The blight of Japanese knotweed has wiped £20 billion off the current value of the UK property market, Environet revealed in new research released last week which was reported in The Independent, Moneywise Magazine and a host of other media outlets.

Based on our historical data over twenty years of nation-wide operation and a recent survey conducted in partnership with YouGov, we estimate that 4 to 5% of the UK’s houses are currently affected, directly or indirectly (e.g. neighbouring an affected property), by knotweed.

Consequently, approximately 850,000 to 900,000 UK households are suffering an average reduction in value of around 10%, knocking almost £20 billion (£19.8 billion) off property values.

Mortgage lenders will typically refuse loans against affected properties unless there is a professional knotweed management plan in place with an insurance-backed guarantee. Sellers are also required by law to inform potential purchasers if the property is affected by Japanese knotweed, which can act as a deterrent. The word “affected” could include those properties that have been previously treated and even those that are clear but threatened by knotweed on an adjoining property.

Environet MD Nic Seal said, “Japanese knotweed is the problem that just keeps growing. For most people in the UK, their home is their biggest asset and often the ‘pot of gold’ they are relying on in retirement, but Japanese knotweed is having a serious impact on values by deterring buyers and making homes difficult to sell, even if the knotweed has been successfully treated.

“Those affected by knotweed should seek to resolve the issue as quickly as possible by appointing a professional removal firm and securing a ten-year insurance-backed guarantee for the work, which can be passed onto a buyer and their mortgage lender. DIY attempts at treatment will usually only make things worse and can even hasten the spread of the plant.”

Homeowners worried about the threat of Japanese knotweed, which is now rife across the UK with particular hotspots in Wales, the South West, London and the South East, can protect themselves by purchasing a specialist Japanese knotweed indemnity policy through Countrywide Legal Indemnities. Starting at less than £100, the policy covers you for the cost of treatment, repairs, legal costs arising from third party claims and any diminution of the property’s value, should knotweed arise.