Enhancing Japanese knotweed control and longterm habitat restoration post-treatment

Japanese knotweed (Reynoutria japonica var. japonica) is among the most problematic invasive plants worldwide due to its negative impacts on recipient ecosystems and propensity for rapid spread, eliciting substantial control costs. As global focus on sustainability intensifies in response to the current biodiversity and climate crises, the value of long-term ecological monitoring and planning is increasingly evident. As a long-lived perennial species, the Japanese knotweed invasion highlights this. While empirical evidence of effective knotweed management measures is growing, there is conflict between knotweed management and the wider ecological and environmental impacts of this. The focus of this thesis was to investigate the long-term effects of Japanese knotweed invasion and subsequent management in a large-scale, long-term knotweed field trial, and assess avenues for post-treatment restoration. Long-term plant community dynamics, soil abiotic characteristics and microbial diversity were investigated at a site in Taff’s Well, south Wales, UK. This study found that the diversity of native flora was largely limited to ruderal species, potentially hindering regeneration potential of native plants. The rare hybrid Conolly’s knotweed (x Reyllopia conollyana) was also discovered in the seed bank. Secondary invasion by Himalayan balsam also occurred. Native species diversity significantly increased in response to knotweed management (i.e., herbicide application). Soil abiotic characteristics showed varied response to knotweed treatment; spatial variation explained changes to most parameters measured. There was no discernible effect of knotweed cover or herbicide application on soil microbial diversity. Restoration trials found that active restoration may be a valuable tool for enhancing post-knotweed community recovery. Native species abundance and target community composition can be enhanced by revegetation. While functional composition of revegetation seed mixes did not enhance knotweed suppression, this may provide insight into the coexistence of native and invasive species to enhance biodiversity. Physical covering using geotextiles increased native species abundance but did not suppress knotweed. Finally, a comparative life cycle assessment (LCA) of knotweed treatment methods found that the simplest and most effective knotweed management method (annual glyphosate foliar spray) elicited the lowest environmental impacts to produce. LCA of restoration methods found that geotextile matting and revegetation contributed the greatest environmental impacts. The results of this thesis can inform sustainable, long-term knotweed management and enhance practical alignment with priorities around ecological and environmental sustainability.