Treating and removing Japanese knotweed

Japanese knotweed is controlled in one of two main ways: a herbicide programme run over several years, or excavation, in which the plant and the surrounding soil are dug out and removed. Sifting the soil, burial on site and root barriers are used in some cases.

Several of the rules below apply in England only; Wales, Scotland and Northern Ireland have separate guidance.

Herbicide treatment

In the typical programme the PCA describes, treatment continues until regrowth stops and the site is then monitored before a completion certificate may be issued. Quoted timescales vary because they measure different things. GOV.UK guidance for England says treatment usually takes at least 3 years, while RICS puts the minimum at four years of treatment and monitoring. The PCA describes that minimum as two years of each, and says herbicide-based management plans run for about 4 to 10 years.

Research cited by RICS (Jones and others, 2018) found control readily achievable for stands under 100 m² when it is carried out properly for long enough, normally 3 to 5 years.

Herbicide does not always kill every piece of rhizome. RICS notes that some treatments can push it into dormancy, and later disturbance, such as digging foundations, can then trigger regrowth. The PCA adds that regrowth after treatment can be stunted to a few centimetres, with small, narrow leaves that are easily missed, which is why the monitoring years matter.

DIY treatment in England

GOV.UK guidance allows only approved herbicides, applied by someone who holds a certificate of competence or works under the direct supervision of someone who does. A COSHH assessment may be needed. Protected sites such as Sites of Special Scientific Interest (SSSIs) need permission from Natural England, and treatment near water needs the Environment Agency's agreement, applied for online or by form. The guidance advises against treating knotweed yourself without the right skills and experience, and a DIY programme leaves you without the management plan and guarantee that a mortgage lender may ask for.

Excavation

Excavation removes the visible plant and the rhizome around and below it. RICS says the dig typically extends 2 to 3 metres beyond the visible stand and 1 to 2 metres deep; in its example, a 2 m x 2 m stand might mean excavating an area of 6 m x 6 m, up to 72 m³ of soil. That volume drives the cost of excavation: even a small infestation, RICS warns, can cost several thousand pounds in waste charges alone.

Soil or plant material contaminated with knotweed may be classified as controlled waste. If it cannot be dealt with on site, GOV.UK guidance for England says it must go to a landfill or incineration site with the correct permit, carried by a registered waste carrier. You must check beforehand that the receiving site holds a permit for invasive plant material and tell the Environment Agency where it was sent. Loads must be covered, vehicles cleaned afterwards, and knotweed soil must not be mixed with other surplus soil or sold as topsoil. The PCA regards at least two full growing seasons of follow-up inspection as best practice after excavation or screening.

Sifting and screening

Sifting, or screening, separates rhizome from excavated soil. In England, the Environment Agency's regulatory position statement RPS 178 allows screened soil to be reused only on the original site, in a restricted area, and never within 50 metres of watercourses or protected areas, on boundaries with neighbours, on lawns, gardens or amenity areas, or in places used by people or livestock. That leaves little scope for reuse on most domestic plots.

Burial on site

GOV.UK guidance and RPS 178 allow knotweed to be buried in England on the site where it grew, at least 5 metres deep, or 2 metres if sealed in a geotextile membrane. The membrane must be undamaged, large enough to keep joins to a minimum, securely sealed, UV resistant and able to last at least 50 years. RPS 178 adds that the burial area should have low habitat value, be unlikely to be disturbed and lie more than 7 metres from a neighbouring landowner's site, with a management plan in place. The Environment Agency must be notified in advance; GOV.UK specifies at least one month.

The PCA considers burial impractical for most residential properties, and RICS says that while it may cut waste costs on large sites, it is unlikely to suit standard homes.

Root barriers

A root barrier is a vertical membrane set into the ground to stop rhizome crossing a line, such as a boundary with knotweed close by on neighbouring land. The PCA says a barrier is not normally a complete solution on its own and is usually used alongside other treatment. RICS adds that barriers may not be viable or effective for many homes, because the deep digging involved can be expensive, disruptive and legally complicated around boundaries and party walls. Any guarantee after a barrier goes in depends on a supporting programme of monitoring and herbicide.

Management plans and guarantees

A Japanese Knotweed Management Plan (JKMP) records what a specialist will do and when. RICS says it should contain a description and accurate record of the infestation with a scaled plan, the contractor's details and methods, a treatment schedule updated after each treatment, and a completion certificate once the plan, including a 2-year no-growth period, is finished.

For lending purposes, RICS also looks for costs paid upfront by the current owner, a plan that covers the whole property and transfers to later owners, and an insurance-backed guarantee (IBG) of at least five years after completion, with third-party insurance keeping it valid if the contractor stops trading. According to the PCA, completion is normally combined with a guarantee covering regrowth for 2, 5 or 10 years.

Methods to avoid

The PCA advises against cutting knotweed down and covering it over, and against putting it in council compost. Burning is unreliable: GOV.UK says crowns and rhizomes may survive it, so in England the burial or disposal rules still apply to what is left, and individuals must check with their council that burning is allowed. In England, dead brown canes may be composted on site only if they are cut, not pulled, at least 10 cm above the crown.

Biological control

CABI has been trialling a psyllid insect, Aphalara itadori, as a natural control. Defra approved a colder-climate line for release in 2021, and it was released at a hybrid (Bohemian) knotweed site in April 2023. Early results were encouraging, but this line prefers hybrid knotweed to Japanese knotweed, and in its March 2024 update CABI said it planned to look for a line suited to Japanese knotweed itself. For now this is research, not a treatment you can arrange for a home or garden.

Choosing a specialist

If you are not yet sure the plant is knotweed, start with how to identify it or the indicative photo check.

RICS says further investigation should be carried out by an accredited member of a recognised trade body, and as of 2022 it names two: the Property Care Association (PCA) and the Invasive Non-Native Specialists Association (INNSA). You can check a company on the PCA member search or the Find Member search on the INNSA website. The PCA's find-a-specialist tool can be filtered by invasive weed control.

Questions to ask a specialist

• Which trade body do you belong to, and does your membership cover invasive weed control?
• Which method do you recommend for this stand, and why?
• How long will the programme run, including monitoring?
• Who will apply the herbicide, and do they hold a certificate of competence?
• In England, if the stand is near water, who will obtain the Environment Agency's agreement?
• If soil leaves the site, which registered waste carrier and permitted site will you use?
• Will I receive a written management plan with a scaled plan, treatment schedule and completion certificate?
• How long is the guarantee, is it insurance-backed, and does it transfer to a buyer?
• What happens if it grows back during or after the programme?

Every specialist we work with is a PCA member whose membership we have confirmed, and any survey, quotation or work is agreed directly with the specialist you choose.