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Japanese Knotweed - A Guide

Mature Knotweed Plant

Japanese knotweed (Fallopia japonica or Polygonum cuspidatum) is a non-native, invasive species of plant, introduced into the UK as an ornamental garden plant in the mid-nineteenth century. It has since spread across the UK, particularly along watercourses, transport routes and derelict land areas. The vigorous growth can damage buildings, structures and hard surfaces and once it becomes established it can be particularly hard to control. Within its native range, Japanese knotweed rarely causes problems but without its natural enemies it out-competes our native plants and animals. The spread of Japanese knotweed is a serious threat to our countryside, and the native plants and animals that rely upon it. Jim Knight, Parliamentary Under-Secretary for Rural Affairs, Landscape and Biodiversity, stated in a debate in the House of Commons on 11th October 2005 that removing Japanese knotweed from the UK using conventional methods would cost around £1.56 billion.

Description

Japanese Knotweed is a rhizomatous (i.e. produces underground stems) perennial. It has distinctive, hollow, bamboo-like stems, covered in purple speckles which often reach 2-3m in height. The leaves of the mature plant are up to 120 mm in length with a flattened base and pointed tip and are arranged on arching stems in a zig-zag pattern. Giant knotweed (Fallopia Sachalinensis) is also found in the UK. It is similar to Fallopia japonica but grows up to 4-5 metres in height and has much larger, elongated leaves up to 200-400mm in length. Both plants flower late in the season, August to October, with small creamy-white flowers hanging in clusters. The underground rhizomes are thick and woody with a knotty appearance and when broken reveal a bright orange-coloured centre. The rhizome system may extend to, and beyond, a depth of at least 2m and extend 7m laterally from a parent plant.

Plants have distinctive heart shaped leaves

During winter, the leaves die back, leaving orange/brown coloured woody stems. Stem and leaf material decomposes slowly, leaving a deep layer of plant litter. During March to April, the plant sends up new shoots which are red/purple in colour with rolled back leaves. These shoots grow rapidly due to stored nutrients in the extensive rhizome system; growth rates of up to 40 mm a day have been recorded.

Knotweed regrowth from plant litter

Only female knotweed (F. japonica var japonica) plants have been recorded to date in the UK. Although seeds are produced, they are not true Japanese knotweed seeds but hybrids, which rarely survive. This means that in the UK the plant is mainly spread through rhizome fragments or cut stems. Greenhouse trials have shown that as little as 0.7 grammes of rhizome material (10 mm in length) can produce a new plant within 10 days. Cut fresh stems have also been shown to produce shoots and roots from nodes when buried in soil or immersed in water. Rhizome material may remain dormant for long periods, possibly as long as 20 years.

Legalities

It is not an offence to have Japanese knotweed on your land and it is not a notifiable weed. It is an offence to cause the spread of Japanese Knotweed under Section 14(2) of the Wildlife and Countryside Act 1981 (WCA 1981) which states that 'if any person plants or otherwise causes to grow in the wild any plant which is included in Part II of Schedule 9, he shall be guilty of an offence.' Japanese knotweed is one of the plants listed in the Schedule. Anyone convicted of an offence under Section 14 of the WCA 1981 may face a fine of £5,000 and/or 6 months imprisonment, or 2 years and/or an unlimited fine on indictment.

The Environmental Protection Act 1990 (EPA 1990) may also be applicable. It contains a number of legal provisions concerning 'controlled waste' and defines soil containing, or likely to contain, fragments of Japanese Knotweed plant material as 'controlled waste'. Anyone holding, treating, keeping or disposing of such waste are required to do so in an appropriate manner, without causing any pollution of the environment or harm to human health. Waste must be handled responsibly and in accordance with the law at all stages between its production and final recovery or disposal. Soil suspected of containing Japanese Knotweed must be taken to an appropriate landfill facility were it will be buried down to a depth of at least 5 metres. It must not be reused for construction or for landscaping purposes.

Allowing Japanese knotweed to grow onto other peoples property may be regarded as a private nuisance under common law, but this would be a civil matter.

Responsibilities

Managing Japanese knotweed is the responsibility of the owner/occupier of a site. Neither the Environment Agency nor the Local Authority is responsible for controlling Japanese knotweed, other than that growing on their own land. If the Knotweed is affecting your land the best solution is to co-operate with your neighbours to try and control the problem together, by sharing costs or labour, for instance. Wherever possible it is best to encourage cooperation and support within the community to control and prevent any further spread of the weed.

The Environment Agency is responsible for regulating waste and may take enforcement action if the law is not complied with. They also give approvals under the Control of Pesticides Regulations 1986 for use of pesticides in or near water.

Removal and treatment

Given its aggressive nature, effective treatment of Japanese Knotweed can take years. It is susceptible to certain herbicides including glyphosate, which is found in the common weedkiller 'Roundup biactive'. Glyphosate is a translocated herbicide, which means the weed takes the herbicide down into its rhizome (roots). Other chemicals that are also effective include triclopyr, imazayr, and picloram. It will take at least three years of herbicide treatment before the Knotweed has been fully eradicated. The Control of Pesticides Regulations 1986 require any person who uses such chemicals to take all reasonable precautions to protect the health of human beings, creatures and plants, safeguard the environment and in particular avoid the pollution of water. For application of pesticides in or near water approval from the Environment Agency must be sought before use. Failure to do so may result in legal action.

If herbicide is not used cutting, mowing or grazing of the plant will gradually weakens it but this method may take up to 10 years to be effective. Cutting should be undertaken at least once a month during the growing season. Pulling is more effective because it removes the crown and some of the rhizome. Any plant material arising from such activities must be disposed of in the appropriate manner, as discussed above.