Japanese knotweed was introduced into Britain in the 19th century as an ornamental plant. Over time it has become widespread in a range of habitats, including roadsides, riverbanks and derelict buildings. It outcompetes native plants and animals.
What Japanese knotweed looks like
- lush green in colour
- shovel shaped leaves
- a stem that's bamboo like in appearance
- produces white flowers around September or October
- can grow by 10cm a day
How Japanese knotweed spreads
It spreads through its crown, rhizome (underground stem) and stem segments, rather than its seeds. The weed can grow a metre in a month and can cause heave below concrete and tarmac, coming up through the resulting cracks and damaging buildings and roads. Studies have shown that a 1cm section of rhizome can produce a new plant in 10 days. Rhizome segments can remain dormant in soil for twenty years before producing new plants.
What to do if you see Japanese knotweed
If you have Japanese knotweed on your land you may be causing a private nuisance to surrounding properties. Using our guidance you should control the Japanese knotweed to prevent further spreading.
If Japanese knotweed on a neighbouring property is causing a nuisance to you, we would always recommend that you co-operate with the landowner and seek to control the problem amicably, rather than resort to legal action. This is an issue under Common Law and the Environment Agency has no powers in this situation.
The Wildlife and Countryside Act 1981 states that it is an offence to "plant or otherwise cause to grow in the wild" any plant listed in Schedule nine, Part II of the Act. This lists over 30 plants including Japanese knotweed, giant hogweed and parrot's feather. The police are responsible for investigating this offence and each police force has a wildlife liaison officer who can be contacted.
The Environment Agency are responsible for ensuring that knotweed waste is managed and disposed of in accordance with the knotweed code of practice.
Cornwall Council have produced a guide which explains who is responsible for the legal enforcement of invasive plants. This can be downloaded from their website.
How to control Japanese knotweed
If you wish to remove the Japanese knotweed before the area is used for development you should closely follow the advice we provide in the knotweed code of practice, or employ a contractor to do so for you.
If you are using a contractor to remove the waste for you, they must be registered with us as a waste carrier.
To find a contractor to remove Japanese knotweed, and other invasive plants, please search our Waste Directory:
If you don't need to remove the knotweed urgently, a combination of herbicide treatment and careful excavation should eventually remove the problem.
Japanese knotweed is sensitive to a range of herbicides. The most effective time to apply herbicides to Japanese knotweed is in late Summer. This is much more damaging to the underground rhizome system than applying herbicides in Spring.
A qualified person should carry out the treatment and contractors must have a National Proficiency Tests Council (NPTC) certification. Our permission is needed before herbicides can be used in or near watercourses.
Information on getting our permission to use herbicides is on our invasive species page:
Herbicide treatment may have to be used for at least three years before Japanese knotweed stops growing back. Even when the plant stops growing back, any soil removed from the area is likely to have dormant rhizome and must be disposed of as described within the knotweed code of practice.
Site trials have shown that combining digging and herbicide treatment is more effective in controlling Japanese knotweed than just applying herbicides to the plant. This because the digging breaks up the rhizome, which stimulates leaf production, making the plant more vulnerable to herbicide treatment.
Cutting can be used to reduce underground biomass. Treating fully-grown stems in late summer provides the most effective herbicide control. If it is not possible to treat the mature stems (eg because of spray-drift), the stems can be cut in the spring and the re-growth treated in late summer. This reduces the height of the canes. The plant should be cut at the base of the stem. Cutting methods that produce fragments, such as flailing, should be avoided as just a small part of the stem can produce a new plant.
Studies have shown that with four cuts a year the plant loses vigour and underground biomass. The first cut should be carried out when the first shoots appear and the last cut should be done when the plant before it dies back in the autumn (September or October). Annual cutting will be required. Cut stems should be thoroughly dried before they are burnt or taken to landfill.
Whenever japanese knotweed is moved from a site precautions must be taken to ensure that it doesn't spread to other areas. It will also need to be disposed of properly.
Disposing of Japanese knotweed
Contact your local authority to see if they offer a collection service or have a list of sites which can accept Japanese knotweed waste:
Find a site in your area which accepts japanese knotweed waste by using our Waste Directory to search for sites accepting invasive weeds (eg Giant hogweed):
Otherwise you can look in the Yellow Pages or local directory to see if there are waste management sites or a waste carrier in your area who can take it. You will need to check that they are authorised to take the waste.
Japanese knotweed waste can be burnt on site under controlled conditions. If the waste is burnt, such burning must take into account any local by-laws for nuisance or pollution that may occur as a result of the activity. Anyone working on the site should use protective clothing and face visors.
Businesses who intend to burn Japanese knotweed waste should inform us before any burning takes place by calling 03708 506 506*.
Any businesses burning Japanese knotweed waste may need a D7 exemption for burning waste in the open.
Private individuals who intend to burn Japanese knotweed waste don't need to inform us but they should inform the local authority as a matter of best practice.
Soil containing Japanese knotweed material and burnt remains of Japanese knotweed may be buried on the site where it was produced. The material should be covered with a root barrier membrane and then buried at least 5 metres deep with inert fill or topsoil.
Anyone burying Japanese knotweed waste should inform us at least one week before burial takes place by calling 03708 506 506*.
Further advice on managing Japanese knotweed
Further advice on managing Japanese knotweed in your garden is available from the Cornwall Knotweed Forum website:
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