Ecologists from AECOM and the university of Leeds have carried out the most extensive research to date assessing the potential of Japanese knotweed (Fallopia japonica) to cause structural damage compared to other plants.
Now recognised as one of the most problematic weeds in the UK and Ireland, Japanese knotweed is known to have a range of negative environmental impacts. In the UK, it is widely believed to pose a significant risk of damage to buildings that are within 7m of the above-ground portions of the plant – the so-called ‘7m rule’ – due to its underground shoots, known as rhizomes.
When identified in homebuyers’ surveys, mortgage lenders often require evidence that a treatment programme is in place, entailing significant expense for sellers. The stigma associated with the plant means that property values can be affected, even after action is taken.
Dr Mark Fennell, Principal Ecologist at AECOM, who led the research, said: “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”.
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 Dr Karen Bacon, from the university of Leeds’ School of Geography, said: “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”.
Professor Max Wade, Technical Director – Ecology at AECOM, and co- author of the paper, said: “We hope our research will inform discussions around the advice currently offered about Japanese knotweed by providing more information about the reality of its impact on built structures”.
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 that it was less likely to cause damage than many other common species.