The scourge of Japanese knotweed! Pest wipes nearly £12bn off UK house prices

New research from Environet UK has revealed that the blight of Japanese knotweed has wiped an estimated £11.8 billion off the current value of the UK housing market, as the invasive plant continues to spread across the country making homes much more difficult to sell.

Environet, the invasive plan specialists, claim that approximately 4% of homes are affected by Japanese knotweed, either directly or indirectly (i.e. neighbouring an affected property). This impacts their value by an average of 5%.

As a result, around 890,000 households across the country are suffering a typical fall in price of £13,200 due to knotweed. In total, this is knocking some £11.8 billion off the value of the nation’s homes.

To combat this reduction in property values and manage the risk posed by the plant, buyers and sellers of affected properties are turning away more and more from traditional herbicide treatments and opting for excavations instead.

This is where the plant’s extensive rhizome system is dug out of the ground. Nearly 70% of customers opted for excavation of Japanese knotweed during 2021, an increase of 10.6% on the previous year, according to Environet data. 

Meanwhile, the proportion of herbicide treatments has dropped by 17.7% over the same period. A decade ago, herbicide was by far the most popular way of dealing with knotweed on residential properties, but more potent herbicides were available at the time which have now been banned.

The company says removing the root system from the ground is the only way to deal with Japanese knotweed decisively with minimal chance of regrowth. In spite of the lower costs at play, herbicide treatment is increasingly being recognised by homeowners as a control method. Although above-ground growth may disappear, the root system beneath the ground is often 'induced into dormancy' meaning it’s capable of regrowing in the future. This is particularly the case if the ground is disturbed by landscaping or building work. 

Currently under consultation is the guidance from the Royal Institution of Chartered Surveyors (RICS) relating to the assessment of Japanese knotweed, with amendments expected to be published next month. The draft guidance places a strong emphasis on ‘control’ of the plant through herbicide treatments. However, evidence suggests homeowners are already turning away from these methods in order to protect the value of their homes.

“With knotweed wiping £11.8 billion 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," Nic Seal, founder and managing director of Environet, said.

“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.”

Envivronet claims excavation costs can be reduced by opting for a combination treatment and removal method, DART™, which removes the bulk of the knotweed crowns and rhizome system.

This, in turn, massively reduces the plant’s vigour and stimulating growth of any small pieces of remaining rhizome, making them more susceptible to herbicide treatment the following spring.

The excavation element can be carried out during the winter months, allowing for full use of gardens during the summer, the firm adds.

Earlier this year, we reported how cases of Japanese knotweed have increased by 28% in the UK as the plant continues to cause misery, uncertainty and frustration for many investors.