Invasive Plants of California's Wildland

Cheatgrass (Bromus tectorum) typically is a short grass. Seedlings are bright green with conspicuously hairy leaves, which suggests the alternate common name, downy brome. At maturity the foliage and seedheads often become reddish. After maturity the fine herbage is characterized by a light tan reflectance. The nodding open panicles with moderately awned seeds (caryopses) are distinctive. Seeds readily penetrate clothing of passersby.

Cheatgrass is widespread throughout California. It is the dominant annual grass on sagebrush (Artemisia sp.) rangelands on the Modoc Plateau in northeastern California and along the eastern Sierra Nevada to Owens Valley. It is relatively rare in the annual range communities of California west of the Sierra Nevada, but widespread throughout the Great Basin, Snake River Plain, and the Columbia Plateau. Cheatgrass is a weed of croplands, especially winter wheat and alfalfa. In wildlands it is most commonly found in sagebrush/bunchgrass communities, although its distribution extends to higher-elevation juniper, pinyon-juniper, and pine woodlands.

Cheatgrass is native to southern Europe, northern Africa, and southwestern Asia, where it occurs from sea level to 5,000 feet (1,500 m). This grass has been in the shadow of livestock production since ruminants were first domesticated in southwestern Asia (Young et al. 1972). It has spread to Europe, southern Russia, west central Asia, North America, Japan, South Africa, Australia, New Zealand, Iceland, and Greenland. It was first identified in the United States in 1861 in New York and Pennsylvania and was accidentally introduced to northeastern California late in the nineteenth century. In 1900 it was found in Yosemite National Park, and by 1920 it could be found along the Klamath River, near Yreka (Siskiyou County), Santa Barbara, and Upland in the South Coast (Robbins 1940). It now occurs throughout the United States (including Hawaii and Alaska), except for portions of Alabama, Georgia, South Carolina, and Florida. The hairy seed heads are spread by wind, attachment to animal fur or human clothing, or by small rodents. Contaminated grain seed probably was the early method of dispersal. Seeds can also be dispersed as a contaminant in hay and straw or by mud clinging to machinery.

Cheatgrass displaces native vegetation. It outcompetes the seedlings of native and desirable species for soil moisture. In a classic paper Robertson and Pearce (1944) determined that cheatgrass closed communities to the establishment of seedlings of perennial herbaceous species. Subsequently, it has been determined that cheatgrass also interferes with seedling establishment of shrubs such as antelope bitterbrush (Purshia tridentata) and with pine (Pinus sp.) transplants.

Cheatgrass changes the frequency, extent, and timing of wildfires. The early-maturing fine-textured herbage of cheatgrass increases the chance of ignition and the rate of spread of wildfires. Repeated wildfires lead to the loss of native shrubs and continued cheatgrass dominance (Young and Evans 1978, Young et al. 1987). Slow-moving fauna such as desert tortoises also are sometimes killed in the rapidly moving fires (Lovich, pers. comm.).

Cheatgrass establishes by seeds only. The plant typically flowers from mid-April through June. It has a tremendous seed production capacity, with a potential in excess of 300 seeds per plant, depending on plant density. Plants as small as one inch (2.5 cm) in height may produce seed. Seed production is so abundant that many seeds do not find safe sites for germination. These seeds can remain dormant in the soil for two to three years. Seeds can withstand extremely high soil temperatures. As a general rule, there are twice as many viable cheatgrass seeds in seedbanks as there are plants established in a given year. The dormancy can be broken by gibberellin treatment or nitrate enhancement of the seedbed. Through the mechanism of acquired seed dormancy cheatgrass enjoys the ecological benefits of continuous germination.

In keeping with its flexible flowering traits, cheatgrass can germinate in fall and act as a winter annual. The primary limit to germination is adequate fall, winter, and/or spring moisture. If fall precipitation is limiting and spring moisture is adequate, germination may be delayed until the following spring. Seeds germinate best in the dark or in diffuse light, and they readily germinate at a wide range of temperatures. They do not need to be in contact with bare soil to germinate, and a litter cover generally will improve germination. However, seeds will germinate more quickly when covered with soil, and seedlings rapidly emerge from the top one inch (2.5 cm) of soil. No emergence occurs from seeds buried four inches (10 cm) below the surface.

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Shoot growth occurs in early spring and continues until soil moisture is exhausted. Cheatgrass grows rapidly and may produce dry matter at a rate of 2.9 g/mm2/day. However, growth varies widely from year to year, with practically nothing one year and tons per acre in subsequent years. Extensive root growth can occur during fall and winter. Cheatgrass will produce roots to depths of seven to eight inches (18-20 cm) before sending out far-reaching lateral roots. These lateral roots are one key to survival of this plant. One study showed that cheatgrass has the capability to reduce soil moisture to the permanent wilting point to a depth of twenty-eight inches (70 cm), reducing competition from other species.

Mechanical methods: Mechanical fallows are effective in controlling cheatgrass and establishing herbaceous perennial seedlings. The fallow process accumulates moisture and nitrate to aid in seedling establishment. Tillage in spring after cheatgrass is established is effective if sufficient moisture remains for perennial seedling establishment. Mowing has been shown to reduce seed production when the stand is mowed within one week after flowering. This reduces seed production, but does not eliminate it because plants that develop later and escape mowing will produce seed.

Insects and fungi: No insects or fungi have been approved by the USDA for use on cheatgrass. Research into the biological control of cheatgrass is limited. Cheatgrass is often infected with a head smut fungus (Ustilago bulleta Berk.) that, when severe, may reduce seed yield. Some research has been conducted on pink snow mold (Fusarium nivale) as a biological control agent, but information has yet to be released. In addition to these molds and smuts, over twenty diseases of cheatgrass have been reported.

Several effective herbicide techniques used in the past are no longer available. The registrations for these herbicides have either been lost or not renewed because of cost to the manufacturing companies. Glyphosate (as Roundup®, Rodeo®) applications control cheatgrass, but its effectiveness is limited by the environmental conditions during the cold early spring when glyphosate should be applied. Several newer herbicides are being tested for selective control of cheatgrass in perennial broadleaf seedling stands.