Plants perennial; not rhizomatous. Culms 50-110 (145) cm, erect to spreading; nodes (3) 4-5 (6), usually glabrous, sometimes pubescent; internodes usually glabrous. Basal sheaths often retrorsely pilose; culm sheaths glabrous, often tufted-pilose near the auricle position, midrib of the culm leaves not abruptly narrowed just below the collar; auricles absent; ligules 0.4-2 mm, glabrous, rounded, erose, ciliolate; blades 10-35 cm long, 3-12 mm wide, flat, glabrous. Panicles 10-20 (25) cm, open, nodding; branches ascending to spreading or drooping, filiform. Spikelets 15-25 (40) mm, elliptic to lanceolate, terete to moderately laterally compressed, with (4) 6-10 (15) florets. Glumes usually glabrous, sometimes pubescent; lower glumes 7.5-12.5 mm, 1 (3) -veined; upper glumes 8.9-11.3 mm, 3-veined, often mucronate; lemmas 9-14 (16) mm, elliptic, rounded over the midvein, margins more or less densely pilose on the lower 1/2 or 3/4, lower lemmas in a spikelet glabrous across the backs, uppermost lemmas with appressed hairs on the backs, apices obtuse, entire; awns (2) 3-5 mm, straight, arising less than 1.5 mm below the lemma apices; anthers 1.6-2.7 mm. 2n = 28.
Bromus richardsonii grows in meadows and open woods in the upper montane and subalpine zones, at 2000-4000 m in the southern Rocky Mountains, and at lower elevations northwards. Its range extends from southern Alaska to southern California and northern Baja California, Mexico; it is found as far east as Saskatchewan, South Dakota, and western Texas. Specimens with pubescent nodes and glumes are apparently confined to the southwestern United States.
"decumbent" is not a number.