Flora of the Southeastern United States
2022 Edition

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Key to Cyperus

1 Floral scales spirally imbricate.
..2 Leaves many, cauline, coriaceous, closely spaced distally; culms arising from long, thick, branched rhizomes; inflorescence dense, capitate; [FL, beaches and coastal dunes]; [section Remirea]
..2 Leaves basally disposed; inflorescence of 1-5 sessile ovoid spikes, or more diffuse.
....3 Spikelets many, in densely globose to pyramidal heads, these borne on elongate rays or sessile; rhizomatous perennials, forming extensive floating mats; [section Oxycaryum]
....3 Spikes [actually consisting of numerous highly reduced, spirally imbricate spikelets each with 1-2 (-3) scales] 1-5, terminal and capitate on the culm; terrestrial, often diminutive annuals; [subgenus Lipocarpha].
1 Floral scales 2-ranked and distichously imbricate.
......4 Spikelets reduced, 1(-2)-flowered with 2-3 scales, flattened, ovoid or lanceoloid, mostly < 5 mm long; rays absent, spikes 1-4, very dense, ovoid to oblong; stigmas 2; achenes biconvex; [subgenus Kyllinga].
......4 Spikelets 1-many-flowered (if all only 1-3-flowered then subterete to quadrangular in cross-section and ellipsoid to linear); rays present or absent, inflorescence mostly not as above.
........5 Stigmas 2; achenes laterally flattened, lenticular (to compressed-subterete), borne with an edge to the rachilla, spikelets thus laterally flattened.
..........6 Achene emarginate or obcordate at apex; rachilla deciduous, basally articulate; spikelets ovate-lanceolate, with 4-6(-8) fertile scales, these shortly but distinctly mucronate [introduction, FL]; [subgenus Queenslandiella]
..........6 Achene obtuse to truncate at the apex, sometimes apiculate; rachilla persistent; spikelets (at least the larger) lanceolate to linear, usually with 8 or more fertile scales, obtuse to acute, or (C. pumilus) excurved-cuspidate; [subgenus Pycreus]
........5 Stigmas 3 (2 in subg. Juncellus species, sometimes 2 and 3 in C. alopecuroides); achenes either trigonous or dorsiventrally compressed and borne with a face to the rachilla; spikelets terete or quadrangular to strongly compressed-quadrangular or compressed-subterete.
............ 7 Rachilla corky-thickened, transversely articulated at the base of each scale, the spikelets at maturity separating into 1-fruited dispersal units, each with 1 scale, the rachilla segment and wings, conspicuously clasping the achene (spikelets visibly “jointed”, at maturity breaking apart); spikelets linear, terete or scale tips slightly spreading.
............ ..8 Spikelets 2-8, rays absent (the spikelets in a single digitate cluster at the top of the culm, appearing lateral); bracts 2-3, longest erect, filiform, ≤ 1 mm wide [pine rocklands, s. peninsular FL]; [section Filiformes, apparently allied with C. fuligineus]
............ ..8 Spikelets 10-60, rays usually present and well-developed; bracts 4-10, spreading to ascending, mostly leafy, ≥1mm wide. [widespread, often weedy]; [section Diclidium]
............ ....9 Tip of each scale not reaching above the base of the next distal scale on the same side of the rachilla, and usually ending short of it; achene linear-oblong, 1.5-2 mm long, ca. 3× as long as wide.
............ ....9 Tip of each scale surpassing the base of the next distal scale on the same side of the rachilla; achene ellipsoid or obovoid-oblong, 1-1.5 mm long, ca. 2× as long as wide
............ 7 Rachilla not transversely articulated; scales persistent or deciduous; rachilla wings present or absent, not corky, clasping the achene or not; spikelets ovoid to linear, compressed, quadrangular, or terete.
............ ......10 Spikelets in dense glomerulate heads or digitate clusters, the rachis essentially absent; spikelets strongly laterally compressed; [subgenus Pycnostachys].
............ ......10 Spikelets in spikes (sometimes sessile and capitate), arranged along a well-developed rachis, this sometimes obscured but evident upon dissection; spikelets quadrangular or terete to moderately compressed.
............ ........11 Longest bract vertical, appearing as a continuation of the culm; rays absent, spikelets 2-7(-14), in a single digitate cluster; achenes dorsiventrally compressed, with the achene face toward the rachilla, the spikelets subterete (elliptic in cross-section) [rare waif, brackish marshes]; [subgenus Juncellus, section Laevigati]
............ ........11 Longest bract not appearing as a continuation of the culm; rays present or absent; inflorescence not as above (sometimes sessile and capitate, but then with 15 or more spikelets); achenes trigonous or dorsiventrally compressed; [subgenus Cyperus].