A Potential Solution to the Challenge of Controlling Invasive Annual Grasses While Maintaining a Sagebrush Overstory

Abstract

Managing invasive annual grasses in sagebrush steppe communities with a shrub overstory can be challenging because herbicides, commonly in liquid formulations, applied to control annual grasses may be intercepted by shrub canopies. Poor control of annual grasses and damage to wildlife-important shrubs, such as sagebrush, may occur if herbicides are intercepted by shrubs. Aerial broadcast application of imazapic, a pre-emergent herbicide, in granular formulation may overcome these challenges in annual grass-invaded rangelands with a shrub overstory. However, granular imazapic effectiveness at controlling annual grasses and effects on shrubs have not been investigated in rangelands with a shrub overstory. We investigated the effects of aerial application of granular imazapic (124 g ai·ha⁻¹) in the summer (pre-emergent) at five sites for two growing seasons postapplication. Granular imazapic substantially reduced annual grass cover and density in plant communities with a shrub overstory and did not negatively impact sagebrush. Control of invasive annual grasses with granular imazapic did not increase the density of perennial herbaceous vegetation, likely because of low perennial vegetation abundance prior to treatment and slow response to increased resources. Although granular imazapic application resulted in slight increases in perennial forb and Sandberg bluegrass cover, it does not appear to be an effective stand-alone treatment to shift herbaceous vegetation from annual- to perennial-dominated, at least not in plant communities with low abundance of co-occurring perennial grasses and forbs. However, its ability to temporarily reduce invasive annual grasses without damaging the shrub overstory may make it a valuable tool in efforts to restore perennial dominance that include additional treatments and seeding perennial grasses and forbs. More intensive treatment and seeding combinations need to be tested before they can be recommended for practice.