Invasive Alien Species in Northern Ireland

Fallopia sachalinensis, Giant knotweed

Fallopia sachalinensis (F.Schmidt)
A vigorous hardy herbaceous perennial similar to the commoner Japanese knotweed (F. japonica) found on waste ground, river banks, lakesides, old gardens, etc. Introduced into Ireland as an ornamental for large gardens.
Similar to Japanese knotweed. The plant has an extensive underground rhizome system from which new herbaceous shoots arise each April and which can reach four or more metres in height. These die back completely each winter, leaving a thicket of dried canes. Living stems are hollow with few branches and bearing leaves which are large, heart-shaped and pointed, up to about 43cm long by 17cm broad. Flowers are small, white, attractive, borne in terminal clusters from July onwards, and are functionally either male or female. Individual plants produce flowers of one sex only.
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.
Country of origin
It is native to the islands of Sakhalin (Russia), the southern Kuril chain, and Hokkaido and Honshu (Japan).
Current distribution
Widely naturalised across Ireland and Great Britain. Also in many European countries, North America and Australia.
Location in Ireland
The earliest report of this plant outside gardens anywhere in Ireland or Great Britain dates from 1896, at Lisburn, County Antrim. In 1901 or 1902 it was reported from Howth, County Dublin. It is now frequent across Ireland in a variety of situations, some of which are old demesnes (like Gosford Park, County Armagh) where it must have been planted originally as an ornamental. It is much less common, however, than Fallopia japonica.
Life cycle
The plant is a conventional sexually reproducing species in Ireland, with separate male and female flowers on separate male and female plants. It is not known, however, just how effective spread by seed has been in Ireland. It is most probable that spread to new sites has been by dispersal of fragments of rhizome or stem in soil, which are capable of forming new plants. Local expansion of colonies is by vegetative growth of the rhizomes, and as a result colonies can cover large areas of ground.
The male plant is capable of fertilising the female Japanese knotweed (Fallopia japonica) clone which occurs in Ireland to form a hybrid (Fallopia x bohemica) which has been recorded in a few sites in Ireland in similar habitats to those occupied by the parent species. There is evidence that this hybrid is even more vigorous than either of its parents.
Wildlife and habitat impacts
Giant knotweed impacts on habitats in the same way as Japanese knotweed. It forms dense stands which obliterate virtually all other plant species (though bluebells can survive inside such stands since they grow and flower before the summer shoots grow fully). The dieback each winter leaves large areas of bare ground and in riverside situations this could result in river bank erosion – this has been reported in Belgium.
In the French Alps it colonises alluvial deposits of alpine rivers along which spread is by dispersal of rhizome fragments. The plant is classed as a noxious weed in California, USA and as a State Prohibited Weed in Victoria, Australia.
Human impacts
There are no directly adverse effects on man, except the damage to habitats and gardens where the plant gets out of hand. The species has a number of economic uses, such as an animal and human food and medicinal uses, but neither of these are exploited in Ireland.
Knotweed can cause damage to buildings and other structures and therefore can have economic as well as environmental implications.
Key vectors
Research to date suggests that the primary vector is contaminated soil containing rhizome fragments, or by natural water-borne spread of rhizome fragments from riverbank colonies.
What you can do as an individual
No individual action recommended
Advice to specific groups relevant to this species
Landowners or land managers, and contractors, should ensure that soil from colonies of giant knotweed is not dumped at new sites, thereby spreading the plant.
Management measures
Currently the most effective method of control is repeated spraying with herbicides over a number of years, which gradually reduces the vigour of the plant. This is carried out in early autumn, when the herbicide in thought to have the most impact on the plant. New sites and larger stands may also be sprayed in early summer as well, to stunt the growth before the autumn spraying. (Quoted from the web site of the Exmoor Knotweed Control Project).
Further information
Exmoor Knotweed Control Project
CABI Bioscience page on Fallopia including information on hybridisation
NeoFlora – Invasive gebietsfremde Pflanzen in Deutschland (produced by
Bundesamtes für Naturschutz , Bonn)
Cornwall Knotweed Forum: Descriptions, photographs and comparisons with F. japonica
Lousley, J.E. and Kent, D.H. (1981). Docks and Knotweeds of the British Isles. BSBI Handbook No. 3. Botanical society of the British Isles, London.
Text written by:
Paul Hackney, Keeper of Botany, Ulster Museum