Japanese knotweed: The £20 Billion garden menace

This article has been reviewed by our Japanese knotweed expert:
Ben Lindley, Company Director at Japanese Knotweed Ltd. Last reviewed on 15th December 2025.

For UK homeowners, few phrases strike as much fear as “Japanese knotweed.” This highly invasive plant is more than just an unsightly weed; it’s a silent threat to your property’s value and a potential legal and financial nightmare.

According to published research data, approximately 5% of homes across the UK are affected by Japanese knotweed, either directly or via an infestation on a neighbouring property. The total cost to the UK economy is estimated to be in the billions, reflecting the severe impact on homeowners and businesses alike.

In this blog we list the reasons why knotweed should never be ignored! For most property owners, the single most important question about Japanese knotweed is:

“Will Japanese knotweed affect my ability to sell or mortgage my property?”

Almost every knotweed-related question ultimately leads back to this one – because it’s about financial risk, legal responsibility, and future uncertainty. Behind it sit the concerns property owners care about most:

Will my lender refuse a mortgage?
Will buyers walk away or reduce their offer?
Am I legally required to declare it?
How expensive will it be to fix?
Can it be properly resolved?

In this article we address the core fear driving property owner enquiries.

The Real Impact on Your Property Value

The primary concern for most homeowners is financial. The presence of Japanese knotweed can drastically affect your home’s marketability and price.

Significant devaluation: According to leading estate agents Japanese knotweed can knock anywhere from 5% to 30% off the value of your home, depending on the severity of the infestation and whether a professional treatment plan is in place.
Buyer aversion: The “knotweed stigma” is a real concern. Research suggests that up to 78% of potential buyers would be put off proceeding with purchasing a property with knotweed, and half of all would-be buyers might walk away entirely without even considering it.
Mortgage troubles: Most mortgage lenders require evidence of a professional treatment plan with an insurance-backed guarantee (IBG) before they will lend on an affected property. Without one, house sales may only be possible to a cash buyer, usually at a heavily discounted price.
The Hidden Physical Damage: While recent studies suggest the plant may not cause as much damage to structures as once believed, its vigorous root system (rhizomes) can still exploit and worsen existing weaknesses – and that’s what causes issues – up to 7 metres away from the original plant.
Aggressive Growth: Knotweed can grow up to 10cm per day in growing season. The costs of repairing the damage it causes quickly add up.

Legal Risks of Inaction

It is not illegal to have Japanese knotweed on your property, but it is illegal to allow it to spread into the wild or onto a neighbour’s land.

Neighbour disputes: Homeowners who fail to control the spread can face costly legal action for private nuisance. Court cases have resulted in landowners being forced to pay thousands of pounds in legal fees for non-disclosure.
Disclosure is mandatory: You are legally required to disclose the presence of knotweed when selling your home using the TA6 property information form. Failing to do so or stating no knotweed incorrectly can lead to an expensive misrepresentation claim.
Official Enforcement: Local authorities can issue Community Protection Notices (CPNs), compelling homeowners to take action or face fines.

Act Fast, Act Professionally

The key to mitigating the risk is prompt, professional action. DIY removal is highly discouraged as even a tiny root fragment the size of a fingernail can regenerate into a new plant, potentially making the problem worse.

Professional treatment, usually involving a multi-year herbicide program or excavation is the recommended route and is a vital investment to protect your primary asset and ensure a smooth future sale.

The bottom line

The facts point to one conclusion for property owners – inaction is the biggest risk, as it can lead to significant legal and financial consequences. And whilst Japanese knotweed can impact property sales and mortgages it doesn’t have to stop them. With early identification, a professional treatment plan, and the right guarantee in place, most properties can be sold and mortgaged without issue. Speak to one of our knotweed experts about it to find out more.

FAQ

Will Japanese knotweed stop me selling my house? Not usually. Most sales proceed as long as knotweed is identified early and a professional treatment plan is in place. Buyers and solicitors want reassurance not perfection.

Can I still get a mortgage if there’s knotweed? Yes, in many cases. Lenders typically require evidence of a professional management plan and, in some cases, an insurance-backed guarantee.

Do I have to declare Japanese knotweed? Yes. Knotweed must be declared on the TA6 property form. Failing to do so can cause legal issues later.

Is Japanese knotweed expensive to deal with? Costs vary depending on severity and treatment method, but early action keeps costs down. Treatment plans are often more affordable than people expect.

What should I do if I think I have knotweed? Get it checked by a specialist as soon as possible. Early identification gives you more options and prevents unnecessary delays or complications.

How We Help

Confirm if it’s knotweed
Free expert identification to remove doubt.
Assess the risk
Get a survey and clear advice on how it affects your property.
Put a plan in place
Treatment or removal options that satisfy lenders and buyers.
Provide long-term reassurance
Insurance-backed guarantees where required.