Disentangling a Knotty Problem

Jones, D., Bruce, G., Fowler, M.S. et al. Optimising physiochemical control of invasive Japanese knotweed. Biol Invasions 20, 2091–2105 (2018). https://doi.org/10.1007/s10530-018-1684-5

Summary

Itadori knotweed (aka Japanese knotweed; Fallopia japonica var. japonica), which is an infamous invasive plant in Europe and North America that amplifies and exacerbates impacts of climate-change-enhanced flooding, has had major economic impacts. A field study in the United Kingdom found best results from a multiple-stage glyphosate-based treatment, although no mechanical (cutting, covering, excavation) or herbicide (soil spray, foliar spray, and stem injection) treatments eliminated the knotweed entirely after three years. As a chemical control oriented project, mechanical control methods alone and variations on combined herbicide/mechanical methods were not assessed with the same variety of permutations given to chemical control. Nevertheless, some treatment combinations appeared to be more effective than others.

Take Home Points

• Efficient control methods can be identified through field trials - many commonly advised management techniques (cut and fill, summer cutting, and excavation), were found to “add equipment and labour costs and increase environmental impacts, without improving control compared to spraying alone”, while the non-glyphosate herbicides tested (especially synthetic auxins) were not found effective enough to justify continued use.

• Timing of treatment matters. The phenological stage of the plant was a deciding factor in efficacy; these stages will change with the habitat and climatic conditions any given knotweed is found in.

Management Implications

• Climate change is shifting phenology - managers will want to reconsider timing of treatment, such as lengthening the end of growing season treatment window, and determining the window for, and viability of, incorporating the “transitional phenological source–sink stage” (summer/stage 3, see Fig. 1) into management plans.

• Knotweed management, as with all control measures, needs to be considered in context, weighing different ecological, economic, and other objectives. For example, though Jones et al. (2018) determine that stem injection of glyphosate is no more effective in controlling knotweed than spraying, the increase in herbicide used via stem injecting was approximately 20 times more and not weighed against other considerations that can arise when spraying is considered, such as non-target impacts to other vegetation, pollinators, amphibians, and other species. With increasing changes in phenology and life histories, such considerations can be considered alongside efficacy in management decisions.

Keywords: Invasive plant, Knotweed, Mechanical Control, Chemical control, Herbicide, glyphosate, phenology