Reducing herbicide input and optimising spray method can minimise non-target impacts on native grassland plant species.

Invasive plants threaten biodiversity worldwide and effective management must control the target invader while conserving biodiversity. Herbicide is often used to control invasive plants, but potential negative impacts on biodiversity have led to spot spraying being recommended over boom spraying to minimise chemical exposure on non-target species. We examined the influence of herbicide application methods on off-target plant communities in threatened temperate grasslands of south-eastern Australia, where spraying with the broadleaf herbicide fluroxypr is commonly used to control St John's Wort, Hypericum perforatum L. It is well established that fluroxypr effectively controls H. perforatum but few studies have examined its impact on native forbs. A spray drift experiment using water-sensitive cards indicated that ground surface coverage was higher for spot spraying (91-99%) than for boom spraying (5-31%). We established a replicated, three-year, before-after-control-impact experiment across 48 1m 2 quadrats to determine how three herbicide application methods (spot spray, fine boom and coarse boom) affected non-target native forbs - the group most likely affected by broadleaf herbicide. This experiment was conducted in grasslands where H. perforatum was almost absent, so responses would reflect direct impacts of the chemical, rather than structural changes resulting from removal of the target invader. Spot spraying decreased the probability of occurrence of native leguminous forbs, while increasing the occurrence of exotic leguminous forbs and the richness of all exotic species and exotic annual forbs. Spot spraying reduced the occurrence of the native Desmodium varians and the abundance of the native Chrysocephalum apiculatum. During this three-year study, native species appeared to be impacted either directly by fluroxypr or indirectly by increased competition with exotic species. Where herbicide application is deemed crucial in these grasslands, we recommend boom spraying when H. perforatum density is moderate to high. Spot spraying should only be used when density of H. perforatum is very low. Given regional variation in H. perforatum density, spatial scale of invasion, soil depth and conservation values, we present a decision tree to assist managers in evaluating the costs and benefits of chemical control, indicating situations where alternative or modified methods could be used. This article is protected by copyright. All rights reserved.