Japanese Knotweed

Waterford City & County Council has commenced a project to control Japanese Knotweed in public open space and on public roads.

What is Japanese Knotweed ?
Japanese knotweed is a non-native invasive perennial (plant that lives more than one year) species in Ireland. Since it was introduced as an ornamental plant in the 19th Century from Japan, it has spread across the UK and Ireland, particularly along watercourses, transport routes and infested waste areas. It is rhizomatous (produces underground stems) with distinctive branched hollow, bamboo-like canes that can grow to over 3m in height. Red/purple shoots appear early in spring but as the canes grow, the leaves unfurl and the plant turns green. The mature canes are hollow and have a characteristic pattern of purple speckles. Flowering occurs in late summer/autumn and consists of creamy white flowers.

How does Japanese Knotweed affect other plants?
Japanese knotweed has vigorously invaded natural habitats and out competes native plants. It forms tall thickets that exclude all other vegetation, shading the area below. Native plants can rarely compete with this invasive species and local plant biodiversity is reduced. Rivers, hedgerows, roadsides and railways form important wildlife corridors for native plants and animals to migrate and disperse along, and large infestations of Japanese knotweed can block these routes for wildlife.

Japanese knotweed can also seriously damage buildings, hard surfaces and infrastructure, but usually only where there are existing weaknesses. Once established underneath or around the built environment, it can be particularly hard to control, growing through concrete and tarmac and other hardstandings if any cracks exist. When Japanese knotweed colonises riverbanks, it can damage flood defence structures and reduce the capacity of channels to carry flood water.

In light of the economic and environmental damage associated with this species, the risk assessment process undertaken as part of the Invasive Species Ireland project identified Japanese knotweed as one of the highest risk (most unwanted) non-native invasive species in Ireland. The Wildlife (Amendment) Act 2000 states that anyone who plants or otherwise causes to grow in a wild state in any place in the State any species of (exotic) flora, or the flowers, roots, seeds or spores of (exotic) flora shall be guilty of an offence. The control of Japanese Knotweed growing on private property is the responsibility of the property owner.

Further information on Japanese Knotweed is available on the Invasive Species Ireland website.

Images of Japanese Knotweed

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Japanese Knotweed
Japanese Knotweed
Japanese Knotweed

Japanese Knotweed

To report Japanese Knotweed
please use our Reporting Tool above

or alternatively email japaneseknotweed@waterfordcouncil.ie
or call 0761 10 20 20