Long-term responses of wild licorice in native grassland to herbicides and mowing

Abstract

Wild licorice is increasing in abundance across sandy grasslands of the Canadian Prairies, yet little is known about how to manage this plant. We conducted two field studies documenting the long-term impact of different herbicides applied one-time in either spring or summer, and annual mowing in either spring or summer, on the abundance of wild licorice for up to five growing seasons (2016-20). Herbicides led to variable reductions in licorice density and biomass between sites and over time, with the greatest reductions coinciding with summer rather than spring herbicides during the first two years post-treatment. Greater and more consistent decreases were evident from the application of aminopyralid and aminocyclopyrachlor, although all herbicides, including dicamba and 2,4-D, exhibited some suppression of licorice out to four years after treatment. Reductions in wild licorice also varied with growing conditions, as drought reduced licorice biomass across all treatments, including the non-treated control. Unlike herbicides, annual mowing, particularly in summer (late July; 9-10 leaf stage), increased wild licorice total annual biomass produced for up to two growing seasons, coincident with above-normal rainfall. With the onset of drought in year three, mowing effects dissipated and did not reappear through year five. Finally, while initially effective in reducing wild licorice, hand-pulling effects on licorice were short-lived, and the selective removal of neighboring herbs led to an increase in licorice during drought. Overall, these results highlight the challenge of managing wild licorice populations in grasslands and provide insight into the strategies that may aid the management of this species.