Japanese knotweed-not such a problematic plant

Japanese knotweed-not such a problematic plant

Homeowners whose property values are weighed down by Japanese knotweed could have found a reprieve. A new study suggests the plant is relatively harmless and refusing mortgages on properties where Japanese knotweed is in evidence is out of proportion to the risk associated with the plant.

Research by the University of Leeds and engineering firm Aecom, assessed the potential of Japanese knotweed (Fallopia japonica) to cause structural damage compared to other plants.

Japanese knotweed (Fallopia japonica)

The Seven Meter Rule

In the UK, Japanese knotweed has been historically thought to pose a significant risk of damage to buildings that are within seven metres of the above-ground elements of the plant – the so-called ‘seven metre rule’. This rule takes into account that due to its underground shoots, known as rhizomes, the plant can spread significantly under the surface and warrants investigations into the reach of the plants roots. It is believed that in London the presence of this plant can cause a reduction in value of up to 25% on a property.

Once knotweed is identified in homebuyers’ surveys, mortgage lenders often require evidence that a treatment programme is in place. Specialist treatment can cost between £2,000 to £5,000 for a three-bedroom semi-detached house.

Property values can be affected, even after action is taken to control it, so this new research should help to alleviate homebuyers fears when faced with the invasive plant.

Dr Mark Fennell, Principal Ecologist at AECOM, who led the study, said: “Our research sought to broaden existing knowledge about the risk to buildings of Japanese knotweed compared to other plants. We found nothing to suggest that Japanese knotweed causes significant damage to buildings – even when it is growing in close proximity – and certainly no more damage than other species that are not subject to such strict lending policies.”

 Unwelcome Visitor

Knotweed was introduced in the 19th century as an ornamental plant and is widely spread across the UK.  The Government has estimated that it would cost £1.5 billion to eradicate in the UK. An infestation plagued Hampstead near the homes of Thierry Henry, Tom Conti and Esther Rantzen.

The report examined 68 homes where knotweed was found, plus 81 additional sites. Ecologists investigated existing theories on knotweed damage and approached surveyors and invasive species experts.

“This plant poses less of a risk to buildings and other structures than many woody species, particularly trees.” states Co-Author of the report Dr Karen Bacon from the University of Leeds.

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”.

The Impact of The Report

Henry Knight, Managing Director from Springtide Capital commentated: ‘While lenders are unlikely to relax their rules around knotweed overnight, this new research into the issue should help inform changes in lending criteria and mould thinking about the severity of the issue, helping to make lending more probable going forward.”

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