Japanese knotweed

Japanese knotweed (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.

Japanese knotweed

Quick facts

Common name Japanese knotweed
Latin name Fallopia japonica (syn. 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).

Appearance

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)
Houttuynia cordata
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, Fallopia 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.

Japanese knotweed is a perennial weed, producing tall canes, up to 2.1m (7ft) in height during the summer. The canes have characteristic purple flecks, and produce branches from nodes along its length. These branches support shovel-shaped leaves.
Bare stems typical of Japanese knotweed's appearance in winter
    Japanese knotweed is a perennial weed, producing tall canes, up to 2.1m (7ft) in height during the summer. The canes have characteristic purple flecks, and produce branches from nodes along its length. These branches support shovel-shaped leaves. Bare stems typical of Japanese knotweed's appearance in winter

    The problem

    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.

    New legislation

    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.

    Control

    Non-chemical controls

    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.

    Chemical controls

    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 Scotts Roundup Tree Stump & Rootkiller. This has label recommendation for controlling Japanese knotweed, instructing it to be applied to the cut canes. Bayer Garden Super Strength Weedkiller also has label control for this weed
    • Alternatively, try other tough formulations of glyphosate (e.g. Scotts Roundup Ultra, Bayer Garden Rootkill, Doff Maxi Strength Glyphosate Weedkiller or Westland Resolva 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

    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.

    Download
    Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners; see sections 3, 4 and 5)

    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.

    The Royal Institute of Chartered Surveyors 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 48.

    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.

    Links

    Chemicals: using a sprayer
    Chemicals: using safely and effectively
    Chemicals: using spot and broad-scale weedkillers
    Weeds: non-chemical control

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    • trueglory

      By trueglory on 30/04/2014

      I have just been told I have Japaneese Knotweed in my back garden, I have plans to build a single story extension on part of the area where the knotweed is, work is due to start in 5 weeks time. Do I need to put the building work off and what are my options to eradicate the problem?


    • ClaudiaJ

      By ClaudiaJ on 08/05/2015

      I think it is important for the RHS to add a caveat about managing knotweed yourself; namely, if you plan to sell your property within the next 5 years, you should probably get it treated by a professional as otherwise you may find that mortgage lenders will not lend on the property, even for a small presence that is far from the building. We very nearly faced an intractable problem when selling because we had treated a small stand of knotweed for 3 years and none was present, but we weren't quite into the second knotweed-free growing season; we had to declare it as having historically been present as the freeholder of the property was aware of it and we felt we should anyway. Very fortunately, our buyer had encountered knotweed before and wasn't concerned and happy to take our word for it (plus it became apparent before exchange that we had got into a second season without regrowth). Otherwise we could have been in a catch 22 where the lenders would view us a being blighted by knotweed, but we wouldn't be able to get a guaranteed treatment from professionals because there was no knotweed to treat! So I think the RHS should highlight that homeowners should be careful of self treatment if planning to sell their home in the foreseeable future.


    • PhBh

      By PhBh on 01/09/2015

      I agree with what Claudia has said; removing knotweed stem growth (by whatever method) will leave you in a mess, or the next owners of your property. Property vendors should bear in mind that they are obliged by law to declare the presence of knotweed on the property, even if it's no longer visible. It's worth mentioning that someone who knowingly covers up the existence of Japanese knotweed is likely to find a solicitor's letter on their door mat at some point in the future, claiming for the cost of treatments and guarantees. On another note, there's also a factual error in the text; Japanese knotweed does go to seed but the seeds are infertile as we only have male plants in 'the wild' in the UK. I wonder where we'd be if we had female knotweed plants here too... Trueglory, this is probably way too late, but yes, and contact a specialist who carries the proper accreditations to excavate and remove it.


    • dextrous63

      By dextrous63 on 01/11/2015

      We are in the process of selling our London home and have begun chemical treatment of knotweed through the use of a licensed company. When the stems are cut down, we would like to burn them but London prohibits garden fires, so how do we get round this dilemma since we are not permitted to remove the stems and burn them elsewhere either?


    • Emspe

      By Emspe on 02/12/2015

      Hi, We moved into our house and unfortunately identified JKW in the spring (the surveyors missed the dead canes). Whilst we are currently in discussions with the surveyors we are hoping to initially lay a patio in the contaminated area (right next to the house) and then in 10 years build an extension on the spot. We have had 2 seasons of professional spraying but I was hoping to get some advice on: 1) Should we lay a patio but not over the contaminated area until we have had 4/5years of treatment and it confirmed that there has been no re-growth (will we be okay to build on this spot in 10 years without digging and removing the soil as the roots will be dead?). 2) Pay to dig and sift and re-bury onsite (we have a very long garden so can do so). Whilst this will clear the area I'm concerned that they want to dig down to 4m next to the house, which to me sounds very risky, and then we would have to think about our neighbours as it would be within 3m of their property. If we go with option 2 do we really need to dig to 4m? This sounds excessive as the foundations would not go below 1.6m. How deep does the root network go? Thanks for your help!


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