Downy brome, also known as cheatgrass, is an invasive annual grass with the scientific name Bromus tectorum. This plant originated in Eurasia and was introduced to North America, with its first recorded presence in the United States in 1861. Since then, it has become one of the most widespread and problematic invasive plants across the continent, particularly in the Intermountain West, where it can dominate ecosystems like the sagebrush steppe. Its rapid growth and prolific seed production enable it to quickly colonize disturbed areas and expand into surrounding landscapes.
Identifying Downy Brome
Downy brome is identified by its distinct physical traits through different growth phases. Young plants display soft, fine hairs covering both their leaf sheaths and blades. The leaves measure between 2 to 4 millimeters wide and can extend up to 20 centimeters long, often appearing rolled in the shoot and unrolling in an upward twisting pattern. As the plant matures, it develops a distinctive purplish-red hue before drying to a straw-like tan color.
Mature plants reach heights of 6 to 24 inches (15 to 60 cm), sometimes up to 36 inches. Its primary identifier is a drooping, branched seed head (panicle), 1.5 to 8 inches long. These panicles are soft, often purplish-tinged, and contain numerous spikelets with long, sharp awns, measuring up to 0.7 inches (1.8 cm). The presence of these awns and the overall hairy texture help distinguish it from other grasses.
The Prolific Life Cycle
Downy brome functions as a winter annual, a life cycle contributing to its invasive success. Seeds germinate in the late summer or autumn following the first significant rains, often when soil temperatures are around 20 degrees Celsius (68 degrees Fahrenheit). These young seedlings then overwinter in a semi-dormant, vegetative state, allowing their root systems to develop extensively beneath the soil surface even in cold conditions, as low as 37 degrees Fahrenheit (3 degrees Celsius).
As early spring arrives, often several weeks before native perennial plants emerge from dormancy, downy brome plants resume rapid shoot growth. This early growth allows it to utilize available soil moisture and nutrients before other vegetation competes. By mid-May to mid-June, the plants mature quickly, producing thousands of seeds per plant. After setting seed, the plants die off by early summer, leaving behind dry, flammable biomass.
Ecological and Agricultural Damage
Downy brome damages ecological landscapes and agricultural systems. Ecologically, its main impact is increasing wildfire frequency and intensity. As downy brome matures and dries out by early summer, it creates a continuous, fine layer of highly flammable fuel across the landscape. This dense, dry biomass allows fires to spread rapidly and intensely, shortening the natural fire return interval in affected ecosystems like the sagebrush steppe.
The altered fire regime leads to a displacement of native perennial vegetation, which is often less fire-tolerant and slower to reestablish after frequent fires. This shift results in a loss of biodiversity as downy brome outcompetes and replaces native plant communities. Downy brome can also reduce the diversity of other annual species, with populations rapidly increasing after a fire.
In agricultural settings, downy brome threatens crop production and livestock health. It is a problematic weed in grain fields, especially winter wheat, where its similar life cycle allows it to compete directly for resources. Infestations can lead to significant yield reductions in crops like winter wheat.
Beyond crop competition, downy brome also reduces the economic value of rangelands and pastures. While young plants offer some forage value, mature downy brome becomes unpalatable, and its sharp awns can cause physical injury to livestock, irritating their mouths, eyes, and nostrils. The presence of these awns also reduces overall feed intake and can contaminate the fleece of fiber-producing animals.
Control Methods
Controlling downy brome involves a combination of management strategies. Chemical control options include applying pre-emergent herbicides in the fall to prevent germination or post-emergent herbicides in late fall or early spring to target established seedlings. Herbicides like imazapic, sulfosulfuron, and rimsulfuron have shown effectiveness against downy brome, though careful application is needed to avoid harming desirable perennial grasses. Glyphosate can also be used selectively at low rates in perennial grass stands.
Mechanical control methods can be effective for smaller infestations or as part of a larger strategy. Hand-pulling individual plants or small patches in early spring before seed set can prevent further spread. Mowing can reduce seed production if performed between flowering and seed development, though repeated mowing may be necessary as plants can resprout. In agricultural fields, shallow cultivation shortly after germination or deep plowing can significantly reduce populations, provided subsequent plowing is avoided for several years.
Cultural control practices focus on creating conditions unfavorable for downy brome while promoting desirable vegetation. Targeted livestock grazing in early spring, when downy brome is still green and palatable, can reduce its biomass and seed production. Crop rotation, particularly including spring-planted crops, can disrupt the winter annual life cycle of downy brome by allowing tillage or herbicide applications to destroy seedlings before the next crop is sown. Establishing competitive perennial grasses in pastures and field borders can also help suppress downy brome by outcompeting it for resources. Integrated Pest Management (IPM), a multi-pronged approach, often yields the most effective long-term control.