Schweinitz’s flatsedge

Cyperus alterniflorus Schweinitz 1824, not R. Brown 1810

Herbs, perennial, rhizomatous. Culms trigonous, (10–)20–50 cm × (0.6–)1–2 mm, glabrous or more often scabridulous, either in distal 1/2 or over entire length; basal sheaths, 1–3, light brown, 1–4 cm, papery. Leaves flat to V-shaped, (2–)20–35 cm × 2–6 mm, margins and keel minutely scabridulous; proximal leaves bladeless. Inflorescences: spikes ± umbellate to slenderly (infrequently broadly) ovoid, 10–25 × 7–16 mm; rays 3–5, 1.5–8(–15) cm; bracts 3–7, erect (or declined at most 25–30° from vertical), (2.5–)8–20(–25) cm × (0.5–)1.5–6 mm; rachilla ± deciduous, wingless. Spikelets (1–)5–10(–20), oblong to linear-oblong, quadrangular, 7–10(–18) × (2.8–)3.2–4(–4.5) mm; floral scales deciduous, (1–)5–10(–14), spreading, laterally stramineous or dull whitish, sometimes also red-spotted, medially green, laterally 2–3-ribbed, medially 3-ribbed, broadly ovate to oblong-ovate, (2.3–)2.8–3.2 × (2–)2.4–3.2 mm, apex broadly rounded, often clear-erose, cusp 0.1–0.4 mm in proximal scales, 0.3–1 mm in distal scales. Flowers: anthers 0.8–1.4 mm; styles 0.1–0.3 mm; stigmas 1.5–3 mm. Achenes light to dark brown or nearly black, broadly ellipsoid, infrequently obovoid, 2–2.4 × 0.9–1.4 mm, base cuneate to ± stipelike, apex obtuse to broadly rounded, surfaces papillose. 2n = 166.

Fruiting late spring–early summer. Riverbanks, sand bars, lakeshores, sand dunes, sandy openings in woods; 0–1000 m; Alta., Man., Ont., Que., Sask.; Colo., Ill., Iowa, Ky., Mass., Mich., Minn., Mo., Mont., N.J., N.Mex., N.Y., Ohio, Okla., Pa., S.Dak., Tex., Utah, Wash., Wis., Wyo.; Mexico (Coahuila).

Cyperus schweinitzii is introduced, but not naturalized, in Massachusetts and Washington.