Spikelets usually with 1-numerous subtending branches; spikelets falling entire, the bristles persistent; florets 1 or 2, upper bisexual, lower sterile or male. Glumes unequal, the lower broader. Lemmas 2, lower sterile, upper fertile, transversely rugose. Paleas 2, unequal, lower minute or absent.
| | Key to the species | |
| 1 | Robust plants with broad, plicate (pleated), palm-like leaf-blades, usually hairy, 30–90 cm long and 3–12 cm wide, narrow-elliptical; panicle open with spreading branches; most spikelets without bristles except for those on the ends of branches | Setaria palmifolia |
| Leaf-blades neither pleated (or only slightly so) nor palm-like; panicle contracted, usually spike-like; most or all spikelets subtended by 1 or more bristles | 2 |
| 2 | Bristles retrorsely barbed, spikelets usually subtended by 1 bristle; upper glume equal to the spikelet | Setaria verticillata |
| Bristles antrorsely barbed; spikelets subtended by 1-many bristles or some spikelets without bristles; upper glume ± shorter than the spikelet
Back to 1 | 3 |
| 3 | Panicle narrow but neither dense nor conspicuously bristly; not all spikelets subtended by a bristle and then by 1 bristle only | Setaria paspalidioides |
| Panicle narrow and cylindrical or linear-lanceolate in outline; always dense and bristly; spikelet(s) subtended by 1 or more bristles
Back to 2 | 4 |
| 4 | Spikelets in clusters subtended by usually >1 bristle, retained in the inflorescence on threshing; fertile floret separating from the rest of the spikelet; panicle often lobed | Setaria italica |
| Spikelets not retained in the inflorescence at maturity; fertile floret falling enclosed by glumes and lower lemma; panicle not lobed; spikelets ± in clusters
Back to 3 | 5 |
| 5 | Nodes bearded | Setaria incrassata |
| Nodes glabrous
Back to 4 | 6 |
| 6 | Upper glume as long as the spikelet, the back of the fertile lemma not exposed | Setaria viridis |
| Upper glume 50–75% the length of the spikelet exposing the rugose back of the fertile floret
Back to 5 | 7 |
| 7 | Panicle linear-lanceolate in outline; spikelets 3–4 mm long on branches to 2.2 cm long; ligule 2–3 mm long; cane-like perennial | Setaria australiensis |
| Panicle linear, cylindrical; spikelets usually not >3 mm long; branches <1 cm long; ligule to 1.5 mm long
Back to 6 | 8 |
| 8 | Plants perennial; bristles evenly coloured | 9 |
| Plants annual; bristles usually pale at the base
Back to 7 | 10 |
| 9 | Plants forming large tufts to 2 m tall with strongly compressed and keeled leaf-sheaths (especially at the base of the plant); 1–4 spikelets on each branch; panicle to 35 cm long; bristles brownish | Setaria sphacelata |
| Plants with wiry rhizomes to 4 cm long, not forming tall tufts, culms spreading or ascending, to 1.2 m long; spikelets solitary on each panicle branch; panicle to 10 cm long; bristles yellow or purplish
Back to 8 | Setaria parviflora |
| 10 | Leaf-blades 2–4 mm wide; culms slender; spikelets 2.5–3 mm long; bristles evenly coloured; panicle 2–5 cm long | Setaria surgens |
| Leaf-blades 4–10 mm wide; culms stout at the base; spikelets 3–3.5 mm long, bristles pale at the base; panicle 1–15 cm long
Back to 8 | Setaria pumila |
