanese knotweed by digging up or cutting the plant rarely succeeds unless combined with herbicide applications. Fragments of the rhizomes or aerial shoots can regenerate, so must be destroyed by burning. Riverside colonies may spread by fragments floating downstream. The Centre for Aquatic Plant Management (CAPM) recommends control by herbicides as the best option.
Transport of soil away from the site containing fragments of Japanese knotweed should be avoided; it might introduce the species to uninfected sites.
Further information
Centre for Aquatic Plant Management, Information Sheet 5
Wildlife Protection Order
The Habitat (Water Fringe) Regulations 1994
See the Invasive Species Ireland web site for further information -
Text written by:
Paul Hackney, Keeper of Botany, Ulster Museum