SNPA Strapline

Invasive Species

Japanese Knotweed

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Japanese Knotweed (Fallopia japonica)

Japanese Knotweed is an invasive plant in a league of its own. It is a major nuisance in Britain and in many other parts of the world where it has been introduced. The vigorous growth overwhelms and displaces native plants. The plant also causes serious economic damage: dead stems can cause blockages to watercourses that lead to flooding, and the shoots are so strong that they can penetrate tarmac and cause structural damage to buildings.

Each spring, the plant produces a dense clump of hollow stems with heart-shaped leaves from a perennial root system (rhizomes). Clusters of white flowers are produced at the end of summer.

Knotweed can survive in a wide variety of habitats and is especially common along linear habitats such as rivers and roads, and on ungrazed ground. The key feature to its success is the way it can reproduce from tiny pieces of root or stem broken off and transported from the parent plant. Human activity is generally involved. Earth moving activities on a site with Japanese Knotweed will soon disperse fragments around the site, and all too often off the site too through deliberate movement of contaminated soil, or, accidentally, by fragments caught up in the tracks and tyres.  Flail mowing produces and scatters viable fragments. Rivers also disperse the plant by carrying eroded material downstream especially in floods.

Japanese Knotweed is such a serious problem that it is illegal to plant it or to cause it to grow in the wild. A license is required to transport soil contaminated with Japanese Knotweed. It can then only be taken to designated disposal sites.

Control Strategy

Blanket control of Japanese Knotweed would be prohibitively expensive. The emphasis has to be on preventing further spread. Active control is mainly undertaken where it is causing a problem or where early action is likely to prevent colonisation of large areas (for example the headwaters of an otherwise uninfected river). Treating whole river catchments by working downstream, progressively eliminating the knotweed, gives the best chance of long-term success.

Control

Small clumps in gardens - For small areas, repeated physical cutting of the shoots and/or careful excavation of the root system will eventually eliminate the plant. The material removed should be thoroughly dried and then burnt. It must not be composted, or taken away from the site. Herbicide treatment is the other main option.

Larger stands - Herbicide application is the most practicable solution. Treatment is most effective at the end of the summer but the growth may also need to be cut or sprayed earlier in the season if the stems would otherwise become too tall to spray effectively. Repeat treatments are always necessary. Details of herbicide techniques are given in the links below. Use of herbicide in riparian areas will require permission from the Environment Agency.

Development sites - Where Japanese Knotweed is present on a development site, Snowdonia National Park Authority must be satisfied that appropriate management is in place before planning permission is granted. Total eradication of knotweed using herbicides is rarely achievable in less than three years. Other options such as burial or removal of soil from site can be very costly. It is strongly recommended that a management plan is drawn up and control is undertaken as early as possible.