Downy brome inhibits revegetation efforts following ecosystem disturbance. Imazapic is a commonly used herbicide for downy brome management, but more information is needed regarding effective application timing for restoration efforts. We wished to determine (1) if native species establishment exhibited a tradeoff between downy brome competition and injury from herbicide and (2) if this differed between pre- and postemergent applications of imazapic. We used a standard replacement series design and overlaid herbicide treatments. Nine weeks after planting, aboveground biomass was harvested and relative yield (RY) indices calculated. Both imazapic applications reduced downy brome biomass by 91% or more (P < 0.05). Imazapic caused drastic reductions in native biomass but less than what was caused by downy brome competition (P < 0.05). Natives were less injured by a pre- than postemergent application (P < 0.05). In situations where downy brome may impact restoration efforts, pre-emergent applications of imazapic at 70 g ai ha⁻¹ (0.06 lb ai ac⁻¹) may reduce downy brome with less negative impacts on newly-seeded native grasses than post-emergent applications. Ensuring sufficient proportions of native species seeds on restoration sites may reduce downy brome.

Nomenclature: Imazapic; blue grama; Bouteloua gracilis (Willd. ex Kunth) Lag. ex Griffiths; downy brome; Bromus tectorum L.; needle-and-thread; Hesperostipa comata (Trin. & Rupr.) Barkworth; western wheatgrass; Pascopyrum smithii (Rydb.) Á. Löve).

Management Implications: Downy brome, a highly competitive invasive grass in the western US, inhibits revegetation efforts following disturbance. While mature perennial grasses may be good competitors against downy brome, seedlings are not. Downy brome control and reseeding efforts may be equally important to revegetation success. Imazapic is a commonly used herbicide for downy brome management, but more information is needed regarding effective application timing for restoration efforts.

We reduced downy brome biomass by 91% or more with a relatively low imazapic rate applied either pre- or post-emergent. Since both application timings provided similar downy brome reduction, managers should focus on the application timing that maximizes native perennial grass production. While imazapic reduced perennial grass growth, downy brome competition caused greater reductions. A pre-emergent application led to less perennial grass biomass reduction overall. Thus, if seeding natives in the fall, it may be beneficial to preemptively apply imazapic instead of reacting after downy brome seedling emergence. Ensuring sufficient proportions of native seeds at a site may also help reduce downy brome. Native seedlings exerted competitive interference upon downy brome and reduced downy brome biomass more than expected from reductions in downy brome seeding.

Our study continues to highlight the need to control downy brome infestations at early stages prior to the loss of mature native perennial grasses – not only to facilitate brome control, but to facilitate perennial grass dominance. Established perennial grasses are better able to withstand the stressors imposed by herbicide and downy brome competition. When the reseeding of native perennials is necessary, it may be desirable to use high seeding rates to increase the proportion of natives in the seedbank or methods to reduce downy brome seedbanks and competition.