Leaves with sheath with margins usually fused when young, hairy; ligule membranous; blade rolled in bud, usually narrow-linear, hairy or glabrous, often lax.
Spikelets laterally compressed or only slightly so, rachilla disarticulating above the glumes and between the lemmas, several–many-flowered, upper florets often imperfect. Glumes unequal, acute, persistent. Lemmas 5–9-nerved, usually 2-toothed at the apex; awn, if present, arising from behind the teeth or dorsally near the tip, straight, bent or recurved; callus short. Paleas usually shorter than the lemma, the 2 keels ciliate or scabrid. Ovary with a terminal, hairy appendage and lateral styles; grain usually adherent to palea and tightly enclosed within the lemma.
This genus has been divided several times in the past and there is still doubt about the names of some of the commonest weedy species. Key from Wheeler et al. (1990).
| | Key to the species | |
| 1 | Awns 3.5–6 cm long; lemmas more than 2 cm long | Bromus diandrus |
| Awns, if present, to 3.5 cm long; lemmas not usually more than 2 cm long | 2 |
| 2 | Lemmas strongly compressed and keeled, often unawned | 3 |
| Lemmas only slightly compressed if at all, rounded on the back, keel not as prominent; lemmas usually awned
Back to 1 | 4 |
| 3 | Panicle compact with spikelets borne on short, erect branches; spikelets and leaves often dull, blue-green; common on sheep camps | Bromus brevis |
| Panicle more open with spikelets borne on long, often drooping branches; spikelets and leaves bright green; found in disturbed areas but not usually sheep camps. (Bromus stamineus also keys out here.)
Back to 2 | Bromus catharticus |
| 4 | Plants rhizomatous; leaves glabrous; lemmas awnless or with short awns, 1–3 mm long | Bromus inermis |
| Plants without rhizomes, leaves usually hairy; lemmas with awns more than 4 mm long
Back to 2 | 5 |
| 5 | Lower glume 1-nerved; spikelets wedge-shaped due to spreading of upper lemmas at maturity | 6 |
| Lower glumes 3–5-nerved; spikelets usually lanceolate in outline as upper lemmas not usually divaricate
Back to 4 | 9 |
| 6 | Panicle open with spreading, often drooping branches; spikelets nodding, not usually purplish | 7 |
| Panicle dense and brush-like to moderately open but branches stiffly erect; spikelets often purplish
Back to 5 | 8 |
| 7 | Spikelets (including awns) usually 4–6 cm long; lower branches of panicle with only 1 or 2 spikelets (panicle sometimes reduced to a raceme); annual or biennial | Bromus sterilis |
| Spikelets (with awns) usually less than 4 cm long; lower branches of panicle with 4 or more spikelets; annual
Back to 6 | Bromus tectorum |
| 8 | Peduncle pubescent; panicle dense and much contracted; panicle branches 0.1–1 cm long and usually not visible | Bromus rubens |
| Peduncle glabrous; panicle less dense with divaricate but always erect branches; panicle branches 1–3 or more cm long with most branches visible
Back to 6 | Bromus madritensis |
| 9 | Panicle compact with spikelets erect on only slightly spreading branches | 10 |
| Panicle open with spreading branches (panicle sometimes erect when first emerged but branches spread later), spikelets often drooping
Back to 5 | 13 |
| 10 | Awn arising from c. 75% of way from lemma base to apex, twisted (often geniculate) and bent away from the spikelet; panicle dense, ovoid. (Bromus squarrosus also keys out here.) | Bromus alopecuros |
| Awn usually subterminal, arising from top 25% of lemma (if arising 75% of way from lemma base then panicle open), straight, usually not twisted but may curve away from spikelets; panicle less dense than Bromus alopecuros
Back to 9 | 11 |
| 11 | Lemmas glabrous or with short hairs less than 0.5 mm long; spikelets and panicle often narrow | Bromus racemosus |
| Lemmas hairy with hairs more than 1 mm long; spikelets and panicle broader than Bromus racemosus
Back to 10 | 12 |
| 12 | Spikelets turgid, the lemmas broad, 8–11 mm long, often tapering abruptly to the tip, awn usually straight | Bromus hordeaceus |
| Spikelets more compressed with narrower elliptic lemmas 7–8 mm long, awns often curving outwards. (Bromus coloratus & B. hankianus also key out here.)
Back to 11 | Bromus molliformis |
| 13 | Spikelets not usually exceeding 2.5 cm (including awns); lemmas 6–9 mm long, awns to 8 mm long, straight; pedicles not curved | Bromus secalinus |
| Spikelets 2–4.6 cm long including awns; lemmas 10–15 mm long (sometimes spreading at maturity), awns more than 8 mm long, straight or bending away from the spikelet; pedicles slender and curved. (Bromus japonicus also keys out here.)
Back to 9 | Bromus arenarius |
