New evidence suggests that refusing mortgages on properties close to Japanese knotweed is out of proportion with the risk posed by the invasive plant.
Aecom and University of Leeds joined forces to carry out more detailed research into the potential for Japanese knotweed (Fallopia japonica) to cause foundation damage compared with other plants.
The new research comes just a week after Network Rail lost a court appeal over damages awarded to homeowners in Maesteg, Wales after Japanese knotweed encroached on their land from the rail infrastructure company’s property.
According to Aecom, the problematic weed is known to have a range of negative environmental effects and is widely believed to pose a significant risk of damage to buildings within 7m of the above ground portions of the plant due to its underground shoots, known as rhizomes.
The new 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,” said Aecom principal ecologist Mark Fennell.
He added that the 7m rule, although based on the best information previously available, was not a statistically robust tool for estimating how far the plant’s rhizomes are likely to reach underground.
Co-author University of Leeds’ School of Geography lecturer Karen Bacon 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. Japanese knotweed is capable of damaging built structures, but where this occurs, it is usually because an existing weakness or defect has been exacerbated.”
The authors assessed the three main mechanisms by which plants are known to cause structural damage: subsidence (usually caused by plants and trees drying out clay soils around foundations); collapse and impact (usually caused by trees falling on buildings) and accumulating pressure due to growth (usually caused by the plant’s main trunk and secondary thickening of the roots in close proximity to the trunk).
Their survey of 51 contractors and 71 surveyors, reporting on 122 properties where Japanese knotweed was present, showed that reports of defects or structural damage to residential properties were rare.
A case study looked at 68 pre-1900 residential properties located on three streets in northern England, chosen because they had been abandoned for at least 10 years, were already in a state of disrepair, and so represented a “worst case” scenario in terms of susceptibility to damage from unchecked plant growth.
While knotweed was identified within 7m of 18 of the properties, it was linked to less damage than the trees, climbers and shrubs (such as buddleia, which is also non-native and invasive in the UK) also found there.
In a separate survey, of 26 contractors who provided records of 81 excavations, results showed that Japanese knotweed rhizomes rarely extended more than 4m from above-ground plants. Rhizome spread was generally less than 2.5m – well below 7m.
The researchers also found no support in the literature for the idea that Japanese knotweed is a major cause of damage to property and, overall, established it was less likely to cause damage than many other common species.