Plants perennial; not rhizomatous. Culms 90–150 cm, erect; nodes 1–2(3), pubescent or puberulent; internodes usually puberulent, occasionally glabrous, pilose to densely pubescent below the nodes. Sheaths densely pubescent to pilose, with hairs to 1 mm, collars pilose, with hairs to 2 mm; auricles absent; ligules 0.5–2.5 mm long, sparsely to densely pubescent, obtuse, erose; blades 7.5–16.5 cm long, 3–12 mm wide, flat, densely pubescent on both surfaces. Panicles 5–16 cm, open; branches erect, ascending and appressed to slightly spreading. Spikelets 25–35(45) mm, terete to moderately laterally compressed, with 3–7 florets. Glumes sparsely to densely pubescent; lower glumes 5–8(9) mm, 1(3)-veined; upper glumes (7)8–9 mm, 3-veined, sometimes mucronate; lemmas 10–14 mm, elliptic, rounded over the midvein, backs sparsely to densely pubescent, margins pubescent, apices entire; awns 3.5–7 mm, straight, arising less than 1.5 mm below the lemma apices; anthers 3–6 mm. 2n = unknown.
Bromushallii grows in southern California on dry, open or shaded hillsides, rocky slopes, and in montane pine woods, from 1500–2700 m.