Herbs, perennial, cespitose, bases cormose; rhizomes knotted, beaded. Culms trigonous, (3-)10-50 cm × 0.4-1.2 mm, glabrous. Leaves flat, 5-40 cm × 1-3.5 mm. Inflorescences: spikes rather densely ovoid to globose, 1.2-3.5 cm; rays 0 or 1-4, 1-6 cm; rachis 1-3.5 mm; bracts 2-4, horizontal to reflexed, flat, 6-25 cm; rachilla ± deciduous, wingless. Spikelets 15-60, compressed, oblong-lanceoloid, (3-)6-22 × 2.5-4 mm; floral scales deciduous, 5-22, off-white to light reddish brown, laterally 3-5-ribbed, ovate-elliptic, 2.5-4 × 2-2.6 mm, margins loosely spreading or clasping achene, apex entire to mucronate, mucro 0.05-0.2 mm. Flowers: anthers 0.3-0.6 mm; styles 1 mm; stigmas 1-1.5 mm. Achenes dark brown or black, sessile, oblong-ellipsoid to ellipsoid, 1.7-2.2 × 0.8-1.2 mm, apex obtuse, apiculate, surfaces puncticulate. 2n = 166. Cyperus lupulinus was studied in detail (B. G. Marcks 1974). Ranges of the two subspecies overlap somewhat. Cyperus lupulinus subsp. lupulinus is found chiefly in the Great Plains, and subsp. macilentus is centered in the Northeast. It is seldom difficult to assign specimens to subspecies. The hybrid of Cyperus lupulinus with C. schweinitzii is C. ×mesochorus Geise. It is occasionally encountered with the two parent species in the north-central states (Illinois, Indiana, Michigan, Minnesota, and Wisconsin) and has been recorded once from Quebec. The hybrid is similar in size to C. schweinitzii; it has fewer rays, inflorescence bracts 30-45º above horizontal, and floral scales with mucros 0.4-0.5 mm.
Perennial from short, tuberous-knotty rhizomes; stems 1-5 dm, smooth, rather obtusely angled, 0.4-1.2 mm thick; lvs light green, 1-3.5 mm wide, flat or folded, shorter than the stem; invol bracts 2-4, widely spreading to more often deflexed, the margins of these and the cauline lvs scabrous; infl usually a single subglobose or hemispheric sessile spike, or seldom with a few rays to 7 cm, each bearing a similar but smaller spike; spikelets drab-green or light reddish-brown, very crowded, radiating from the axis, flattened, 2.5-4 mm wide, (3-)6-22-fld; scales oblong-elliptic, 2-3.5 mm, in half-view from the side about a third as wide as long, multinerved, obtuse or with a minute mucro 0.1-0.4 mm, readily deciduous; rachilla-joints sharp-edged but not winged; anthers 0.3-0.6 mm; achene trigonous, with flat or only slightly concave sides, 1.4-2.2 mm, about half as wide, or a bit wider; 2n=ca 166. Dry woods and fields; abundant; Me. and s. Que. to Minn., S.D., and Colo., s. to Fla. and Tex. (C. bushii; C. macilentus; C. filiculmis var. m.)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
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