4. Cyperus L. (umbrella sedge)

Plants annual or perennial, sometimes with rhizomes, tubers, and/or thickened stem bases. Aerial stems 1 to many per plant, erect to spreading, solid, unbranched, mostly triangular in cross-section, glabrous or roughened. Leaves few to many, basal or near the stem base, all usually with well-developed leaf blades, the sheath usually with a ligule appearing as a short, white, scaly ridge, the leaf blade spreading to ascending, flat or somewhat folded lengthwise, the margins sometimes inrolled. Inflorescences terminal, varying from single, short, sessile, headlike or more elongate spikes to umbels or panicles of spikes, subtended by several erect to spreading or reflexed, leaflike bracts, composed of numerous sessile spikelets. Spikelets sometimes flattened, 2-ranked, with 3 to many alternating scales, the 2 basal scales sterile and somewhat reduced, the fertile florets 2 to many, or, if reduced to 1, then the inflorescence noticeably branched and the branches with short to long stalks. Perianth (bristles or scales) absent. Stamens 1–3. Styles not expanded at the base, not persisting on the fruit as a tubercle . Stigmas 2 or 3. Ovaries and fruits naked, without a perigynium (saclike covering). Fruits biconvex or 3-angled, often somewhat flattened. About 650 species, worldwide.

Cyperus has been treated variously in the botanical literature as a single variable genus, composed of six or more subgenera, or, in a restricted sense, with some or all of the subgenera elevated to generic status. The relationships between these groups are not well understood, and it seems likely that in the future more detailed studies will result in a classification somewhere between these two extremes. The present treatment follows that of Tucker (1987, 1994) in recognizing only two genera, Cyperus and Kyllinga, because of the unique spikelet morphology of the latter group.

Users should be aware that a number of interspecific hybrids in the genus have been reported from Missouri. The key to species below does not accommodate such plants, which sometimes will not key to either putative parent. Because these hybrids often grow within mixed populations of the putative parents, collectors should be careful to note other species of Cyperus occurring in proximity to the plants they sample, which may provide clues to the identity of specimens with unusual morphology.