Plants perennial; usually rhizomatous, sometimes cespitose, rhizomes short to long-creeping. Culms 50-135 cm, erect or ascending, sometimes geniculate, usually single or few together, sometimes clumped; nodes 2-7, pubescent or glabrous; internodes glabrous or pubescent. Sheaths pilose, villous, or glabrous; auricles sometimes present on the lower leaves; ligules to 4 mm, glabrous, truncate or obtuse, erose; blades 7-30 cm long, 2.5-8.5 (9) mm wide, flat, pubescent or glabrous on both surfaces, sometimes only the adaxial surface pubescent. Panicles 10-24 cm, open or contracted, erect or nodding; branches erect to spreading. Spikelets 16-32 (45) mm, elliptic to lanceolate, terete to moderately laterally compressed, sometimes purplish, with 4-14 florets. Glumes glabrous or hairy; lower glumes (4) 5-10 mm, 1 (3) -veined; upper glumes (5) 7.5-13 mm, 3-veined; lemmas 9-16 mm, lanceolate, rounded over the midvein, sparsely to densely hairy throughout, or on the margins and lower portion of the back, or along the marginal veins and keel, apices subulate to acute, entire or slightly emarginate, lobes shorter than 1 mm; awns usually present, sometimes absent, to 7.5 mm, straight, arising less than 1.5 mm below the lemma apices; anthers 3.5-7 mm. 2n = 28, 56.
The range of Bromus pumpellianus extends from Asia to North America, where it includes Alaska, the western half of Canada, the western United States as far south as New Mexico, and a few other locations eastward. It is sometimes treated as a subspecies of B. inermis. It differs from that species primarily in its tendency to have pubescent lemmas, nodes, and leaf blades.
Two subspecies that differ in morphology and distribution are described below. Both strongly resemble the recently introduced Bromus riparius, differing in the case of B. pumpellianus subsp. pumpellianus in having longer rhizomes, or, in the case of B. pumpellianus subsp. dicksonii, in having a more restricted distribution. It is possible that the description and distribution of B. pumpellianus may be based in part on misidentification of B. riparius, as many taxonomists may have been unaware of the introduction of the latter species to North America.
|1||Panicles usually open; plants cespitose, sometimes shortly rhizomatous; culms ascending, often geniculate; nodes glabrous or pubescent; plants of the Yukon River drainage||Bromus pumpellianus subsp. dicksonii|
|1||Panicles contracted to open; plants rhizomatous; culms erect; nodes usually pubescent; plants of the range of the species||Bromus pumpellianus subsp. pumpellianus|
"decumbent" is not a number.