| 1. Spikelet or spikelet cluster subtended by 13 bristles; lower floret palea generally < 2/3 × lemma |
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| 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 |
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| 3. Spikelet 2.53 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 415 bristles; lower floret palea generally well developed |
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| 4. Spikelet ± 2.83.5 mm, generally 1.52 mm wide; annual |
| S. pumila |
| 4' Spikelet generally < 2.8 mm, width generally = or < 1.5 mm; perennial herb |
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| 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
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