Japanese Knotweed

Japanese Knotweed was introduced into our gardens early in the 19th century. It was recommended as being suitable for wild gardens and shrubberies, but gardeners must have soon become fed up with its invasive nature and dumped it over the garden fence. It was first noted in the wild in London in 1900, and from there it has spread rapidly through Britain.  Once established it is very hard to eradicate.

On a more positive note, it can act as a shelter for woodland species like Celandines, which flower and seed before the Japanese Knotweed develops in Spring. The flowers also come late in the season, and are likely to be a valuable nectar source for insects. However, conservationists are generally agreed that the harm caused by the plant to valuable wildlife habitats, especially along watercourses, outweighs these benefits.

Japanese Knotweed is a herbaceous perennial - it dies back every year but emerges again from underground roots, or rhizomes, in late spring. The new stems are thick, hollow and mottled red on green, and grow at a phenomenal rate to make dense thickets. Its frothy mass of tiny, creamy green flowers appear in late in summer and autumn. The whole plant dies back after the frosts, but the bare yellowy-brown stems remain conspicuous into winter.

To prevent Japanese Knotweed from damaging important habitats, it is necessary to control or remove it. This is not easy, because it can regenerate from cut stems and tiny pieces of rhizome. There is no simple way to eradicate it. Attempting to dig out the plants either by hand or JCB usually makes the situation worse, unless the process is strictly controlled or is followed up by herbicide treatment. Cutting or pulling stems is not effective against large clumps, and using herbicides can cause harm to adjacent habitats.

Small stands and plants growing amongst other vegetation are best kept under control by repeated pulling of stems. Ideally this should be done several times a year.

Larger clumps will need repeated herbicide treatment. Combined with controlled digging over, this can be very effective. Close to watercourses, the only two herbicides that can be used are approved formulations of Glyphosate or 2,4-D amine; on a nature reserve, Glyphosate is preferable since it is less toxic to other species and is quickly broken down. Permission from the Environment Agency is needed if spraying within 10 metres of a watercourse.

If taken off site, the rhizome (underground stems), stems (above ground growth) and soils in which it grew must be taken to a landfill site, that is licensed to accept such waste. On site, rhizomes and living stems can either be buried deeply (this is not usually possible as the recommended depth is a minimum of 5 metres) or burned. Dead stems can be left to rot down.

Under the Wildlife and Countryside Act 1981, any person who plants Japanese Knotweed or otherwise causes it to grow in the wild is guilty of an offence. Under the Environmental Protection Act 1990, Japanese Knotweed is classed as controlled waste and must be disposed of safely at a licensed landfill site according to the EPA (Duty of Care) Regulations 1991.

• Soil containing rhizome material should be regarded as contaminated and if removed from site must be disposed of at a licensed landfill site where it will be buried to a depth of at least five metres.
• You could face prosecution for failing to prevent the spread of Japanese Knotweed to your neighbouring property
• Inappropriate management practices, or the spread of waste and soil containing Knotweed, should be reported to the local council.

Should queries relate to private land the enquirer should be directed to the Environment Agency who publish the following:

• Japanese Knotweed, Guidance for Developers & Hauliers
• Japanese Knotweed, Guidance for Householders & landowners