Japanese Knotweed Removal
Japanese Knotweed is an aggressive, non-native plant notorious for its deep, fast-spreading rhizome root system, which can cause structural damage.
If your property is affected, the problem is not just physical; it is financial. Mortgage lenders will refuse to lend unless you provide clear proof of a professional, fully-funded Knotweed Management Plan (KMP). Tackling knotweed requires specialists because:
- The plant is classed as "controlled waste" under UK law; illegal disposal can lead to heavy fines.
- Eradication must adhere to RICS risk categories to satisfy lender requirements.
- DIY removal is highly ineffective and risks spreading the plant further, which is a criminal offence under the Wildlife and Countryside Act 1981.
How Japanese Knotweed affects a property sale
When you sell or buy a property in England or Wales, the presence of Japanese Knotweed is assessed against the framework established by the Royal Institution of Chartered Surveyors (RICS) and must be disclosed on the TA6 Property Information Form.
RICS Risk Assessment
Lender policy is now based on RICS's current, nuanced guidance (updated 2022), which moved away from the strict "7-metre rule." A surveyor now assesses risk based on the potential for damage and spread, categorising the infestation:
- Categories A & B (High Risk): Knotweed is present on-site and poses a significant or potential risk to structures. A Knotweed Management Plan (KMP) and Insurance-Backed Guarantee (IBG) are mandatory to obtain a mortgage.
- Categories C & D (Lower Risk): Knotweed is either present but far from the property, or found only on neighbouring land. While a KMP may not be immediately required, the lender may still insist on action if the risk is deemed too high.
The TA6 Form
The seller has a legal duty to disclose the presence of Japanese Knotweed on the TA6 form. Lying or answering "Don't Know" when you are aware of the plant can lead to a misrepresentation claim, resulting in significant damages for the seller after the sale is complete.
Biological Control: The UK Government has licensed the use of a Japanese bug (aphalara itadori) to combat the weed.
However, this method is slow and not a commercially viable or available option for residential homeowners seeking to satisfy mortgage lender requirements today.
What are the methods of Japanese Knotweed removal?
Removing Japanese Knotweed requires a professional, specialist approach to satisfy lenders and comply with the law.
The treatment method chosen depends heavily on the site, the size of the infestation, and whether an immediate solution is needed or if a long-term treatment can be waited for.
Chemical Treatment (Herbicide) | This is the most common method. It involves the repeated application of specific herbicides, often glyphosate-based, directly to the plant's leaves or stems. However, this is a slow process; full eradication requires a multi-year programme (typically 3 to 5 years) to ensure the deeply buried rhizomes are destroyed. Professional, repeated chemical treatment can cost £1,000s but is usually covered by the KMP fee. |
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Excavation and Off-Site Removal (Digging Out) | For immediate removal, usually required for development or construction, the contaminated soil and rhizomes are physically dug out. This process must be done to exacting standards; a piece of root the size of a thumbnail (0.8g) is enough to regenerate. Critically, this material is classified as controlled waste under the Environmental Protection Act 1990. It can ONLY be moved and disposed of at licensed landfill sites by a registered waste carrier. Failure to comply puts the owner at risk of a huge fine or even imprisonment. |
On-Site Burial or Containment | Where excavation is necessary but off-site disposal is too costly, professionals may bury the material on site at a depth of at least 5 metres or contain it using specialist geo-textile root barriers. This must be conducted under strict controls and documented fully within the KMP and Completion Certificate to remain compliant with Environment Agency rules. |
What a Knotweed Management Plan (KMP) must contain
If you are buying or selling, a mortgage lender will insist on a fully-funded KMP and an Insurance-Backed Guarantee (IBG) issued by a specialist who is accredited by the Property Care Association (PCA) or similar trade body. This plan must include:
- Knotweed Survey and Report: Detailed findings, drawings, and an accurate map recording the contamination area and the RICS risk category presented to the property.
- Treatment Programme: A defined, scheduled treatment and monitoring plan, typically lasting 5 years or more, which sets out the exact method (chemical, excavation, etc.).
- Insurance-Backed Guarantee (IBG): A transferable guarantee, usually for 5 or 10 years, that protects the buyer and lender. It ensures that if the specialist company ceases trading, an underwriter will fund any required re-treatment if the knotweed regrows.
- Completion Certificate: Issued after the treatment period has ended and no regrowth has been observed for a minimum of 2 years. This formal certificate is essential evidence for future mortgage or sale purposes.
- Receipted Invoice: Evidence that the KMP and the guarantee are fully funded and paid for, satisfying the lender's condition before they release mortgage funds.
While you can attempt chemical treatment, no mortgage lender will accept a self-managed plan.
For any property transaction to proceed, you require a professionally prepared KMP and an Insurance-Backed Guarantee (IBG) from a specialist company accredited by the Property Care Association (PCA).
Attempting to remove it yourself also risks causing it to spread, which is a criminal offence.
Japanese Knotweed costs: Survey vs Management Plan
Knotweed survey cost
A specialist survey to test for and confirm the presence and extent of Japanese Knotweed typically costs a few £100s.
The final figure depends on the scale of the property and its grounds. This is the starting point if knotweed is suspected but not confirmed.
Knotweed Management Plan (KMP) cost
This is the cost of the full, guaranteed eradication or containment programme. For a typical residential infestation of 50 square metres or less, you can expect the cost to be structured as:
- A 5-year KMP with herbicide treatment is likely to cost around £1,000.
- Extending the Insurance-Backed Guarantee (IBG) to 10 years may increase the overall cost to between £1,500 and £2,000, depending on the provider.
These prices are proportionate to the size of the affected area; larger or complex infestations, or those requiring full excavation, will be significantly more expensive.
Andrew started his career in 2000 working within conveyancing solicitor firms and grew hands-on knowledge of a wide variety of conveyancing challenges and solutions. After helping in excess of 50,000 clients in his career, he uses all this experience within his article writing for SAM, mainstream media and his self published book How to Buy a House Without Killing Anyone.
Caragh is an excellent writer and copy editor of books, news articles and editorials. She has written extensively for SAM for a variety of conveyancing, survey, property law and mortgage-related articles.
