| | 1. Lower stems densely hairy |
| A. californica var. californica |
| |
| |
| | 3. Lemma awns < 2.5 cm; glumes 5–10 mm |
| A. adscensionis |
| | 3' Lemma awns > 3.5 cm; glumes 20–34 mm |
| A. oligantha |
| |
| | 4. Base of 1° inflorescence branches abruptly spreading or ascending from main axis |
| |
| | 5. Lower inflorescence branches ascending, upper appressed |
| A. purpurea var. parishii |
| | 5' All inflorescence branches spreading |
| |
| | 6. Leaf blade base glabrous; lemma tip much twisted; anthers ± 1 mm |
| A. divaricata |
| | 6' Leaf blade base sparsely long-hairy; lemma tip slightly twisted or straight; anthers 1–2 mm |
| A. ternipes var. hamulosa |
| | 4' Base of 1° inflorescence branches appressed to main axis, tips sometimes spreading or ascending |
| |
| A. purpurea |
| | 8. Upper glume generally > 16 mm; lemma ± linear |
| var. longiseta |
| | 8' Upper glume generally < 16 mm; lemma gradually tapered to tip |
| var. purpurea |
| |
| | 9. Mature inflorescence branches and spikelet stalks slender, curving to drooping, appearing S- or U-shaped |
| var. purpurea |
| | 9' Mature inflorescence branches and spikelet stalks generally stiff, straight, erect to spreading |
| |
| | 10. Tip of lemma generally < 0.2 mm wide; awns delicate, generally < 0.2 mm wide at base |
| var. nealleyi |
| | 10' Tip of lemma generally > 0.2 mm wide; awns stiff, generally > 0.2 mm wide at base |
| |
| | 11. Inflorescence generally 3–14 cm; leaf blade generally < 10 cm |
| var. fendleriana |
| | 11' Inflorescence generally 15–30 cm; leaf blade generally > 10 cm |
| |
| | 12. Inflorescence dense, reddish, fading to straw-colored; lower branches with 8–18 spikelets |
| var. parishii |
| | 12' Inflorescence somewhat loose, tan to brown, fading to straw-colored; lower branches with 2–10 spikelets |
| var. wrightii
|