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Japanese knotweed

(Fallopia japonica)

Japanese knotweed © Laurie Campbell

Japanese knotweed © Laurie Campbell


A non-native invasive plant.

Japanese knotweed inhabits both urban and rural areas, for example it can dominate along road verges, railway land but waste ground, or heavily disturbed ground are particularly vulnerable to infestations. It is also densely colonises along including river banks, woodlands, grasslands and coastal habitats. It grows through walls, tarmac and concrete and causes huge problems wherever it grows.

This plant is one of the species we keep track of in our Wildflowers Count survey - click here to find out how you can help out.

Habitat

It can colonise most habitats, including river banks, woodlands, grasslands and coastal habitats.

What's the problem?

Japanese knotweed is one of the most pernicious weeds in Britain. It reproduces from tiny fragments of rhizome and any soil contaminated with Japanese knotweed fragments must be disposed of at registered sites to prevent its spread. In 2003, the Government estimated that it would cost £1.56 billion to control this plant across the country. It is considered more of a problem in human-dominated environments such as urban areas and railway lines than in natural habitats.

A hybrid of this knotweed Fallopia japonica x Fallopia sachalinensis is also considered invasive and has received a ***** critical risk rating from the Rapid Risk Assessment.

Removing Japanese knotweed

There is no quick solution for the eradication of Japanese knotweed. Effective control can be achieved if Japanese Knotweed is cut or sprayed in early summer, and then sprayed again in late summer, just before the winter dieback. A systemic weedkiller for dicotyledons should be used. It will take about three years until complete dieback and destruction of the rhizome has been achieved. Digging up Japanese knotweed can be attempted, but this must be done very carefully to avoid spreading stem fragments and increasing the problem. New plants can grow from a piece of rhizome the size of a little finger and the crown, located at the base of the stem will also produce new plants. The digging should be carried out by excavating the areas immediately surrounding the Japanese Knotweed, ideally to a depth of 3m and to a distance of 7m from the edge of colony. However if this is impractical another chemical-free option is to put a membranous liner over the Japanese Knotweed and re-plant on top of it, although a depth of at least 1 metre of soil is required.

Japanese knotweed is classified as controlled waste and must be properly disposed of. It should never be composted but if possible burnt on site. If it has to be removed from site, follow Environment Agency guidelines and ensure very careful removal from the site in sealed bags to take to a licensed landfill site. Always inform the waste site manager that the material contains Japanese knotweed.