Japanese knotweed ‘poses less risk to buildings than trees’

sellers of houses with knotweed may still struggle.

Alastair McKee, a broker at One77 Mortgages, said one study might not be enough to do the trick. He added: “It will take a while to change the public perception of how damaging it is. It is deep in the public’s psyche.”

Mark Montaldo of Cobleys Solicitors, which specialises in knotweed cases, said academic studies would be largely irrelevant in the face of public opinion. He said: “It’s how it is interpreted by the public that is important. Back in the days of mad cow disease, academics said you couldn’t get it from eating British beef. It didn’t matter – sales of British beef plummeted.”

Mr Montaldo said his experience was that knotweed could reduce sale values by as much as 20pc.

Knotweed is not completely harmless. The research said it crowded out other plants, reducing diversity. It creates dense clusters of foliage that can block paths and harm riverbanks by exacerbating erosion.

Dr Karen Bacon of the University of Leeds said: “The negative impact of Japanese knotweed on such factors as biodiversity and flooding risks remains a cause for concern. But this plant poses less of a risk to buildings and other structures than many woody species, particularly trees.”

The findings echo the views of many knotweed removal firms. Japanese Knotweed Solutions said the plant was not a risk to the structure of well-maintained properties. It said the plant could grow into a house through bricks, doors and windows, but usually only of ill-maintained houses.

A spokesman for the Royal Institution of Chartered Surveyors backed the research and said that “while knotweed has caused concerns, some are based on misunderstanding”. Knotweed is native to Japan, where it is found growing on the sides of volcanoes.

It was imported to England in 1850 when Bavarian botanist Philipp von Siebold sent some to Kew Gardens in London.

The striking plant proved a hit with Victorians. It was planted in parks and gardens, but also spread of its own accord along railways and waterways.

The plant’s invasiveness was noted as far back as 1898, said Aecom, with this view becoming more mainstream in the Seventies, but knotweed remained on sale in nurseries until at least 1990.

It was only this century that property surveyors and mortgage lenders began to take a hard-line view on the plant.