The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner. For example, where pests, diseases or weeds pose a serious threat to the wider environment, to important heritage specimens, to habitat, or to native wildlife.
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 Tree Stump Weedkiller. This has label recommendation for controlling Japanese knotweed, instructing it to be applied to the cut canes or a foliar spray. SBM Job done Tough Weedkiller (soluble sachet only) and Roundup Ultra also have label control for this weed
- Alternatively, try other tough formulations of glyphosate (e.g. SBM Job done Tough Tree Stump Killer (soluble sachet only), Doff Weedout Extra Tough Weedkiller 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.
Chemicals: using a sprayer
Chemicals: using safely and effectively
Chemicals: using spot and broad-scale weedkillers
Weeds: non-chemical control