What is Japanese Knotweed?

Japanese Knot weed (Fallopia japonica) was first introduced to Britain by the Victorians as an ornamental plant – and was actually awarded a gold medal at a prestigious flower show. It is referred to under its previous name of Polygonum cuspidatum in The English Flower Garden by John Murray. In the 1907 edition it is cited as “easier to plant than to get rid of in the garden”.

In its native countries of Japan, North China, Korea and Taiwan, the weed presents nowhere near the problem it now poses across Europe, America and New Zealand. With its natural habitat being on the slopes of volcanoes, it is no surprise that the less harsh and more fertile environment of Britain has allowed this plant to flourish to extreme proportions. Furthermore, outside of Asia, the plant has no natural biological enemies to check its spread. In Japan, for example, at least 30 species of insect and 6 species of fungi live on the plant.

This plant is perennial and extremely invasive. It thrives on disturbance. The tiniest piece can re-grow, and has been spread by both natural means and by human activity. It soon overruns riverbanks, railway embankments, road verges, gardens and hedgerows, threatening the survival of other native plant species and in turn insects and other animal species.

In riparian areas, high water flows disperse fragments of the plant downstream where new colonies form. In the past, fly-tipping and transportation of soil containing rhizome fragments have been a major cause of spread, particularly in the urban environment. Green waste recycling schemes are also sites of potential contamination which is a cause for concern. Local Authorities, including those in Devon, are desperate to find ways of eradicating this serious pest.

Huge sums are being spent in the UK controlling the weed. In 2004, a DEFRA review of non-native species policy stated that a conservative estimate for the costs involved in eradication would be £1.56bn. The aggressive spread of the plant following its first escapes into the wild in the 19th Century resulted in it occurring in most parts of the UK (except Orkney) and eventually being listed in the Wildlife and Countryside Act 1981 as a pest species. All parts of the plant are considered as controlled waste under the Waste Act.  Old stems remain in place as new growth appears


In the early spring red/purple shoots appear from the ground and grow rapidly forming canes. As the canes grow the leaves gradually unfurl and turn green.

The plants are fully grown by early summer and mature canes are hollow with a distinctive purple speckle and form dense stands up to 3 metres high.

The plant flowers in late summer and these consist of clusters of spiky stems covered in tiny creamy-white flowers. These provide a good source of nectar for insects. The seeds are rarely fertile and in Britain the plant spreads mainly by vegetative means.

The canes can arise from the rhizome which grows underground, from an existing crown, where previous growth has taken place, or from a cut stem. During the late autumn/winter the leaves fall and the canes die and turn brown. The canes remain standing throughout the winter and can often still be seen in new stands in the following spring and summer.

The rhizome is the underground part of the plant. It is knotty with a leathery dark brown bark and when fresh snaps like a carrot. Under the bark it is orange or yellow. Inside the rhizome is a dark orange/brown central core or sometimes it is hollow with an orange, yellow or creamy outer ring, although this is variable. Young rhizome is very soft and white. The ‘knots’ are nodes, spaced at 1-2cm intervals where there are often small white fibrous roots or buds emerging. Each of these ‘knots’ can potentially become a new plant if the rhizome is cut up (e.g. through digging).

Invasive species
In the U.S. and Europe, Japanese knotweed is widely considered an invasive species or weed. It is listed by the World Conservation Union as one of the world’s 100 worst invasive species.

The invasive root system and strong growth can damage foundations, buildings, flood defences, roads, paving, retaining walls and architectural sites. It can also reduce the capacity of channels in flood defences to carry water.

It is a frequent colonizer of temperate riparian ecosystems, roadsides and waste places. It forms thick, dense colonies that completely crowd out any other herbaceous species and is now considered one of the worst invasive exotics in parts of the eastern United States. The success of the species has been partially attributed to its tolerance of a very wide range of soil types, pH and salinity. Its rhizomes can survive temperatures of −35 °C (−31 °F) and can extend 7 meters (23 ft) horizontally and 3 meters (9.8 ft) deep, making removal by excavation extremely difficult. The plant is also resilient to cutting, vigorously re-sprouting from the roots. The most effective method of control is by herbicide application close to the flowering stage in late summer or autumn.

Japanese knotweed has a large underground network of roots (rhizomes). To eradicate the plant the roots need to be killed. All above-ground portions of the plant need to be controlled repeatedly for several years in order to weaken and kill the entire patch. Picking the right herbicide is essential, as it must travel through the plant and into the root system below. Glyphosate is the best active ingredient in herbicide for use on Japanese knotweed as it is ’systemic’; it penetrates through the whole plant and travels to the roots.

Digging up the rhizomes is a common solution where the land is to be developed, as this is quicker than the use of herbicides, but disposal of the plant material is difficult, governed by law in the UK, where it is classed as controlled waste.

More ecologically friendly means are being tested as an alternative to chemical treatments. Soil steam sterilization [15] involves injecting steam into contaminated soil in order to kill subterranean plant parts. Research has also been carried out on Mycosphaerella leafspot fungus, which devastates knotweed in its native Japan. Research with Mycosphaerella has been relatively slow, due to its complex life cycle

In the UK, it is an offence under section 14(2) of the Wildlife and Countryside act 1981 to “plant or otherwise cause to grow in the wild” any plant listed in Schedule nine, Part II to the Act, which includes Japanese knotweed. Over £150m is spent annually on Japanese knotweed control, and a decision was taken on 9 March 2010 in the UK to release into the wild a Japanese psyllid insect, Aphalara itadori. Its diet is highly specific to Japanese knotweed and shows good potential for its control .

Knotweed Do’s and Don’ts
To help prevent the spread of this highly invasive plant look at the Do’s and Don’ts in this section. Causing Japanese Knotweed to spread is illegal so it is important to avoid activities which risk this happening.

•Do make a plan to eradicate Japanese Knotweed from your site.
•Do follow the Environment Agency’s Code of Practice available from their Devon Ecological Appraisal Team 08708 506 506 or their website.
•Do use herbicides safely and effectively.
•Do obtain the approval of the Environment Agency prior to treatment if you intend to use a herbicide in or near water on 08708 506 506.
•Do follow the manufacturer’s instructions regarding protective clothing and the safe and effective use of herbicides.
•Do take care to avoid drift, and any damage to non-target plants when applying herbicides. Spraying should be performed during still dry conditions, without rain for 6 hours.
•Do check qualifications – spraying on land which is not your own should be carried out by an approved contractor with a National Proficiency Tests Council Certificate of Competence.

•Don’t flail Japanese Knotweed as this could cause it to spread. Cutting with sharp hooks, slashers. etc or hand pulling is recommended to avoid any dispersal of cut fragments.
•Don’t cause the spread of Japanese Knotweed stem and crowns. If you cut down Japanese Knotweed, it is best to dispose of it on site. Material taken off site must be safely contained and disposed of at a licensed disposal site.
•Don’t try to dig up Japanese Knotweed as this will lead to a significant increase in stem density. Even a tiny fragment of the cut rhizome is capable of regeneration.
•Don’t spread soil contaminated with Japanese Knotweed rhizome. Any soil that is obtained from ground within 7 m of a Japanese Knotweed plant could contain rhizome. The rhizome is highly regenerative and will readily grow into new plants.
•Don’t chip Japanese Knotweed material. Mechanical chippers don’t kill Japanese Knotweed. If you spread the chipped material on soil, Japanese Knotweed could regrow.
•Don’t add Japanese Knotweed to compost. Compost it separately (preferably on plastic sheeting to prevent rooting) so that you can be sure it is dead before you apply it to land.
•Don’t take Japanese Knotweed to recycling centres that receive garden waste as it will contaminate the compost.
•Don’t dump garden waste contaminated with Japanese Knotweed in the countryside.
•Don’t waste time. If Japanese Knotweed appears on your site, treat it immediately. Don’t allow it to become established.
•Don’t break the law. Remember, if you cause Japanese Knotweed to spread you are guilty of an offence under the Wildlife and Countryside Act, 1981.

Advice for Gardeners
•Japanese Knotweed is an attractive plant and was originally brought into the UK in 1825 by the Victorians. Gardeners soon discovered how difficult it was to keep under control and by 1886 the plant had escaped from gardens and had become naturalized throughout the UK.
•Where rhizomes or stems were composted or left on road verges or waste ground the plant started to establish and spread.
•If you suspect you have Japanese Knotweed on your property you should take care not to allow it to spread. Even the smallest piece of rhizome, stem or crown can potentially form a new plant. Compost Japanese Knotweed separately, preferably on strong plastic sheeting so it is not in contact with the ground. Check the compost regularly to ensure it is not sprouting. Ensure that it is fully decomposed before spreading it on the garden. Do not shred or strim the plant as this could cause rapid spread. Mowing is only advised if you have a collecting box for mowings which can then be composted. Do not dig Japanese Knotweed as this is known to increase stem density and it encourages sprouting and spread.
•Hand pulling or cutting the plant is a good method of control but will take several years for the rhizome to be exhausted and die. Leave the material on a plastic sheet to dry and then burn it. Do this on site to prevent spread. The cutting and pulling of stems encourages the plant to send up more shoots which can in turn be pulled.
•You can also use chemical herbicides, glyphosate is recommended but treatment will need to be ongoing and may take several years depending on how established the colony is.
•Avoid digging within 7 meters of a colony of Japanese Knotweed, and avoid moving the soil around the garden as the soil could contain rhizome.
•Do not take Japanese Knotweed material to your local recycling centre, Japanese Knotweed has to be treated as ‘controlled waste’. Do not remove Japanese Knotweed material from the site unless you have made a prior arrangement with a licensed landfill site for deep burial. Treatment on site is the preferred option.

Japanese Knotweed and the Law.
In the UK there are two main pieces of legislation that cover Japanese Knotweed. These are:

Wildlife and Countryside Act 1981
Listed under Schedule 9, Section 14 of the Act, it is an offence to plant or otherwise cause the species to grow in the wild.

Environmental Protection Act 1990
Japanese Knotweed is classed as ‘controlled waste’ and as such must be disposed of safely at a licensed landfill site according to the Environmental Protection Act (Duty of Care) Regulations 1991. Soil containing rhizome material can be regarded as contaminated and, if taken off a site, must be disposed of at a suitably licensed landfill site and buried to a depth of at least 5 m.

An offence under the Wildlife and Countryside Act can result in a criminal prosecution. An infringement under the Environmental Protection Act can result in enforcement action being taken by the Environment Agency which can result in an unlimited fine. You can also be held liable for costs incurred from the spread of Knotweed into adjacent properties and for the disposal of infested soil off site during development which later leads to the spread of Knotweed onto another site.


Ceredigion Council – 01545 570881
Advanced Forestry & Garden Services – 01974 272016
Clean Ground Partnership – 07971 268207

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