Japanese knotweed (Reynoutria japonica syn. Fallopia japonica) is a weed that spreads rapidly. In winter the plant dies back to ground level but by early summer the bamboo-like stems emerge from rhizomes deep underground to shoot to over 2.1m (7ft), suppressing all other plant growth. Eradication requires determination as it is very hard to remove by hand or eradicate with chemicals. New legislation now covers its control – see below.
Latin name Reynoutria japonica (syn. Fallopia japonica, Polygonum cuspidatum)
Areas affected Waysides, beds, borders and paving
Main causes Weed with creeping roots
Timing Seen late spring to autumn; treat in summer
What is Japanese knotweed?
Japanese knotweed is a fast-growing and strong clump-forming perennial, with tall, dense annual stems. Stem growth is renewed each year from the stout, deeply-penetrating rhizomes (creeping underground stems). Although orginally introduced to Britain as an ornamental garden plant, Japanese knotweed is an invasive non-native species and this page looks at how it can be controlled.
In spring, reddish-purple fleshy shoots emerge from crimson-pink buds at ground level. These grow rapidly, producing in summer, dense stands of tall bamboo-like canes which grow to 2.1m (7ft) tall. These canes have characteristic purple flecks, and produce branches from nodes along its length.
Leaves are heart or shovel-shaped and up to 14cm (5½in) in length and borne alternately (in a zig zag pattern) along the stems. The stems die back to ground level in winter, but the dry canes remain for several months or longer.
The creamy-white flower tassels produced in late summer and early autumn reach up to 15cm (6in).
Identification is important. Japanese knotweed can be confused with other plants including:
Fallopia baldschuanica (Russian vine)
Leycesteria formosa (Himalayan honeysuckle)
Persicaria microcephala (e.g. P. microcephala 'Red Dragon')
Several species of Persicaria and Polygonum, including Persicaria lapathifolia and P. maculosa can also be troublesome weeds but are not as invasive. Additionally it should be noted that a less troublesome form of Japanese knotweed is grown in gardens, Reynoutria japonica var. compacta and its cultivars.
See the GB Non-native species Secretariat (NNSS) identification sheets for invasive non-native species including Japanese knotweed.
Members of the Royal Horticultural Society can have knotweed identified via images sent to RHS Gardening Advice. We are unable to accept plant samples.
Although it rarely sets seed in this country, Japanese knotweed can sprout from very small sections of rhizomes. Under the provisions made within Schedule 9 of the Wildlife and Countryside Act 1981, it is an offence to cause Japanese knotweed to grow in the wild.
Fly tipping should be reported to The Environment Agency, free-phone number 0800 807060.
The legal situation
Buying and selling property
Since 2013, the seller is required to state whether Japanese knotweed (Fallopia japonica) is present on their property through a TA6 form - the property information form used for conveyancing. Your conveyancer or solicitor will be able to provide full legal advice, however, here is a summary:
- If you are selling, it is your responsibility to check the garden for Japanese knotweed (bearing in mind that it can die back in winter). The TA6 form asks you to confirm whether your property is affected by Japanese knotweed and, where it is, to provide a management plan for its eradication from a professional company (see Seeking help from the professionals below)
- If you are buying, the presence of Japanese knotweed will be stated in the responses to the TA6 form. This often results in your mortgage lender requiring assurances that it will be eradicated before agreeing the funds. A management plan by a professional eradication company, backed by a transferable guarantee, is usually sufficient. It is most common for this plan to be provided by the seller before the purchase is completed
- Whether a buyer or seller, it is also worth being pro-active and checking the property for Japanese knotweed. Disputes over the identity of a plant, the failure to disclose its presence, or the lack of a management plan can result in delays, increased costs later in the buying process, or even a possible misrepresentation claim after the sale, so this approach will help avoid problems
Contact The Royal Institute of Chartered Surveyors for further information.
An amendment to the Anti-social Behaviour, Crime and Policing Act 2014 includes invasive non-native plants including Japanese knotweed. Here are some key points for how this affects the homeowner:
- It is not illegal to have Japanese knotweed in your garden, but on your property you should aim to control this invasive non-native plant to prevent it becoming a problem in your neighbourhood. If it has a "detrimental effect of a persistent or continuing nature on the quality of life of those in the locality", the legislation could be used to enforce its control and property owners may be prosecuted
- Where problems with Japanese knotweed occur in neighbouring gardens, we suggest that you speak or correspond directly with your neighbours (who may already be taking action to control this difficult weed). These informal steps should be taken before contacting your council to talk about action under the legislation
- Homeowners can consider control themselves for a small, isolated clump (see the Control section below). However, a specialist professional company will be skilled at control, ensure eradication and can dispose of the plant waste at licenced landfill sites
For more information see The Environment Agency Information Note: Japanese knotweed.
When tackling Japanese knotweed, cultural control methods pose some problems.
- Digging out this deeply penetrating plant without professional help, even if feasible, creates problems over disposal as Japanese knotweed is classed as 'controlled waste' under the Environmental Protection Act 1990. This requires disposal at licensed landfill sites
- Specialist Japanese knotweed contractors must be registered waste carriers to safely remove the weed from site but check first before employing their services
- Alternatively, it can be destroyed on site by allowing it to dry before burning
On no account should Japanese knotweed be included with normal household waste or put out in green waste collection schemes.
It usually takes at least three to four seasons to eradicate Japanese knotweed using weedkiller. Professional contractors, however, will have access to more powerful weedkiller that may reduce this period by half.
When using weedkiller, always follow the instructions on the pack to make effective and economic use of the product while minimising risks to people and the environment.
- For home gardeners, perhaps the most effective and simplest method to tackle Japanese knotweed is with a glyphosate-based weedkiller such as Roundup Stump Killer. This has label recommendation for controlling Japanese knotweed, instructing it to be applied to the cut canes or a foliar spray. Roundup Ultra also has label control for this weed
- Alternatively, try other tough formulations of glyphosate (e.g. Doff Weedout Extra Tough Concentrate or Westland Resolva Pro Xtra Tough Concentrate)
- Glyphosate-treated knotweed will often produce small-leaved, bushy regrowth 50-90cm (20in-3ft) in height the following spring. This is very different in appearance to the normal plant and it is essential that this regrowth is treated
- Bear in mind that the home gardener will not get an insurance backed guarantee without using a professional company for the control of Japanese knotweed. This may be important if planning to sell your property in the near future or if a neighbour is threatening litigation from the spread of knotweed from your property
When using glyphosate take care to avoid leaves and other green parts of all garden plants as it is not selective in action. Used with care, glyphosate is safe to use around the base of non-suckering woody plants, as long as the bark is woody, brown and mature. Glyphosate is not active through the soil and there is therefore no risk garden plants will absorb it through their roots.
Inclusion of a weedkiller product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.
Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners; see section 4)
Seeking help from the professionals
Professional companies offer Japanese knotweed removal. They can report on risk for mortgage purposes with suggested treatment plans and offer insurance-backed guarantees where required.
Royal Institution of Chartered Surveyors (RICS) has worked with the Property Care Association (PCA) to establish the PCA Invasive Weed Control Group (IWCG) trade body for Japanese knotweed specialists, which provides a register of vetted consultants and contractors.
Invasive Non-native Specialists Association (INNSA) maintains a membership list of contractors and consultants phone 0800 1300 485.
The British Association of Landscape Industries (BALI) has a directory of members offering invasive weed control.
Trustmark Government Endorsed Standards has a 'Find a Tradesman' scheme including invasive weed controllers.
See our page on hiring contractors for more guidance.
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.