Cheatgrass (Bromus tectorum), an invasive winter annual grass, dominates millions of acres of rangeland in the western United States. This invasive plant degrades native ecosystems by outcompeting desirable perennial grasses for limited soil moisture and nutrients. Cheatgrass dramatically increases wildfire risk because it dries out early, creating a continuous layer of highly flammable fuel. The frequent fires it promotes lead to a self-perpetuating cycle of invasion and fire. Land managers seek control methods that target this aggressive annual grass without harming established perennial grasses.
Principles of Selective Weed Control
Selective control methods exploit the biological differences between cheatgrass and desired perennial grasses. Cheatgrass is an annual, completing its life cycle—germination, growth, and seed production—in a single season, usually germinating in the fall. Perennial grasses are long-lived, surviving multiple seasons and often entering dormancy during cold or dry periods. This difference creates a window of vulnerability for the invader.
The most important physiological difference is phenology, or timing of growth. Cheatgrass is actively growing from late fall through early spring. Established perennial grasses are often dormant during this period, making them less susceptible to soil-active treatments applied at that time. Selective herbicides are designed to target the shallow soil layer where cheatgrass seeds germinate or disrupt the growth processes of fast-growing annuals.
Targeted Chemical Options
Selective control relies on specific herbicides that kill cheatgrass without harming perennial grasses. These chemicals primarily function as pre-emergent herbicides, preventing weed seeds from germinating or establishing root structures.
Imazapic
Imazapic (e.g., Plateau) is a widely used amino acid synthesis inhibitor. It works best when applied before cheatgrass emerges or when seedlings are very small. Established perennial grasses tolerate this chemical, especially during their dormant phase, and it provides residual control for one season.
Indaziflam
Indaziflam (e.g., Rejuvra) is a highly effective pre-emergent option known for its longevity. It inhibits cellulose biosynthesis in new seedlings, preventing cell wall formation and root growth. This herbicide often provides multi-year control because it binds tightly to organic matter in the shallow soil layer where cheatgrass germinates.
Propoxycarbazone-sodium
Propoxycarbazone-sodium is a selective post-emergent option used to control cheatgrass seedlings that have already emerged. This chemical is an amino acid synthesis inhibitor. The application window is narrow and must occur when the cheatgrass is actively growing but still small.
Maximizing Selectivity Through Timing and Application
Successful selective control requires precise timing and method. Herbicide application is most effective when it targets the weed’s vulnerable stage.
The optimal time for applying pre-emergent chemicals like imazapic and indaziflam is in the fall, typically between August and November. This timing ensures the chemical is ready to intercept cheatgrass seedlings as they germinate following the first fall rains. Applying the herbicide during the perennial grass dormancy period minimizes injury risk.
For the herbicide to be active, it must be dissolved and moved into the soil surface, requiring rainfall or irrigation after application. Soil moisture and temperature are factors in efficacy. Using appropriate application techniques, such as proper spray volume, is important to ensure the chemical reaches the soil and avoids non-target damage.
Non-Chemical Management Strategies
Non-chemical strategies can also favor perennial grasses by exploiting the biological weaknesses of the annual invader.
Targeted Grazing
Targeted grazing uses livestock, such as sheep or goats, to heavily graze an area in early spring while cheatgrass is still palatable. The goal is to remove cheatgrass biomass before it produces seeds in late spring or early summer. This method is selective because it occurs before desirable perennial grasses begin their active growth, minimizing harm to them.
Competitive Seeding
Competitive seeding is a restoration strategy that establishes a dense cover of aggressive perennial grasses to outcompete cheatgrass for resources. Species like ‘Sodar’ streambank wheatgrass or ‘Hycrest’ crested wheatgrass are sometimes used. Their robust, established root systems monopolize soil moisture and nutrients, and established perennial grasses can significantly reduce cheatgrass biomass.
Mowing and Tillage
Mowing or tillage can be used for control, but the timing is extremely narrow. It must be executed just before the cheatgrass begins to set seed. Mowing too early allows the plant to regrow, while mowing too late spreads mature seeds. This method is less selective and risks damaging desirable plants if not managed precisely.