Nontarget effects of herbicides on annual forbs and seeded grass in the Great Basin, United States, are partially offset by planting depth and application rate

Abstract

Pre‐emergent herbicides can reduce the abundance of invasive annual plants, but they can also harm native plants, particularly annuals or perennial seedlings, including seeds planted during restoration. We assessed the effects of imazapic and indaziflam on invasive target and nontarget native plants in the Great Basin, a region with extensive invasive annual grasses. We tested nontarget effects on native annual forbs in an agricultural field previously used to grow native annual forbs, which contained a large seed bank. We seeded perennial grass (Elymus elymoides) at multiple depths to determine susceptibility and resistance. Herbicides were applied at full and reduced rates to mimic the effect of litter in natural systems. We observed reductions in most non‐native species in all treatments, but also extensive reductions of native annual forbs, although these were offset at lower application rates, and some species (e.g. Amsinckia tessellata and Microsteris gracilis) were less susceptible than others. Herbicides, particularly indaziflam, reduced E. elymoides emergence, but planting seeds at 2–3 cm depths improved emergence, particularly for imazapic, with 15–68% greater emergence than seeds planted at 1 cm. We suggest surveys for native annual forbs and resistant invaders before applying herbicides and field testing to determine whether reduced rates could provide weed control while maintaining annual forbs. We suggest planting E. elymoides at 2–3 cm when applying herbicides, an approach that may be effective for other species. Herbicide use can be an effective tool, but our results indicate that mitigation of nontarget effects will be needed to maintain native plant diversity.