"We found nothing to suggest that Japanese knotweed causes significant damage to buildings – even when it is growing in close proximity"
A new study found no evidence that Japanese knotweed causes significant structural damage and its ecologists concluded that automatically refusing mortgages on properties where Japanese knotweed is found is "out of proportion to the risk posed by this invasive species".
Ecologists from global infrastructure services firm AECOM and the University of Leeds assessed the potential of Japanese knotweed to cause structural damage compared to other plants.
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 ten 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 seven metres 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.
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.
Dr Mark Fennell, principal ecologist at AECOM, 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.”
Joe Arnold, managing director of Arnold & Baldwin Chartered Surveyors, commented: "Research from the University of Leeds and AECOM, an infrastructure services firm, has found there is no evidence that Japanese knotweed causes significant structural damage. The study, which compared Japanese knotweed to other plants, found there was nothing to suggest the plant causes more damage to other species that are not subject to such strict lending policies.
"Currently, mortgage lenders will often require evidence that a treatment programme is in place to control Japanese knotweed if it is identified at a property and the stigma associated with the plant can negatively impact property prices even after action has been taken.
"So it will be interesting to see whether the results of this study are reflected by a change in mortgage lender criteria and a shift in the way that homeowners and homebuyers think about Japanese knotweed."