Japanese Knotweed and Property: What Buyers and Developers Need to Know

Japanese knotweed has a formidable reputation — often described in tabloid coverage as capable of growing through concrete, breaking through walls, and destroying buildings. The reality is more nuanced and matters for how you approach any construction work.

• **What knotweed does do**:
• •Grows very rapidly during the growing season (April–October) — up to 10cm per day in peak conditions
• •Exploits existing weaknesses in structures — it will grow through cracks in masonry, drains, or paved surfaces that are already compromised
• •Regenerates prolifically from very small rhizome (root) fragments — a 1cm piece of rhizome can produce a new plant
• •Spreads by rhizome (underground root network) rather than seed in the UK — the rhizome network can extend several metres from the visible stems
• •Reduces property value and marketability significantly
• •Can make obtaining a mortgage extremely difficult or impossible without a professional treatment plan in place

• **What knotweed does not typically do**:
• •Penetrate sound, undamaged concrete or masonry — it exploits existing cracks, not create them through solid material
• •Cause structural collapse of a sound building — the structural damage risk is substantially overstated by treatment companies with a financial interest in alarming property owners
• •Spread over boundaries by wind or human contact (unlike many invasive plants) — it primarily spreads through disturbance of rhizome material (often by contractors excavating, mowing, or disturbing the ground)

**Legal position**: Japanese knotweed is listed under Schedule 9 of the Wildlife and Countryside Act 1981 — it is a criminal offence to plant or cause it to grow in the wild. Importantly, you do not need to remove knotweed from your property simply because it exists there — the offence is 'causing it to spread'. However, if knotweed spreads from your property to an adjoining property or land, you may face civil liability, and neighbours can take action under the tort of private nuisance.

The primary practical problem with Japanese knotweed is its effect on mortgage lending — which in turn affects the ability to buy, sell, or remortgage a property.

**RICS Guidance Note (2022)**: RICS updated its guidance note on Japanese knotweed in 2022, introducing a risk-based categorisation rather than the blanket 'not mortgageable' position that previously applied:

*Category 1 (low risk)*: knotweed present in the garden but more than 7m from the main habitable building and not affecting any outbuildings — lender likely to accept without requiring treatment

*Category 2 (moderate risk)*: knotweed within 7m of the main habitable building or outbuildings, or knotweed present in the legal boundary; or minor damage to outbuildings — lender likely to require a professional management plan with a 10-year guarantee from a qualified professional company (PCA registered)

*Category 3 (high risk)*: knotweed causing identifiable damage to a building or outbuilding; or knotweed in a situation where it is causing encroachment on neighbouring property — lender likely to require treatment to be underway, a 10-year guarantee, and may require specialist structural assessment

*Category 4 (very high risk)*: severe damage to the structure of habitable areas — specialist structural report required; some lenders will decline regardless of treatment plan

• **In practice (2025)**:
• Most high-street lenders (Halifax, Nationwide, Santander, NatWest, Barclays) will lend on Category 1 and 2 properties, provided:
• •A professional knotweed management plan is in place from a PCA (Property Care Association) registered company
• •The plan includes a minimum 10-year guarantee backed by an insurance policy (not just the company's promise)
• •The management plan is transferable to a subsequent owner

Category 3 and 4 properties are more problematic and may require specialist lenders or a significant price reduction.

• **Herbicide treatment (glyphosate)**:
• •The most common treatment method for established knotweed
• •Glyphosate is applied as a foliar spray or by stem injection during the growing season (April–October)
• •Requires multiple treatment cycles over 3–5 years to reliably kill the rhizome system
• •Stem injection: a more targeted approach using a lower volume of concentrated herbicide injected directly into each stem — preferred near watercourses (where spray drift is a risk) and in urban settings
• •Cost: £1,500–£5,000 for a standard domestic garden treatment programme over 3 years (PCA registered company, with 10-year guarantee)
• •Timescale: results visible within 2–3 growing seasons; 3–5 year programme required for full treatment; monitoring continues to end of guarantee period

• **Excavation and disposal**:
• •Required where knotweed must be removed quickly (e.g., to enable immediate construction, or where the infested area is small and accessible)
• •Rhizome must be excavated to at least 3m depth and 7m laterally — the minimum extent of a typical rhizome system
• •All contaminated soil and plant material must be disposed of as controlled waste at a licensed facility (it is illegal to dispose of knotweed material in a domestic skip or at a general landfill without appropriate classification)
• •Cost: £5,000–£30,000+ for a domestic garden excavation, depending on the extent of infestation, excavation depth, and disposal costs
• •Risk: excavation carries high risk of spreading the rhizome if not carried out very carefully by experienced operators
• •Not typically used as a standalone treatment — often combined with herbicide pre-treatment to weaken the plant before dig

• **Root barrier systems**:
• •High-density polyethylene (HDPE) root barriers installed vertically around the knotweed zone can contain the rhizome spread during herbicide treatment
• •Useful in urban settings where the rhizome is approaching a structure or boundary
• •Used in combination with herbicide, not as a standalone measure

• **For construction projects**:
• If Japanese knotweed is present on a site where an extension, loft conversion, or significant groundwork is planned:
• •Commission a knotweed survey (professional assessment of extent) before work starts
• •Develop a knotweed management plan specifically for the project — particularly if excavation will disturb the rhizome
• •Ensure any soil or plant material moved during groundworks is classified, manifested, and disposed of as controlled waste — contractor must have appropriate waste carrier licence
• •Never allow excavated knotweed material to be placed in skips with other inert or general waste