Poa secunda might run to this genus because of the weakness of the keel on its glumes and lemmas; it may be distinguished from our species of Bromus by its short (less than 5 mm) awnless lemmas in small few-flowered spikelets, as well as by its open sheath. Bromus as recognized here is sometimes split into several genera. The annual Bromus sterilis and B. tectorum are placed in the genus Anisantha, and the perennial alien B. erectus and B. inermis plus our native perennial bromes, B. ciliatus, B. kalmii, B. latiglumis, B. nottowayanus, B. pubescens, and B. pumpellianus in the genus Bromopsis.
1. First glume with one distinct nerve; second glume with 3 (–5) nerves.
2. Awns (8–) 10–30 mm long, as long as or longer than their lemmas; apex of lemma beyond insertion of awn 1.5–2.7 mm long; annual weeds.
3. Lemmas 16–19 mm long; awns mostly ca. 20–30 mm long; second glume 13–17 mm long.
3. Lemmas (8–) 10–12 (–14) mm long; awns ca. (8–) 10–17 (–19) mm long; second glume 7–12 mm long.
2. Awns absent or up to 7 (–9) mm long, shorter than their lemmas; apex of lemma less than 1.5 mm long; perennials, mostly native (B. inermis and B. erectus introduced).
4. Plants with elongate rhizomes; lemmas (at least when fresh) usually ± flushed with purplish, especially toward the margins, the awns absent or less than 4 (–5.5) mm long; anthers 3.3–4.7 (–6) mm long.
5. Lemmas glabrous or scabrous to short-hispid; culms glabrous or finely pubescent at the nodes; leaf blades glabrous (rarely ± pilose on both surfaces or at least on lower surface); awns absent or up to 2.5 (–3.1) mm long; common introduced species.
5. Lemmas pubescent with distinct long hairs (0.5 mm or more) at least toward the margins; culms usually pubescent with long hairs at or immediately adjacent to the nodes (occasionally glabrous); leaf blades pubescent on upper surface, glabrous or sparsely pubescent below; awns mostly (1–) 1.5–4 (–5.5) mm long; native species localized on beaches or dunes.
4. Plants without elongated rhizomes; lemmas (when fresh) green (very rarely flushed with purple), the larger awns 3–7 (–9) mm long; anthers various.
6. Branches of nearly simple panicle erect or strongly ascending; anthers ca. 4.5–6 (–8) mm long; leaf blades involute; a rare clump-forming adventive of open disturbed ground.
6. Branches of the compound panicle loosely ascending, spreading, or nodding at maturity; anthers ca. 0.9–5 mm long; leaf blades flat; native, in forests, savannas, and thickets.
7. Nodes and also number of leaves usually 8–15; leaf sheaths longer than the internodes, thus overlapping and covering all the nodes, the summit of the sheath with a band of dense pubescence and (when intact) with a pair of prominent tooth-like auricles; anthers 1.5–2.2 mm long.
7. Nodes and leaves usually not more than 6 (–8 in B. ciliatus); leaf sheaths shorter than at least the upper internodes, exposing one or more of them, the summit of the sheath glabrous or pubescent but lacking auricles; anthers various.
8. Lemmas ± uniformly hairy (very rarely glabrous); anthers ca. 2.5–5 mm long; glumes pubescent at least on keel (sometimes only scabrous).
9. Lower and upper surface of leaves both dull; front of upper sheath apices (opposite ligule) with or without long hairs, but these usually not much more prominent than the hairs on the back of the sheath.
9. Lower leaf surface with a conspicuous satiny sheen, much shinier than the upper surface; front of upper sheath apices (opposite ligule) usually with a conspicuous tuft of whitish hairs much more prominent than those on the back of the sheath.
8. Lemmas with long hairs along the margin, especially toward the base, glabrous or only minutely pubescent on the back; anthers 0.8–1.7 mm long; glumes glabrous or at most scabrous to minutely hispid.
1. First glume with 3 (–5) distinct nerves; second glume with 5–7 nerves.
10. Lemmas pubescent all across the back, at least apically; glumes pubescent; awns straight (except in the rare B. hordeaceus subsp. molliformis).
11. Larger awns 2–2.8 (–3.5) mm long; primary branches of inflorescence mostly longer than the spikelets; anthers (0.9–) 1.5–2 mm long; ligule less than 0.7 mm long, glabrous on the back; plant a native perennial.
11. Larger awns 3.5–14 mm long; branches of inflorescence mostly much shorter than the spikelets; anthers 0.5–1.2 mm long; ligule ca. 0.5–2 mm long, pubescent on the back (side next to the blade); introduced annuals.
10. Lemmas glabrous or scabrous on the back; glumes likewise glabrous; awns usually ± divaricate or undulate (or absent).
12. Lemma equaling or slightly shorter than the tip of the mature palea; sheaths glabrous (or occasionally the lowermost with some short hairs); margins of ripe lemmas strongly inrolled, exposing the rachilla; lemmas ca. 7–8.2 mm long, the awns ± undulate, sometimes as long as lemma but usually much shorter, rudimentary, or occasionally absent.
12. Lemma at least slightly exceeding tip of palea; sheaths of at least middle and lower leaves ± densely (though sometimes finely) hairy; lemmas and awns various.
13. Sides of larger lemmas (from middle of back to margin) (2.5–) 3–3.5 mm wide, including a broad hyaline border; tips of at least the upper lemmas in a spikelet exceeding their paleas by more than 2 mm; awns either absent (or less than 1 mm long) or very strongly divaricate (even recurved) at maturity.
14. Awns absent or less than 1 mm long; spikelets broad (the larger 8–12 mm wide) and flat.
14. Awns elongate and very strongly divaricate at maturity; spikelets mostly not so broad and flat.
13. Sides of larger lemmas not over 2.5 mm wide; tips of upper lemmas exceeding paleas by less than 2 mm (occasionally 2.5 mm).
15. Longest awns in a spikelet longer than their lemmas and more than twice as long as awn on lowest lemma of the spikelet; branches of inflorescence lax and flexuous; hairs of sheath very fine and delicate, and tending to be ± crooked or tangled toward their tips (though basically ± retrorse); tip of palea 1–2.5 mm shorter than tip of lemma; anthers 0.5–1.5 mm long.
15. Longest awns in a spikelet about as long as their lemmas or shorter, and usually less than twice as long as awn on lowest lemma of the spikelet; branches of inflorescence rather stiff (whether spreading or ascending); hairs of sheath usually fine but stiffish and straight (spreading to retrorse); tip of palea less than 1.5 mm shorter than tip of its lemma (rarely 2.5 mm in B. racemosus); anthers (0.7–) 1.2–2 (–2.5) mm long.
16. Branches of panicle, at least the lower ones, ± widely spreading or even drooping at maturity, forming a broad open inflorescence usually nodding at the summit; larger lemmas ca. (8–) 8.5–10 mm long, attached ca. 1.5–2 mm apart on the rachilla.
16. Branches of panicle erect or very strongly ascending, the entire inflorescence narrow and compact; larger lemmas ca. 6.5–7.5 mm long, attached ca. 1–1.5 mm apart on the rachilla.