| | 1. Spikelet or spikelet cluster subtended by 1–3 bristles; lower floret palea generally < 2/3 × lemma |
| |
| | 2. Short stiff hairs on bristles pointed downward; main inflorescence axis scabrous |
| S. verticillata |
| | 2' Short stiff hairs on bristles pointed toward tip; main inflorescence axis short-hairy |
| |
| | 3. Spikelet 2.5–3 mm; upper leaf blade surface short-soft-hairy |
| S. faberi |
| | 3' Spikelet ± 2 mm; upper leaf blade surface glabrous |
| S. viridis |
| | 1' Spikelet or spikelet cluster subtended by 4–15 bristles; lower floret palea generally well developed |
| |
| | 4. Spikelet ± 2.8–3.5 mm, generally 1.5–2 mm wide; annual |
| S. pumila |
| | 4' Spikelet generally < 2.8 mm, width generally = or < 1.5 mm; perennial herb |
| |
| | 5. Inflorescence axis < 8 cm; base of flowering stems with hard, knot-like swellings |
| S. gracilis |
| | 5' Inflorescence axis > 8 cm; base of flowering stems not swollen |
| S. sphacelata
|