Plants perennial; not rhizomatous. Culms 70–180 cm, erect; nodes 3–7, pubescent or puberulent; internodes pubescent, puberulent, or glabrous. Sheaths densely pubescent, hairs to 1 mm, collars with hairs to 2 mm, midrib of the culm leaves not abruptly narrowed just below the collar; auricles sometimes present; ligules 1–3 mm, densely pubescent to pilose, obtuse, lacerate; blades (13)18–38 cm long, 3–12 mm wide, flat, sparsely to densely pubescent on both surfaces. Panicles 14–27 cm long, very open, nodding; branches flexuous, usually widely spreading, with spikelets near the tips. Spikelets 20–37(45) mm, elliptic to lanceolate, terete to moderately laterally compressed, with 4–9(10) florets. Glumes pubescent, with prominent veins, margins not bronze-tinged; lower glumes 4–8.5 mm, (1)3-veined; upper glumes 7–10 mm, 3(5)-veined, not mucronate; lemmas 9.5–14 mm, lanceolate, rounded over the midvein, backs pilose, pubescent, or glabrous, margins pilose, not bronze-tinged, apices subulate to acute, entire; awns 3–7 mm, straight, arising less than 1.5 mm below the lemma apices; anthers 3–6 mm. 2n = 14.
Bromus grandis grows on dry, wooded or open slopes, at elevations of 350–2500 m. Its range extends from central California into Baja California, Mexico.