Japanese knotweed is wiping an estimated £21.4 billion from UK property values, with more than 1.5 million homes affected, according to research from invasive plant specialist Environet.
Research conducted with Censuswide in January 2026 indicates that 7% of UK homes, equivalent to more than 1.58 million properties, are affected by the invasive plant.
Environet said property values are typically reduced by around 5%, equivalent to £13,500 per home, underlining the potential impact on housing wealth and saleability.
Mortgage lenders will typically refuse loans against properties affected by Japanese knotweed unless a professional knotweed management plan is in place with an insurance-backed guarantee.
Sellers are also legally required to declare its presence, which can deter buyers even after an infestation has been treated. Environet said this continuing “knotweed stigma” continues to affect property values.
Separate research conducted by Environet with YouGov found that almost a third, 32%, of British adults would not buy a property affected by Japanese knotweed under any circumstance. A further 31% said they would consider doing so if there was a professional treatment plan in place and an appropriate discount on the price.
Environet said sellers are increasingly moving away from cheaper herbicide treatment, which controls but does not fully remove the plant, and are instead choosing excavation, where the root system is dug out of the ground.
The company’s data shows that in 2015, 74% of customers opted for herbicide treatment, compared with 49% in 2025, a fall of 25 percentage points.
Emily Grant, director at Environet, said: “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.”
Andrew McColl, chairman of the Residential Property Surveyors Association, said: “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.”
A 2023 report by CABI, the Centre for Agriculture and Bioscience International, estimated that Japanese knotweed costs the UK economy more than £245 million each year.
Environet said that, while some reports of the damage caused by the plant have been overstated, it can spread rapidly and grow through asphalt, patios, cavity walls, drains and cracks in concrete.
The knotweed growing season is now underway, with mature plants able to reach up to three metres in height by late summer.
According to Environet’s live Japanese knotweed heat map, Exposed, which is populated with more than 100,000 known incidences of knotweed across the country, Bristol, Merseyside, Greater Manchester, Greater London and Lancashire are among the worst affected areas.
Environet said there are, on average, 1.1 occurrences of knotweed per square mile in England, rising to 3.9 in Wales.
