Bromus tectorum
| Downy brome | ||||||||||||||
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Bromus tectorum |
Downy brome (Bromus tectorum) is native to Eurasia and the Mediterranean region. 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. By 1928, downy brome had reached its present range. It occurs throughout the United States (including Hawaii and Alaska), except for portions of Alabama, Georgia, South Carolina, and Florida. It grows on rangelands, pastures, prairies, fields, waste areas, eroded sites, and roadsides. It is most abundant in the Great Basin and Columbia Basin of the western United States.
Downy brome grows in many climatic areas. It is found primarily in the 150-560 mm precipitation zone. Downy brome will grow in almost any type of soil. Research has shown that it is most often found on coarse textured soils and does not grow well on heavy, dry, and/or saline soils. Downy brome has been found growing on B and C horizons of eroded areas and areas low in nitrogen. It grows in a relatively narrow range of soil temperatures. Growth starts at 2.0-3.5°C and stops when temperatures exceed 15°C. Litter promotes germination and establishment of seedlings.
Downy brome is a winter annual. It germinates in the fall, over winters as a seedling, then flowers in the spring. If fall precipitation is limiting and spring moisture is adequate, the seeds will germinate in the spring, and the plants will flower that summer. Downy brome is cleistogamous and self-fertile, with no evident out-crossing. The plants typically flower from mid-April through June. Florets are dispersed within a week of maturity by wind, attachment to animal fur, or small rodents. Seeds move as a contaminant in hay, grain, straw, and machinery. An abundant seed producer, downy brome has been reported as having a production potential in excess of 300 seeds per plant. Under optimal conditions, it may produce 450 kg of seed per hectare with about 330,000 seeds/kg. Seed production per plant is dependent on plant density. Plants as small as 2.5 cm in height may produce seed.
Seeds will maintain a high viability in dry storage, lasting in excess of 11 years. In the field, under buried conditions, seeds will loose their viability in 2 to 5 years. The primary limit to germination is adequate fall, winter, and/or spring moisture. Seeds can withstand extremely high soil temperatures. Seeds germinate the best in the dark or in diffuse light. 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. Seedlings rapidly emerge from the top 2.5 cm of soil, and a few plants emerge from depths of 8 cm. No emergence occurs from seeds buried 10 cm below the surface.
Downy brome possesses an extensive root system that grows through the spring. It will produce roots to depths of 18-20 cm prior to sending out far-reaching lateral roots. These lateral roots are one of the keys to the survival of this plant. One study showed that downy brome had the capability to reduce soil moisture to the permanent wilting point to a depth of 70 cm, reducing competition from other species. Downy brome grows rapidly and may produce dry matter at a rate of 2.9 g/mm2/day.