Herbs, annual, cespitose or single-stemmed. Culms trigonous, easily compressed, 30-75 cm × 1-5 mm, basally soft, glabrous. Leaves 1-3, usually withered at anthesis, V-shaped, 5-35 cm × 2-8 mm. Inflorescences: spikes loosely ovoid, 5-25 × 8-22 mm; rays (3-)5-11, 0.4-20 cm; 2d order rays occasionally present, 5-28 mm; bracts 3-7, horizontal to ascending at 30°, (2-)8-35 cm × (1-)4-10 mm. Spikelets 6-60, linear to lanceoloid, 7-30 × 2-3 mm, margins appearing serrate; floral scales 6-24, loosely overlapping, light brown to reddish brown, with conspicuous clear border, medially greenish, 5-7-ribbed, ovate-obovate, (1.4-)1.7-2.3 × 1.4-2 mm, apex acute to obtuse. Flowers: stamens 2-3; anthers 0.4 mm, connective apex reddish, to 0.1 mm; styles 0.1-0.3 mm; stigmas 0.7-1.5 mm. Achenes jet black to dark reddish brown, sessile, obovoid, broadly rounded, 1.2-1.6 × 0.6-1.1 mm, surfaces minutely punctate.
Cyperus flavicomus has long been called C. albomarginatus (see G. C. Tucker 1994). The closely related (perhaps not distinct) C. macrostachyos Lamarck, a pantropical subspecies, has been reported from Central America; the relationship of the two species needs further study.
Cyperus flavicomus has been introduced in New York; it has not persisted there (R. S. Mitchell and G. C. Tucker 1997).
Annual to 8 dm, stouter than the next 5 spp.; lvs elongate, often surpassing the stem; bracts 3-7, to 3 dm נ8 mm; some spikes sessile, others on 1-6 rays to 15 cm; spikelets very numerous, 1-2 cm נ2-3 mm, scales very broadly oblong-ovate, 1.4-1.7 mm, as wide or slightly wider when flattened, 5-nerved near the center, broadly hyaline-margined, rounded and ±erose at the tip; stamens 2 or 3; achenes lenticular, black, obovate, 1.2-1.6 mm wide, one edge fitting into a groove in the persistent rachilla (as in the next 5 spp.). Wet soil; trop. Amer., extending n. along the coast to se. Va. and inland to Ky. (C. albomarginatus; C. sabulosus, misapplied)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.