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17 January 2019
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The weed is less of a risk to buildings than other woody plants © Shutterstock

11 July 2018 | Herpreet Kaur Grewal

Japanese knotweed, a notorious non-native species of plant in the UK known for causing structural damage, poses “less of a risk to buildings and other structures than many woody species”, according to a study.


Ecologists from global infrastructure services firm AECOM and the University of Leeds have carried out extensive research assessing the potential of Japanese knotweed (Fallopia japonica) to cause structural damage compared with other plants.


The plant is recognised as one of the most problematic weeds in the UK and Ireland and is known to have a range of negative environmental effects.


In the UK, it 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’ – owing to its underground shoots, known as rhizomes.


The research team looked for evidence of the perceived threat in previous research literature, surveyed invasive species control contractors and property surveyors, and assessed 68 residential properties where Japanese knotweed was found and examined data collected when knotweed was removed by excavation from an additional 81 sites.


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 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 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.


Dr Mark Fennell, principal ecologist at AECOM, who led the research, 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.”


Professor Max Wade, technical director of 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 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).