1 Culms, rays, and leaves bluish-glaucous, papillose; leaves harshly scabrid on margins and midrib; spikes with numerous well-developed branches or secondary rays; spikelets dark reddish brown at maturity

1 Culms, rays, and leaves green, not papillose; leaves glabrous to scabridulous; spikes solitary or with a few reduced basal branches; spikelets various.

  2 All but the uppermost spikelets strongly (-weakly in some forms of C. hystricinus) reflexed; scales (3.8-) 4-4.9 (-5.4) mm long, terminal involute and prolonged distally, midrib often bent downward; achenes linear to linear-oblong, (2.2-) 2.5-3 (-2.3) mm long; [section Umbellati, subsection Longistylis, C. retrofractus complex]

    3 Spikes mostly at least 2× as long as wide, slightly (or not) narrowed to base, spikelets reflexed to spreading; culms, leaves, and bracts glabrous except for marginal prickles on leaves and bracts; rachilla wing clasping achene for its entire length (concealing it in abaxial view); floral scales golden-brown when mature

    3 Spikes mostly about as long as wide and strongly narrowed to the base, spikelets reflexed; culms scabrous, bracts scabridulous and also often hirtellous; rachilla wing clasping achene for <3/4 its length; floral scales greenish to brown or reddish-purple-striped.

      4 Rays scabrid throughout their lengths; leaves and bracts puberulent; longest ray equal to or longer than the longest bract; culms roundly trigonous to subterete; terminal scale spinose distally; scales 1-2 (-3); spikes tight, burlike, turbinate

      4 Rays glabrous or rarely with a few hairs just below the spike; leaves and bracts with hairs confined to abaxial midrib; longest ray shorter than longest bract; culm sharply trigonous; terminal scale involute but not spinose; scales (2-)3-6; spikes often loose, sometimes more elongate

  2 Spikelets ascending to spreading, or reflexed in the lower half of spike; scales 1.8-5.3 mm long, terminal scale not modified as above; achenes ellipsoid to narrowly oblong, 1.2-2.4 (-2.6) mm long (linear-oblong in C. refractus).

        5 Spikes densely cylindric, all sessile or 1-3 (-5) lateral spikes on short horizontal rays; spikelets ellipsoid (rarely oblong-ellipsoid), with 1-2 (-4) scales; [section Umbellati, subsection Brevistyli].

          6 Spikes dense, the rachis obscured when pressed; achenes lacking a stipitate base, apiculate at the apex; [native of Mexico and Central America, introduced northward along the Gulf and Atlantic coasts]

          6 Spikes relatively loose, the rachis usually visible when pressed; achenes cuneate to a stipitate base, broadly rounded to acute at the apex; [native, s. TX, uncertain reports]

        5 Spikes oblong to ovoid (cylindric in C. tetragonus), primarily on well-developed rays; spikelets oblong-lanceoloid to linear, with (1-) 2-8 scales.

             7 Spikelets very closely spaced, obscuring the rachis in life and when pressed; [section Umbellati, subsection Brevistylis]

               8 Spikes broadly and appearing (because of longer spikelets) relatively loosely ellipsoid to ovoid-cylindric, 2.5-7 cm long × 1.5-6 cm wide; spikelets (11-) 12-25 (-28) mm long; achenes 2.2-3 mm long (if < 2.5 mm long, noticeably falcate).

                 9 Spikes dense, ellipsoid to obovoid, convex, with 50-90 spikelets; scales 3-6 (-7), each 4-4.6 mm long, not red-purple striate; achenes 3-4× as long as wide, falcate

                 9 Spikes loose, broadly ovoid or ellipsoid to ovoid-cylindric, with 13-75 spikelets; scales 4-8(-11), each 4.5-5.3 mm long, red-purple striate; achenes 4-5× as long as wide, straight

               8 Spikes tightly globose or to oblong, not more than 2.5 cm long or 2 cm wide; spikelets 2-8 mm long; achenes < 2.3 mm long, straight.

                   10 Spikes uniformly convex throughout, globose or globose-ellipsoid, so dense as to appear nearly smooth in silhouette; scales 3.5-4.6 (-4.8) mm long, acute

                   10 Spikes cylindric, oblong, ellipsoid, or ovoid, mostly less dense; scales 1.8-3.6 mm long (or, if longer, mucronate).

                     11 Terminal scale of spikelet reduced, sterile, involute, with a minute but distinct mucro to 0.3 mm long, usually uncinate (hooked); fertile scales (if more than one) not overlapping on the same side of the rachilla, usually red-brown or orangeish laterally at maturity; leaf blades 1-3 mm wide; [Cyperus retroflexus complex]

                       12 Fertile floral scales 1.9-3.0 (-3.3) mm long; rachilla wing usually membranaceous throughout; rachilla lacking lateral nerves; longest spikelets 2.8-5.8 (-8.0) mm long; terminal sterile floral scale of spikelet often much reduced, less than 2/3 the length of fertile floral scales; longest peduncle less than 2.7 (-3.9) cm long; plants diminutive, culms 3-35 (-45) cm tall

                       12 Fertile floral scales (2.8-) 3.0-3.9 mm long; rachilla wing usually chartaceous beyond clasped achene angle, border membranaceous; rachilla with two lateral nerves, one along each side of the median; longest spikelets 4.9-9.0 (-11.3) mm long; terminal sterile floral scale usually not greatly reduced, 2/3 or more the length of fertile floral scales; longest peduncle (0.5-) 2.4-6.8 cm long; plants more robust, culms usually >25 cm tall

                     11 Terminal scale of spikelet not differentiated; scales acute, greenish-hyaline to brownish or straw-colored laterally; scales overlapping; leaf blades 1.5-5 mm wide.

                          13 Spikes relatively loosely ovoid, 8-20 mm wide, with (5-) 10-50 (-70) spikelets, lacking basal branches; spikelets (2.2-) 3.5-10 mm long, angular in ×-section, greenish-hyaline to pale yellow-green; bracts horizontal

                          13 Spikes densely oblong-ellipsoid to ovoid, 6-8 (-10) mm wide, with (13-) 30-200 (-274) spikelets, occasionally with short basal branches; spikelets 2.2-4 (-4.5) mm, subterete, straw-colored to brown at maturity; bracts horizontal or ascending to erect; [Cyperus retrorsus complex]

                            14 Achenes oblong-fusiform, stipitate, narrowed to both ends; spikes densely oblong to oblong-cylindric, often 3× or more as long as wide; scales ascending (the tips free), usually straw-colored; bracts horizontal

                            14 Achenes oblong, usually abruptly constricted at the ends; spikes ovoid to oblong, to 2× as long as wide (rarely longer in C. retrorsus); scales appressed, usually reddish-brown at maturity; bracts ascending to erect (sometimes horizontal in C. nashii).

                              15 Spikes > 1.5× as long as wide, each with (42-) 50-200 (-274) spikelets; achenes 0.4-0.5 (-0.6) mm wide; stems moderately cespitose; [widespread and weedy]

                              15 Spikes < 1.5 × as long as wide, each with (13-) 30-92 spikelets; achenes 0.5-0.7 mm wide; stems single to slightly cespitose; [Florida scrub, northward and westward in coastal dunes]

                                16 Rays 8-27; anthers 0.8-1.0 mm long; achenes 1.6-1.7mm long × 0.6-0.7 mm wide

                                16 Rays 3-6 (-8); anthers 0.2-0.5 (-0.7) mm long; achenes 1.3-1.6 mm long × 0.5-0.6 mm wide

             7 Spikelet attachments spaced apart at least proximally, rachis not obscured when pressed and usually also visible in life.

                                  17 Fertile floral scales 1-3 (-5), not overlapping on the same side of the spikelet (if more than one); terminal scale reduced, sterile and involute, mucro usually uncinate; achenes (1.7-) 1.9-2.6 mm long; [section Umbellati, Cyperus retroflexus complex]

                                    18 Longest spikelets 10-21 mm long, strongly contorted; distal scales with scabrid midrib and mucro 0.6-1.9 mm long; longest floral scale of spikelet (3.5-) 3.7-4.8 mm long; anthers 0.5-1.3 mm long; achenes > 3× as long as wide; [TX]

                                    18 Longest spikelets usually < 10 mm long, at most somewhat flexuous with curved tips; distal scales with glabrous midrib and mucro 0.1-0.3 (-0.5) mm long; longest floral scale of spikelet (2.1-) 2.5-3.5 (-4.0) mm long; anthers 0.3-0.5 (-0.6) mm long; achenes 2-3 (-3.3)× as long as wide; [more widespread]

                                  17 Fertile floral scales 3-8, reaching the next scale or overlapping on the same side of the spikelet; terminal scale not involute and sterile, mucro straight; achenes various.

                                       19 Scales 3.2-5.3 mm long, acute; spikes broadly ovoid-cylindric to ellipsoid, spikelets 5-30mm long; achenes narrowly oblong to linear, 3-6× as long as wide; culm bases cormose.

                                         20 Spikelets ±terete; scales 4.5-5.3 mm long, persistent, laterally reddish brown or tinted with reddish brown if paler and stramineous; achenes usually at least half as long as floral scale; achenes linear-oblong, 2.6-3 mm long, 5-6× as long as wide, > 0.5 mm wide; anthers 1-1.5 mm long; spikes usually broadly and convexly ellipsoid or ovoid, usually unbranched (or rarely with 1-2 reduced proximal branches); [section Umbellati, subsection Brevistylis]

                                         20 Spikelets flattened; scales 3.2-4.5 (-6) mm long, laterally golden yellow to stramineous; achenes less than half as long as floral scale; achenes oblong, (1.5-) 1.8-2.4 mm long, 3-4× as long as wide, < 0.4 mm wide; anthers 0.3-0.5 mm long; spikes usually parallel-sided, ovoid-cylindric, frequently branched; [section Strigosi]

                                           21 Plants annual, 2-3 dm tall; culm 0.5-1 (-1.5) mm in diameter; floral scales with excurved aristate apex

                                           21 Plants perennial, ≥ 3 dm tall; culm 1.8-3 mm in diameter; floral scales not aristate

                                             22 Plants > 10 dm tall; floral scales more loosely arranged, the tips gaping distally

                                             22 Plants 3-8 (-11) dm tall; floral scales appressed to slightly gaping distally

                                       19 Scales (2-) 2.5-3.2 (-4) mm long, acute to cuspidate; spikes narrowly cylindric to ovoid, spikelets mostly 4-8mm long (if longer, scales mucronate); achenes ellipsoid, 2-3× as long as wide; culm bases not cormose.

                                               23 Spikelets ellipsoid, 2-4× as long as wide; achenes ellipsoid; spikes narrowly cylindric, 4-13 mm wide; [section Tetragoni]

                                               23 Spikelets oblong to linear, 4-10× as long as wide; achenes ellipsoid to narrowly oblong; spikes narrowly to broadly cylindric to oblong or ellipsoid, ≥ 12 mm wide (C. retroflexus occasionally as narrow as 8 mm wide, but not cylindric).

                                                 24 Floral scales pale olivaceous to yellowish, not mucronate (rarely with a mucro to 0.1 mm long); spikes cylindric, loose, 6-10 spikelets per 5 mm span of the rachis; culms 0.5-1 mm in diameter; leaves 0.8-2.3 (-3) mm wide; [more widespread]; [section Tetragoni]

                                                 24 Floral scales straw-colored to reddish-brown, acute or mucronate, mucro up to 0.2 mm long; spikes loosely to densely ovoid to oblong, spikelets 8-21 per 5 mm span of the rachis; culms 1-2.6 mm in diameter; leaves (1-) 3-6 (-10) mm wide; [TX].

                                                    25 Floral scales mucronate, mucro at least 0.2 mm long; spikes densely ovoid to loosely ellipsoid-oblong (to 1.5× as long as wide); rachis, spikelets, bracteoles, peduncles, and prophylls usually conspicuously reddish brown striate or maculate (at 10× magnification); [section Strigosi].

                                                      26 Spikes loose, spikelets 8-12 per 5 mm span of the rachis, divaricate; scale color not bilaterally variable, scales yellowish to light brown; spikelets stipitate, stipe 0.3-0.5 mm long; spikes oblong to cylindric

                                                      26 Spikes denser, spikelets 11-21 per 5 mm span of the rachis (at least in the upper portion), often the upper spikelets ascending; scale color bilaterally variable, reddish or reddish-brown to whitish; spikelets estipitate or stipe to 0.2 mm long; spikes ellipsoid to oblong