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| 1 Spikelets solitary, not associated with rudimentary spikelets or naked pedicels. |
| ..2 Panicles usually composed of a terminal pair of branches, sometimes with 1 (-5) additional branches below the terminal pair. |
| ....3 Upper glumes pubescent on the back or margins. |
| ......4 Spikelets 1.3-1.9 mm long; upper glumes pilose along the margins |
| ......4 Spikelets 2.4-3.2 mm long; upper glumes sparsely pubescent on the back |
| ....3 Upper glumes glabrous. |
| ........5 Spikelets elliptic, acute or acuminate at the tip |
| ........5 Spikelets ovate to broadly elliptic, obtuse to broadly acute at the tip. |
| ..........6 Spikelets 1.9-2.3 mm long; leaf blades flat |
| ..........6 Spikelets 2.5-4.0 mm long; leaf blades flat or longitudinally folded |
| ..2 Panicles with 1-70 branches, if > 1, the branches arranged racemosely. |
| ............ 7 Panicle branches 7-70, the axes extending beyond the outermost spikelets; panicle branches disarticulating at maturity |
| ............ 7 Panicle branches 1-6, terminating in a spikelet; panicle branches persistent. |
............ ..8 Upper florets olive to dark brown |
| ............ ..8 Upper florets pale to tan. |
| ............ ....9 Axes of panicle branches not broadly winged, 0.6-1.3 mm wide. |
| ............ ......10 Spikelets orbicular, 2.8-3.2 mm wide |
| ............ ......10 Spikelets slightly longer than broad, 2.0-2.5 mm wide |
| ............ ....9 Axes of panicle branches broadly winged, 1.8-3.3 mm wide. |
| ............ ........11 Spikelets 3.2-4.0 mm long; upper lemmas with a few short hairs at their tips |
| ............ ........11 Spikelets 1.7-2.1 mm long; upper lemmas glabrous |
| 1 Spikelets paired, or at least the second nonfunctional spikelet represented by a naked pedicel. |
| ............ ..........12 Spikelets 1.0-1.3 mm long |
| ............ ..........12 Spikelets 1.3-4.1 mm long |
| ............ ............ 13 Margins of upper glumes and lower lemmas pilose. |
| ............ ............ ..14 Panicle branches 2-7; spikelets 2.3-4.0 mm long |
| ............ ............ ..14 Panicle branches (4-) 10-30; spikelets 1.8-2.8 mm long |
| ............ ............ 13 Margins of upper glumes and lower lemmas glabrous or pubescent (if pubescent, the hairs not pilose), but neither ciliate-lacerate, winged, nor pilose. |
| ............ ............ ....15 Upper florets olive to dark brown. |
| ............ ............ ......16 Panicle branches 10-28 (or more). |
| ............ ............ ........17 Plants annual; axes of panicle branches broadly winged, the wings about as wide as the central portion; [common native] |
| ............ ............ ........17 Plants perennial; axes of panicle branches narrowly winged, the wings narrower than the central portion; [rare exotics]. |
| ............ ............ ..........18 Axes of panicle branches 0.5-1.2 mm wide; spikelets 1.1-1.8 mm wide |
| ............ ............ ..........18 Axes of the panicle branches 1.0-1.7 mm wide; spikelets 1.8-2.4 mm wide |
| ............ ............ ......16 Panicle branches 1-10 (or to 28 in P. boscianum, keyed under both leads). |
| ............ ............ ............ 19 Plants annual. |
| ............ ............ ............ ..20 Spikelets 1.3-1.8 mm wide, broadly elliptic to orbicular, glabrous; panicles with 1-10 (-28) branches, the axes 0.7-2.3 mm wide |
| ............ ............ ............ ..20 Spikelets 1.7-2.4 mm wide, broadly obovate, shortly pubescent; panicles with 1-5 branches, the axes 0.8-1.3 mm wide |
| ............ ............ ............ 19 Plants perennial. |
| ............ ............ ............ ....21 Plants cespitose, rhizomes poorly developed; culms 10-20 dm tall; panicle branches ascending, divaricate, or reflexed. |
| ............ ............ ............ ......22 Leaves 7-18 mm wide |
| ............ ............ ............ ......22 Leaves 2.5-4 mm wide |
| ............ ............ ............ ....21 Plants not cespitose, rhizomatous; culms 1-15 dm tall; panicle branches ascending. |
| ............ ............ ............ ........23 Rhizomes long, evident |
| ............ ............ ............ ........23 Rhizomes short, indistinct |
| ............ ............ ....15 Upper florets white, stramineous, or golden brown. |
| ............ ............ ............ ..........24 Lower lemmas with well-developed cross-ribs over the veins; upper glumes absent |
| ............ ............ ............ ..........24 Lower lemmas not ribbed over the veins; upper glumes present. |
| ............ ............ ............ ............ 25 Panicles with 15-100 branches. |
| ............ ............ ............ ............ ..26 Plants annual; upper glumes and lower lemmas rugose |
| ............ ............ ............ ............ ..26 Plant perennial; upper glumes and lower lemmas smooth. |
| ............ ............ ............ ............ ....27 Plant rhizomatous; panicle branch axes 0.9-1.2 mm wide; panicle branches often arcing |
| ............ ............ ............ ............ ....27 Plant cespitose; panicle branch axes 0.3-0.6 mm wide; panicle branches straight. |
| ............ ............ ............ ............ ......28 Panicle branches spreading to reflexed (rarely ascending); leaf blades 10-23 mm wide; axes of panicle branches 0.3-0.4 mm wide |
| ............ ............ ............ ............ ......28 Panicle branches erect to ascending; leaf blades 4.9-6.1 mm wide; axes of panicle branches 0.5-0.6 mm wide |
| ............ ............ ............ ............ 25 Panicles with 1-15 branches. |
| ............ ............ ............ ............ ........29 Spikelets 1.3-2.5 mm long. |
| ............ ............ ............ ............ ..........30 Upper glumes (and usually also the lower lemmas) shortly pubescent. |
| ............ ............ ............ ............ ............ 31 Lower glumes present |
| ............ ............ ............ ............ ............ 31 Lower glumes absent. |
| ............ ............ ............ ............ ............ ..32 Panicles both terminal and axillary, the axillary panicles partially or completely enclosed by the subtending leaf sheath |
| ............ ............ ............ ............ ............ ..32 Panicles all terminal |
| ............ ............ ............ ............ ..........30 Upper glumes and lower lemmas glabrous. |
| ............ ............ ............ ............ ............ ....33 Panicles both terminal and axillary, the axillary panicles partially or completely enclosed by the subtending leaf sheath |
| ............ ............ ............ ............ ............ ....33 Panicles all terminal. |
| ............ ............ ............ ............ ............ ......34 Upper panicle branches erect |
| ............ ............ ............ ............ ............ ......34 Upper panicle branches spreading to ascending. |
| ............ ............ ............ ............ ............ ........35 Upper glumes and lower lemmas 5-veined |
| ............ ............ ............ ............ ............ ........35 Upper glumes and lower lemmas 3-veined. |
| ............ ............ ............ ............ ............ ..........36 Lower sheaths villous or hirsute; spikelets 2.6-3.5 mm long |
| ............ ............ ............ ............ ............ ..........36 Lower sheaths glabrous or sparsely papillose pubescent; spikelets 2.2-2.8 mm long |
| ............ ............ ............ ............ ........29 Spikelets 2.5-4.1 mm long. |
| ............ ............ ............ ............ ............ ............ 37 Spikelet pairs barely if at all imbricate; lower glumes usually present |
| ............ ............ ............ ............ ............ ............ 37 Spikelet pairs imbricate; lower glumes absent or present. |
| ............ ............ ............ ............ ............ ............ ..38 Upper glumes pubescent; lower lemmas usually pubescent. |
| ............ ............ ............ ............ ............ ............ ....39 Lower glumes present |
| ............ ............ ............ ............ ............ ............ ....39 Lower glumes absent |
| ............ ............ ............ ............ ............ ............ ..38 Upper glumes glabrous; lower lemmas usually glabrous. |
| ............ ............ ............ ............ ............ ............ ......40 Upper florets golden brown |
| ............ ............ ............ ............ ............ ............ ......40 Upper florets pale to tan. |
| ............ ............ ............ ............ ............ ............ ........41 Terminal panicle branches erect |
| ............ ............ ............ ............ ............ ............ ........41 Terminal panicle branches spreading to ascending. |
| ............ ............ ............ ............ ............ ............ ..........42 Plants decumbent, rooting at the lower nodes; spikelets obovate to elliptic |
| ............ ............ ............ ............ ............ ............ ..........42 Plants rhizomatous; spikelets orbicular to elliptic. |
| ............ ............ ............ ............ ............ ............ ............ 43 Spikelets 2.9-4.1 mm long; 1.9-3.1 mm wide, suborbicular to elliptic; upper glumes 5-veined; leaf blades flat |
| ............ ............ ............ ............ ............ ............ ............ 43 Spikelets 2.1-3.1 mm long, 2.0-2.8 mm wide, orbicular or nearly so; upper glumes 3-veined; leaf blades laterally folded. |
| ............ ............ ............ ............ ............ ............ ............ ..44 Lower sheaths villous or hirsute |
| ............ ............ ............ ............ ............ ............ ............ ..44 Lower sheaths glabrous or sparsely papillose pubescent |