Herbicides sprayed to inhibit exotic annual grass germination and establishment can have unintended effects, including the release of secondary invaders or damage to native plant species.
USGS researchers investigated how exotic and native plant functional groups, species diversity, and a cheatgrass pathogen (Ustilago bullata) responded to treatment targeting exotic annual grasses. The area was burned in 2016 and then treated with the pre-emergent herbicide imazapic and weed-suppressive bacteria (Pseudomonas fluorescens, strain MB906). Exotic annual grass cover and species richness were initially suppressed with treatment but increased to levels similar to untreated plots by the third year. Spraying to treat a single plant functional group did not release native perennials sufficiently to counteract the simultaneous release of secondary invaders; the treatment increased exotic perennial forb cover and cover of the invasive biennial bulbous bluegrass. Authors suggest planting or seeding may also be needed to achieve management goals in these diversely invaded communities.
Lazarus, B.E., Germino, M.J., 2021, A chemical and bio-herbicide mixture increases exotic invaders, both targeted and non-targeted, across a diversely invaded landscape: Applied Vegetation Science, v. 24, no. 2, e12574, https://doi.org/10.1111/avsc.12574.