Scirpus vahlii Lamarck in J. Lamarck and J. Poiret, Tabl. Encycl. 1: 139. 1791 (as vhalii); Fimbristylis apus (A. Gray) S. Watson; F. congesta Torrey; F. vincentii Steudel; Isolepis vahlii (Lamarck) Kunth; Scirpus apus A. Gray

Plants annual, cespitose, delicate, 4–15 cm, bases soft; rhizomes absent. Leaves polystichous, mostly spreading or excurved, often exceeding culms; sheaths entire, abaxially smooth or sparsely hirtellous; ligule absent; blades filiform, to 0.5 mm wide, somewhat involute, abaxially glabrous or ascending-strigillose. Inflorescences terminal; spikelets sessile or subsessile in single capitate leafy-involucrate cluster; scapes filiform; involucral bracts leafy, setaceous, greatly overtopping inflorescence. Spikelets greenish, cylindric to lanceoloid-cylindric, 5–10 mm; fertile scales narrowly ovate to lanceolate, 1–1.5 mm, acute, glabrous, midrib strong, excurrent as cusp. Flowers: stamens 1; styles 2-fid, slender, bulbous-based, smooth or papillate. Achenes pale, tumidly obovoid, 0.5–0.7 mm, cancellate, pits horizontally rectangular in 5–7 vertical rows per side. 2n = 20.

Fruiting summer–fall. Moist to wet, alluvial or mineralized banks, shores, fluctuating pond and lake edges, often a “drawdown” plant around stock tanks and reservoirs; 0–500 m; Ala., Ariz., Ark., Calif., Fla., Ga., Ill., Kans., Ky., La., Miss., Mo., Nebr., Okla., S.C., Tenn., Tex.; Mexico; Central America; South America.