Japanese knotweed has a bad reputation. Notorious across the UK, this invasive non-native species strikes fear into the hearts of UK property owners and developers who find it growing on their land.

Mark Fennell

Such widespread negativity is largely the result of the substantial economic burden associated with the plant. Japanese knotweed is broadly believed to pose significant risk of damage to buildings that are within seven metres of the above-ground portions of the plant due to its underground shoots. As such, lenders often require evidence that a treatment programme is in place to control it, which can result in significant expense for sellers. The stigma associated with the plant means property values can be affected, even after action is taken to control it.

Little research has been conducted to investigate the impact of Japanese knotweed on the structural integrity of buildings. The seven-metre rule is derived from the Environment Agency’s Knotweed Code of Practice, published in 2006 and based on the best information available at the time. In an attempt to broaden existing knowledge around the plant, the University of Leeds carried out the most extensive research to date on the capacity of Japanese knotweed to cause structural damage compared with other plants.

Japanese Knotweed

Source: Shutterstock/ Erika J Mitchell

It found nothing to suggest it causes significant damage to buildings – even when it is growing in close proximity – and certainly no more than other species that are not subject to such stringent policies. In fact, the plant poses less of a risk to property than many woody species, particularly trees.

While developers and commercial landowners still need to manage Japanese knotweed found on their sites, the research suggests a more pragmatic approach should be embraced. For example, developers and landowners could prioritise management in areas adjacent to residential properties without the fear that Japanese knotweed will cause significant damage elsewhere.

Following site-specific risk assessments, it may be possible to manage the species in a more sustainable and cost-effective way by reducing the total amount of waste generated, which would have positive effects on diminishing landfill space and reduce the quantity of soil resource being destroyed.

Japanese knotweed is still problematic. Legislation around facilitating its spread as well as the duty of care for managing the waste associated with the plant remains. However, it is hoped the new findings will help separate some of the myth from the reality when it comes to this invasive species.

Mark Fennell, principal ecologist at AECOM