Japanese knotweed (Reynoutria japonica) is recognised as one of the most pernicious invasive weeds in Europe and worldwide. It was introduced in Europe in the mid-1800s as an ornamental plant and for fodder, where it rapidly spread via water courses, roads and railways, or through the transport of contaminated soil by humans. Japanese knotweed was not listed in the new EU Regulation to address invasive alien species since there is insufficient evidence that it meets the listing criteria (Reg. No. 1143/2014, 2014). Therefore, it is up to the Member States to impose individual control measures.
The UK has legislation surrounding Japanese knotweed; however, the impacts for property sales mainly stem from lenders classing properties with Japanese knotweed 7m or closer as being ‘at risk’, as per the existing RICS framework. Furthermore, there are indications that the general public perceives Japanese knotweed to be a greater risk to property than some evidence suggests, with potential impacts for property valuations and sales.
Other countries may have less stringent legislation and guidance than the UK regarding Japanese knotweed. Whilst it is treated as an invasive weed, the majority of removal projects aim to reduce the environmental impacts.
The Parliamentary Science and Technology Committee held a one-off oral evidence session in early 2019 to explore the science behind the effects of Japanese knotweed on the built environment. The evidence session highlighted the disparity between approaches and attitudes toward Japanese knotweed in UK compared to other countries; this may be leading to the UK taking an overly cautious approach.
The report produced following the session included several recommendations, one of which was a desk-based research study to review international approaches to Japanese knotweed in the context of property sales. Research and learning from the approaches taken to Japanese knotweed and other destructive plants in other countries will enable future approaches to be built upon a sound evidence base. This research will help ensure that the impacts of Japanese knotweed are proportionate to the physical effects of the plant in the built environment.