Japanese knotweed is wiping an estimated £21.4bn from UK property values with more than 1.5 million homes affected by the invasive plant, new research has found.
Data from Environet suggests 7% of UK homes – around 1.58 million properties – are impacted, with values typically reduced by 5%, or £13,500 per property.
The findings underline the continued drag knotweed places on saleability, with lenders often unwilling to approve mortgages unless a professional management plan backed by insurance is in place.
Sellers are also legally required to declare its presence, adding to what the industry increasingly refers to as a lingering “knotweed stigma”.
BUYER RESISTANCE
Separate research by Environet, conducted with YouGov, shows buyer resistance remains significant. Nearly a third (32%) of British adults said they would not purchase a property affected by knotweed under any circumstances, while 31% would only proceed if a treatment plan and price discount were in place.
That caution is feeding through into seller behaviour. Homeowners are increasingly opting for full excavation – removing the root system entirely – rather than relying on herbicide treatment, which controls but does not eradicate the plant.
Environet’s data shows a marked shift over the past decade. In 2015, 74% of customers chose herbicide treatment, compared with just 49% in 2025.
CLEAR SHIFT
Emily Grant (main picture), director at Environet, says: “For most people, their home is their biggest asset, yet over 1.5 million households are seeing their property value undermined by knotweed. Despite the fact knotweed is now much better understood than it was a few years ago, and many people are now quite pragmatic about it, a significant proportion of buyers still won’t touch an affected property under any circumstance.
“As a result, we’re seeing a clear shift towards permanent removal of knotweed, particularly where a sale is on the horizon and the priority is preserving value and avoiding delays.
“The key for homeowners is early identification and professional management. With the right management plan and guarantees in place, the impact on value and saleability can be managed. Ignoring the problem or attempting a DIY removal will almost always cost more in the long run.”
FINANCIAL IMPLICATIONS
The financial implications can extend well beyond initial treatment costs, particularly where mortgage lending is involved.
Andrew McColl, Chairman of the Residential Property Surveyors Association, says: “Discovering Japanese knotweed can have significant financial implications, as removal and management plans are often costly and may require anywhere from five to ten years to fully implement.
“By ensuring you have accurate information before exchanging contracts, you can better anticipate and mitigate these expenses and potential complications, particularly those that may arise when dealing with lenders.”
BIG ECONOMIC COST
A 2023 report by Centre for Agriculture and Bioscience Internationalestimates Japanese knotweed costs the UK economy more than £245m each year.
Regional data highlights the scale of the issue. Environet’s heat map shows Bristol, Merseyside, Greater Manchester, Greater London and Lancashire among the worst affected areas, with an average of 1.1 cases per square mile across England, rising to 3.9 in Wales.
With the growing season now underway, the plant is expected to spread rapidly over the coming months, reaching up to three metres in height by late summer and continuing to weigh on property values in affected areas.
