Japanese knotweed (Fallopia japonica): an analysis of capacity to cause structural damage (compared to other plants) and typical rhizome extension

Abstract

Fallopia japonica (Japanese knotweed) is a well-known invasive alien species in the UK and elsewhere in Europe and North America. The plant is known to have a negative impact on local biodiversity, flood risk and ecosystem services; but in the UK it is also considered to pose a significant risk to the structural integrity of buildings that are within seven m of the above ground portions of the plant. This has led to the presence of the plant on residential properties regularly being used to refuse mortgage applications. Despite the significant socioeconomic impacts of such automatic mortgage option restriction, little research has been conducted to investigate this issue. The ‘seven-m rule’ is derived from widely adopted government guidance in the UK. This study considered if there is evidence to support this phenomenon in the literature, reports the findings of a survey of invasive species control contractors and property surveyors to determine if field observations support these assertions, and reports a case study of 68 properties, located on three streets in northern England where F. japonica was recorded. Additionally, given the importance of proximity, the seven-m rule is also tested based on data collected during the excavation based removal of F. japonica from 81 sites. No support was found to suggest that F. japonica causes significant damage to built structures, even when it is growing in close proximity to them and certainly no more damage than other plant species that are not subject to such stringent lending policies. It was found that the seven-m rule is not a statistically robust tool for estimating likely rhizome extension. F. japonica rhizome rarely extends more than four m from above ground plants and is typically found within two m for small stands and 2.5 m for large stands. Based on these findings, the practice of automatically restricting mortgage options for home buyers when F. japonica is present, is not commensurate with the risk.

Is Japanese Knotweed inherently damaging? Network Rail Infrastructure Ltd v Williams and Waistell [2018] EWCA Civ 1514

“…31 The presence of Japanese Knotweed could render a property unsaleable, so it is important that the risk is accurately assessed. Recent peer-reviewed research 32 suggests that the situation is not as black and white as the RICS report suggests. The researchers took several approaches to test the so-called seven-metre rule.…”

Regeneration and growth in crowns and rhizome fragments of Japanese knotweed (Reynoutria japonica) and desiccation as a potential control strategy

“…Some studies also suggest that increasing global temperatures may support the spread of hybrids that do set seed (e.g., Groenevld, ois Belzile & Lavoie, 2014;Buhk & Thielsch, 2015), which is likely to increase the difficulties of controlling this species. R. japonica is often found on road verges and alongside railways (Beerling, Bailey & Conolly, 1994;Palmer, 1994;Child & Wade, 2000;Fennell, Wade & Bacon, 2018) but primarily invades disturbed ecosystems, such as riparian zones, resulting in increased flood risk (Booy, Wade & Roy, 2015), significant biodiversity alterations (Gerber et al, 2008;Aguilera et al, 2010;Gillies, Clements & Grenz, 2016) and other impacts. Various management options are available for control of Japanese knotweed, from avoidance to excavation, with herbicide treatment being the optimal option under most scenarios (Environment Agency, 2016).…”

“…The rhizomes are known to twist and turn more than many other species (Smith et al, 2007) and to grow through even small spaces. However, despite the reputation that rhizomes are associated with major structural damage, there is no evidence that they are any more damaging to built structures than other plants (Fennell, Wade & Bacon, 2018). The importance of R. japonica rhizomes for reproduction and spread can be highlighted by observations that only small fragments of rhizome are required for regeneration (Brock & Wade, 1992;Beerling, Bailey & Conolly, 1994;Francis, Riley & Hoggart, 2008); however, there is uncertainty in the literature regarding the minimum size of rhizome fragment that can regenerate successfully.…”

Review of Existing Knowledge and Practices of Tarping for the Control of Invasive Knotweeds

“…The knotweeds' rhizomatous systems can extend 3 m down into the soil. They can expand laterally from aboveground stems over several meters, with most typically less than 2.5 m and rarely up to 4 m [41]. Tarping is also frequently used in production agriculture and by home gardeners as a soil solarization technique.…”