Japanese knotweed, a widespread invasive non-native species in the UK, is seldom out of the news and can strike fear into the hearts of anyone who finds it growing on their property, house owner or developer alike. It is a tall, herbaceous and fast-growing plant that reduces biodiversity and can increase the risk of flooding.
Japanese knotweed – Fallopia japonica – is associated with a significant economic burden in the UK. Recently, a court in Wales ruled that homeowners were entitled to claim damages from Network Rail because rhizomes (the plant’s underground shoots) had extended below their properties. Japanese knotweed is subject to various legislation and mortgage lenders often require that an insurance-backed management plan for the plant is in place when it is present on or near a property before agreeing to a mortgage. The presence of the species can also result in a reduction of a property’s value.
Japanese knotweed’s often quoted abilities to grow through concrete, damage buildings and extend destructive rhizomes seven metres from the above ground portion of the plant are among the most feared features of the plant. But it is exactly these supposed abilities that our new research challenges.
I teamed up with lead author Mark Fennell and co-author Max Wade, both members of the environment and ground engineering team at global services firm AECOM, to present the most comprehensive study to date assessing the ability of Japanese knotweed to cause damage to built structures compared to other plants. What we found is at odds with what is currently accepted and may help to alleviate fears that the plant can grow through concrete or cause major structural damage to buildings.
There are three primary ways plants can damage buildings: indirect damage though subsidence or heave; direct damage though collapse and impact; and direct damage through the accumulated pressure of plant growth. We assessed the evidence that Japanese knotweed can cause each of these types of damage and how the possibility of Japanese knotweed causing damage in these ways compares to other plants.
A detailed survey of the literature revealed that indirect damage caused by plants is only possible on shrinkable clay soils, a type of soil that is not particularly common in the UK. Even where the most shrinkable soils are found, the biology and size of Japanese knotweed makes it less likely to facilitate this type of damage than large trees. This means that the risk of Japanese knotweed causing damage by modifying soil water content is extremely remote and only relevant in areas with exactly the right type of soil.
Our next step was to conduct a survey of members of the Property Care Association and Royal Institution of Chartered Surveyors who have been involved in property surveys and treating or removing Japanese knotweed in the UK. Respondents were asked to report whether they had observed damage to buildings occurring with Japanese knotweed.
Only between 2% and 6% of respondents reported any co-occurrence of Japanese knotweed and structural damage to buildings. Our paper also concluded that where Japanese knotweed is associated with damage, it is likely that the plants will have exacerbated existing damage, rather than being the initial cause of the damage.
Further to this, we asked respondents to report the lateral extension of underground shoots in cases where they had undertaken full excavations of Japanese knotweed. This allowed us to test the “seven-metre rule” commonly used to denote whether Japanese knotweed is likely to pose a threat to buildings.
We found that smaller stands of Japanese knotweed (less than four square metres in area) generally had rhizomes no longer than two metres and not beyond four, while 75% of larger stands had rhizomes extending no further than 2.5 metres. We received only one report of rhizomes over four metres in length. This shows that the fear of Japanese knotweed commonly having seven-metre rhizomes is unfounded and the use of the seven-metre rule is not robust for determining the likely lateral extent of these underground shoots.
We then assessed 68 abandoned properties on three streets in northern England with a significant Japanese knotweed infestation, to determine whether these houses had damage caused by the presence of the plant. Many properties had Japanese knotweed present or nearby but we found no evidence to suggest that the dilapidation of the properties was caused by the plant. In fact, the other plants – particularly some woody trees – were visually associated with more damage. This area represented a near worst case scenario for houses that had been left to the mercy of Japanese knotweed and still we found no evidence for the plant causing damage.
Overall, our study found no support for the commonly suggested ideas that Japanese knotweed routinely damages buildings and that its influence extends seven metres from plants above ground. Nor did we find evidence that it poses a major risk to built structures.
Japanese knotweed remains a serious threat to Britain’s biodiversity, ecosystems and the amenity value of land, but these very real threats should not be confused with what our research shows to be myth more than fact. Japanese knotweed is no more of a risk to solidly built homes and buildings than many plants and less so than many woody species, particularly some large trees.
Our research highlights that the key to tackling invasive species lies in developing a detailed understanding of their biology and further highlights the ongoing need for new research and knowledge to help us to understand and, hopefully, address the emerging challenges presented by invasive non-native species.