Japanese knotweed damage to properties ‘rare’, researchers say

Researchers have found little evidence Japanese knotweed causes significant structural damage to properties.

Ecologists from global infrastructure services firm AECOM and the University of Leeds have carried out the most extensive research to date on Japanese knotweed (Fallopia japonica), which is a notorious non-native species in the UK, and elsewhere in Europe and North America.

Their survey reported on 122 properties where Japanese knotweed was present and showed that reports of defects or structural damage to residential properties were rare.

Researchers also visited 68 pre-1900 residential properties located on three streets in northern England, chosen because they had been abandoned for at least ten years and 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 also found there.

In the UK, Japanese knotweed is widely believed to pose a significant risk of damage to buildings that are within seven metres of the above-ground portions of the plant – the so-called ‘seven metre rule’ – due to its underground shoots, known as rhizomes.

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.’

He added that the seven metre 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: ‘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.’