Tufted annual (or seemingly annual); stems slender and wiry, to 3 dm; lvs few, borne mostly below the middle, the sheath bladeless and merely mucronate-tipped, or the upper ones with a slender blade to 10 cm; bract 2-10 cm, ±erect, like a continuation of the culm; spikelets 2- several, closely clustered, 3-15 mm, at least the larger ones mostly with more than 15 fls; scales hyaline- scarious, with firmer, greenish, shortly excurrent midrib; bristles present or absent; achene blackish, 1.5 mm, finely densely, and conspicuously cross-ridged as seen at 10 tricarpellate and trigonous, or bicarpellate and planoconvex. Swales and shores, tolerant of alkali; interruptedly circumboreal, and nearly cosmopolitan, but local and irregular. Fr June-Sept. Typical S. supinus is tricarpellate and Eurasian; 2n=28. Tricarpellate Amer. plants with greenish brown scales, 2n=50, mainly in c. and w. U.S., but e. as far as O., are var. saximontanus (Fernald) T. Koyama. (S. saximontanus) Bicarpellate plants, with brown scales, 2n=22, mainly in the coastal states from Mass. to Tex., but also inland to Mich. and even Kans., are var. hallii (A. Gray) A. Gray. (S. hallii)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.