Japanese knotweed is not illegal to have on your land, but it is illegal to cause it to spread to neighbouring land (under the Wildlife and Countryside Act 1981 and the Anti-Social Behaviour, Crime and Policing Act 2014). A landlord who allows knotweed to spread from their property to a neighbour's may face civil nuisance claims and, in extreme cases, an Environmental Protection Act 1990 abatement notice from the local authority if the spread constitutes a statutory nuisance.
The practical problem for buy-to-let landlords is primarily mortgage-related. Most mainstream buy-to-let lenders will decline to lend on a property where Japanese knotweed is confirmed within or adjacent to the property — or will require a professional management plan from a specialist company as a condition of the mortgage. This affects both the purchase of a new buy-to-let and the refinancing of an existing property where knotweed is later discovered.
RICS risk categories — how surveyors assess knotweed
The Royal Institution of Chartered Surveyors (RICS) uses a four-category risk framework for Japanese knotweed in residential surveys (RICS guidance note, 2022):
- Category 1 (low risk): Japanese knotweed is within 7 metres of the property boundary but is not within the curtilage of the property itself. Risk to the property is low. Mortgage lenders may still accept the property subject to a management plan
- Category 2 (medium risk): Japanese knotweed is within the curtilage of the property (i.e., within the boundary) but is more than 7 metres from a habitable space, outbuilding, or the boundary. A management plan is typically required by lenders
- Category 3 (high risk): Japanese knotweed is within 7 metres of a habitable space, outbuilding, or the boundary. This is the category most likely to cause mortgage lender refusal without a completed professional management plan
- Category 4 (very high risk): Japanese knotweed is causing or has caused damage to the structure of the building (foundations, walls, drainage). This is the most serious category and typically requires specialist structural assessment in addition to a knotweed management plan. Lenders may refuse to lend even with a management plan in place
- Surveyors are required to flag Japanese knotweed in all RICS HomeBuyer Reports and Building Surveys. The RICS category will directly influence the lender's response and may affect the property's valuation
Legal obligations — controlling and not spreading knotweed
Landlords have legal duties relating to Japanese knotweed at their property:
- Wildlife and Countryside Act 1981 (s.14): It is a criminal offence to 'plant or otherwise cause to grow in the wild' Japanese knotweed. Disposing of knotweed as green waste or allowing it to spread to adjacent land (including through soil movement, water courses, or garden waste) is an offence
- Environmental Protection Act 1990 (s.79): Where knotweed causes a statutory nuisance — typically where it has encroached on neighbouring land to such a degree that it is causing a detriment to the neighbours' use and enjoyment of their property — the local authority can issue an abatement notice requiring the landlord to take remedial action within a specified period
- Anti-Social Behaviour, Crime and Policing Act 2014: Local authorities can issue Community Protection Notices (CPNs) requiring landowners to take action to prevent or remedy knotweed encroachment. Breach of a CPN can result in a fixed penalty notice of up to £2,500 or prosecution with a fine of up to £2,500
- Private nuisance: Neighbours affected by knotweed encroaching from a landlord's property can bring a civil nuisance claim. The Supreme Court confirmed in Network Rail v Williams (2018) that allowing knotweed to encroach on a neighbour's land constitutes private nuisance even without physical damage
- Disposed knotweed is controlled waste: Any soil, plant material, or cutting containing Japanese knotweed must be disposed of as controlled (special) waste at a licensed facility — not put in domestic or garden waste collections
Disclosure — when must landlords tell buyers and tenants?
Disclosure obligations apply at different stages of the property lifecycle:
- On sale of the property: RICS-compliant surveys flag knotweed. The TA6 Property Information Form (used in residential conveyancing) asks sellers to confirm whether they are aware of knotweed at the property. Knowingly answering incorrectly on a TA6 is fraudulent misrepresentation and grounds for rescission of the contract or a claim in damages
- On remortgage: Where a surveyor flags knotweed during a remortgage valuation, the lender may require a management plan as a condition of the mortgage offer. The landlord must arrange and fund this
- To prospective tenants: There is no statutory obligation to disclose knotweed to prospective tenants — but a landlord who knowingly lets a property with severe knotweed (particularly where it is causing structural damage) without disclosure may face claims under the Consumer Rights Act 2015 or common law misrepresentation
- To neighbours: If knotweed has spread (or is at risk of spreading) to a neighbouring property, early voluntary disclosure and an offer to fund a management plan is strongly advisable — it demonstrates good faith and may prevent a nuisance claim
- Specialist knotweed management companies typically provide an 'Insurance Backed Guarantee' (IBG) with their management plans — this is transferable to a purchaser or lender and provides comfort that the knotweed is being actively managed under a professional plan
Treatment options and timescales
Japanese knotweed cannot be eradicated quickly — effective management takes 3-5 years:
- Herbicide treatment: The most common approach is a professional herbicide programme (typically glyphosate-based) applied 2-3 times per year over 3-5 growing seasons. The plant dies back visibly but underground rhizomes remain viable for years — only complete death of the rhizome system constitutes eradication
- Excavation and removal: Physical removal of all knotweed-contaminated soil (often to a depth of 2-3 metres) is the fastest treatment but also the most expensive. All excavated material is controlled waste and must be disposed of at licensed facilities. Excavation may disturb foundations and drainage — a structural survey is advisable first
- Encapsulation (root barrier): Installing a root barrier membrane prevents regrowth into designated areas but does not kill the plant. Often used in combination with herbicide treatment to protect adjacent structures
- Treatment cost: Professional herbicide programmes cost £500-£3,000 per season depending on the extent of the infestation. Full excavation can cost £5,000-£50,000+ for heavily infested sites. An IBG (transferable guarantee) adds to the cost but is essential for mortgage and insurance purposes
- DIY risk: Attempting to control knotweed with domestic herbicides (non-professional strength) is rarely effective against an established infestation and creates a risk of disposal errors (controlled waste offences). Professional INNSA-certified or BASIS-qualified contractors are strongly recommended
Impact on mortgageability — before purchase
Landlords acquiring property should always check for knotweed before exchange:
- Pre-purchase survey: A RICS HomeBuyer Report or Building Survey will flag visible knotweed. However, surveyors do not undertake specialist botanical surveys — knotweed can be dormant (invisible) in winter. Where the property is in an area of known knotweed prevalence, consider commissioning a specialist knotweed survey
- Lender requirements vary: Some lenders (particularly specialist buy-to-let lenders) will lend on Category 1 or 2 knotweed properties without requiring a management plan. Others require a plan regardless of category. A small number will not lend at all where knotweed is confirmed within the curtilage
- Retention at completion: Where knotweed is identified during conveyancing, the buyer's solicitor may seek a retention (a portion of the purchase price held in escrow) until a management plan is in place and the knotweed is under control
- Reduce the purchase price: Where knotweed is identified during the survey, the buyer can renegotiate the purchase price to reflect the cost of a management plan, the IBG, and the likely reduction in future value and marketability
- Acquiring a property with severe (Category 3 or 4) Japanese knotweed without a plan and guarantee in place is a significant risk for a buy-to-let investor — it may prevent refinancing, sale, or further borrowing against the property for years
Frequently asked questions
Do I have to tell my tenants there is Japanese knotweed at the property?+
There is no statutory duty to disclose knotweed to tenants. However, where knotweed is causing structural damage (Category 4) that could affect habitability, non-disclosure could be relevant under the Consumer Rights Act 2015 or common law. Best practice is to commission a management plan, which demonstrates you are actively controlling the plant.
Can I get a buy-to-let mortgage on a property with Japanese knotweed?+
It depends on the RICS category. Category 1 (knotweed within 7 metres of the boundary but not within the curtilage) is sometimes accepted by lenders without a management plan. Categories 2 and 3 typically require a professional management plan with an Insurance Backed Guarantee. Category 4 (structural damage) may result in lender refusal even with a management plan in place.
Is it illegal to have Japanese knotweed on my land?+
No — having knotweed on your land is not illegal. However, allowing it to spread to neighbouring land is a criminal offence under the Wildlife and Countryside Act 1981. Disposing of knotweed as ordinary garden waste is also illegal — it must be treated as controlled (special) waste and disposed of at a licensed facility.
How long does it take to get rid of Japanese knotweed?+
Effective eradication via herbicide typically takes 3-5 years of professional treatment (2-3 applications per growing season). Physical excavation and removal is faster but much more expensive and creates significant controlled waste disposal obligations. There is no quick fix — any company guaranteeing complete eradication in one season should be treated with scepticism.