The blight of Japanese knotweed has wiped an estimated £11.8bn off the current value of the UK housing market, as the invasive plant continues to spread across the country making homes more difficult to sell, research from Environet UK, the invasive plant specialists, claims.

The study estimates that around 4% of homes are affected by Japanese knotweed, either directly or indirectly (i.e. neighbouring an affected property), impacting their value by an average of 5%.

Consequently, approximately 890,000 households across the country are suffering a typical reduction in value of £13,200 due to knotweed, knocking £11.8bn off the value of the nation’s homes, the company calculates.

Many homeowners want to limit the risk posed by the plant by getting rid of them at the earliest opportunity in order to protect the value of their homes.

Guidance from The Royal Institute of Chartered Surveyors (RICS) relating to the assessment of Japanese knotweed is currently under consultation with amendments expected to be published soon. The draft guidance places a strong emphasis on ‘control’ of the plant through herbicide treatments.

Environet founder and MD Nic Seal said: “With knotweed wiping £11.8bn off UK house prices this year, preservation of property values is the top priority for most homeowners. Those selling property are legally required to declare if the property is or has been affected by Japanese knotweed, but if an infestation has been professionally excavated with an insurance-backed guarantee to satisfy mortgage lenders, it is possible to restore the property value to close to the original value.

“Herbicide treatment of knotweed has always been very popular due to the lower costs, but the message is getting through that it’s only a control method and won’t solve the problem definitively. Buyers are much more wary of buying a property which still has knotweed rhizome beneath the ground as there’s no way of knowing whether it’s completely dead. There’s also an environmental cost to using chemicals, which is of growing concern.”