1 Inflorescences dense and spikelike, symmetrical, cylindrical or ovoid, unbranched; [genus Crypsis, or if treated as part of Sporobolus, section Crypsis; subsection Crypsis]

  2 Plant annual; spikelike panicle 0.3-7.5 cm long

  2 Plant perennial; spikelike panicle (10-) 15-45 (-50) cm long

1 Inflorescences loose and open, or if relatively dense, then with discernible branches, and thus lobed or asymmetrical.

    3 Inflorescence an array of spikes, the spikelets closely imbricate in 2 rows along the rachis of the spikes, the spikes alternate along the primary inflorescence axis; [genus Spartina, or, if treated as part of Sporobolus, section Spartina].

    3 Inflorescence a slender or broad panicle.

      4 Florets with a conspicuous tuft of hairs on the callus; [genus Calamovilfa, or, if treated as part of Sporobolus, section Calamovilfa; subsection Calamovilfa].

      4 Florets not conspicuously hairy on the callus.

        5 Inflorescence a contracted, (superficially) spike-like panicle, < 6 cm broad, the branches appressed to strongly ascending.

          6 Inflorescence 2-5 cm long; most inflorescences enclosed by sheaths (or most or all exserted); plant a geniculate annual; [section Clandestini].

             7 Spikelets (1.3-) 1.6-2.8 mm long; grain falling free of the lemma and palea; lemma glabrous

             7 Spikelets 2.3-5 mm long; grain falling enclosed in the lemma and palea; lemma strigose (use 10× or more magnification) or glabrous.

               8 Lemma and palea shorter than the glumes; palea usually shorter than the lemma; lemma glabrous or strigose with hairs < 0.2 mm long; spikelets 2.3 -3.3 (-3.8) mm long; floret (lemma, palea and enclosed grain) 1.6-3.3 (-3.8)× as long as wide

               8 Lemma and palea longer than the glumes; palea usually longer than the lemma; lemma strigose with hairs > 0.2 mm long; spikelets 2.8-5 mm long; floret (lemma, palea and enclosed grain) 2.2-5.7 (-7.5)× as long as wide

          6 Inflorescence 4-15 cm long; most inflorescences exserted to partly enclosed; plant a rhizomatous or tufted perennial or cespitose annual.

                 9 Plant creeping extensively by slender rhizomes; leaf blades cauline, distichous, to 12 cm long; [section Virginicae]

                 9 Plant loosely tufted, from short rhizomes; leaf blades basal or cauline, not distichous, 10-100 cm long.

                   10 Spikelets 1.2-2.2 mm long; first glume 0.5-0.8 mm long; leaves primarily basal; [section Sporobolus].

                     11 Panicle branches appressed, 0.5-2 cm long in the middle of the inflorescence; second glume acute, > ½ as long as the spikelet

                     11 Panicle branches strongly ascending, 2-8 cm long in the middle of the inflorescence; second glume truncate or broadly obtuse, < ½ as long as the spikelet.

                       12 Plants to 7.5 dm tall; leaf blades to 4 dm long and 2.5-3.5 mm wide

                       12 Plants to 17 dm tall; leaf blades to 7 dm long and 6-8 mm wide

                   10 Spikelets 4-8 mm long; first glume 2-5 mm long; leaves cauline and basal; [section Clandestini].

                          13 Lemma pubescent, usually conspicuously shorter than the palea; pericarp loose when moist

                          13 Lemma glabrous, about as long as the palea; pericarp gelatinous when moist.

                            14 Culms (1.4-) 2.0-5.0 mm thick; uppermost leaf sheath (1.3-) 1.5-6.0 mm wide near its base; panicles with 12-35 primary branches, crowded, densely flowered

                            14 Culms 1.0-2.0 (-2.5) mm thick; terminal sheath 0.8-2.0 (-2.5) mm wide near its base; panicles with 8-18 primary branches, lax, loosely flowered.

                              15 Plants cespitose, lacking scaly rhizomes

                              15 Plants forming clonal patches, with scaly rhizomes

        5 Inflorescence an open panicle, > 2 cm broad, the branches ascending to spreading.

                                16 Branches of the panicle verticillate, whorled; spikelets 2.5-4 mm long; [section Triachyrum]

                                16 Branches of the panicle alternate (some occasionally rather randomly subopposite or opposite, but never regularly whorled); spikelets either 4-6.5 mm long, or 1.2-3.0 mm long.

                                  17 Spikelets 1.2-3.0 mm long.

                                    18 Flag blades (the last leaf below the panicle) nearly perpendicular to the culm; sheath apices with a tuft of white hairs; [section Cryptandri]

                                    18 Flag blades ascending to appressed; sheath apices glabrous or with few and scattered hairs.

                                       19 Mature panicle (1-) 2-5 cm wide; [section Pyramidati].

                                         20 Primary panicle branches bearing spikelets to the base

                                         20 Primary panicle branches without spikelets in the lower 1/8 to ½ of their length

                                       19 Mature panicle 12-35 cm wide; [section Airoides]

                                           21 Secondary panicle branches bearing spikelets throughout their length; pedicels mostly spreading, mostly 0.2-0.5 mm long; [section Airoides]

                                           21 Secondary panicle branches bearing spikelets only in the upper 3/4 or 1/2 of their length; pedicels appressed, mostly 0.5-25 mm long.

                                             22 Pedicels 0.5-2 mm long; anthers 1.1-1.8 mm long; [section Airoides]

                                             22 Pedicels 6-25 mm long; anthers 0.3-1.0 mm long; [section Cryptandri]

                                  17 Spikelets 4-6.5 mm long; [section Calamovilfa; subsection Floridani].

                                               23 First glume scaberulous, acuminate or awn-like; spikelets dark gray; base of plant relatively fibrous; grain spherical; [of barrens and prairies of IN, IL, MO, and n. AR, scattered eastwards as a disjunct, almost always inland of the Coastal Plain]

                                               23 First glume glabrous, acute to acuminate; spikelets purplish (fading tan); base of plant smooth and hard, made up of the indurated leaf bases; grain oblong (when present, usually abortive); [of pine savannas and seeps of the Coastal Plain of NC south to c. FL, west to e. TX and se. OK].

                                                 24 Leaves terete or subterete (wiry), oval in cross-section, sometimes irregularly channeled for part of their length (never with any portion above the sheath flat), < 1 mm wide, tending to senesce and turn tan in autumn, the margins generally smooth; culms (including the inflorescence) (2-) 4-7 (-10) dm tall; culms (from base to first inflorescence branch) 1.5-5 dm tall; first glume averaging about 0.7× as long as the second glume (though variable, ranging from 0.5-0.75×)

                                                 24 Leaves flat (sometimes inrolled or folded when dry), plane or v-shaped in cross-section, with free margins their entire length, (0.5-) 1.2-10 mm wide, tending to remain green into the winter (at least until December), the margins at least partially scabrous (except in S. curtissii); culms (including the inflorescence) 3-22 dm tall; culms (from base to first inflorescence branch) (4-) 6-10 dm tall; first glume averaging 0.75-1× as long as the second glume (though variable, collectively ranging from about 0.5-1.2×).

                                                    25 First glume averaging 0.5-0.9× as long as the second glume (though variable, ranging from 0.5-0.95×); pedicels mostly 4-15 mm long, ascending to spreading; culms (including the inflorescence) (3-) 7-16 (-22) dm tall; inflorescence branches initially ascending, later loosely ascending to spreading; leaves 1.2-10 mm wide, mostly (3-) 4-8 dm long, upwardly scabridulous on at least the lower leaf margins; [e. NC southward and westward to se. OK and e. TX].

                                                      26 Pedicels appressed; lemmas 4.4-6.5 mm long; anthers 3.5-5.0 mm long; [w. LA west to se. OK and e. TX]

                                                      26 Pedicels spreading; lemmas 3.0-4.3 mm long; anthers 2.0-3.4 mm long; [se. NC south to c. FL, west to se. AL and FL Panhandle].

                                                        27 Leaves (2.0-) 3-10 mm wide, typically pale bluish-green (sometimes with some yellowish leaves as well), typically widely spreading to recumbent with age; first glume averaging 0.75-0.9× as long as the second glume

                                                        27 Leaves 1.2-3.0 (-4.0) mm wide, yellowish-green to dark green, mostly strictly ascending; first glume averaging (0.5-) 0.6-0.8× as long as the second glume.

                                                          28 Leaves 2.0-3.0 (-4.0), yellowish-green, generally flat or folded; culms (including the inflorescence) usually (10-) 12-16 dm tall; inflorescence usually 3.5-6 dm long; pedicels glabrous or nearly so; [ne. to c. peninsular FL]

                                                          28 Leaves 1.2-2.0 (-3.0) mm wide, dark green, often inrolled above when dry; culms (including the inflorescence) usually 6-12 (-18) dm tall; inflorescence usually 2-3.5 dm long; pedicels often scabridulous; [e. NC, SC, and e. GA]