This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Cyperus Haspan L. Sp. PI. 45. 1753.
Perennial by short rootstocks (sometimes annual?), roots fibrous, culms slender, weak, tufted, 1°-3° high. Lower leaves reduced to membranous acuminate sheaths, those of the involucre about 2, usually less than 1" wide, commonly little exceeding or shorter than the inflorescence; umbel several-rayed, simple or compound, the longer rays 1'-2' long; spikelets few, capitate, linear, acute, many-flowered, 3"-6" long, about \" wide; scales oblong or oblong-lanceolate, reddish-brown, acute, mucron-ulate, keeled, 3-nerved; rachis narrowly winged; stamens 3; style 3-cleft, scarcely exserted; achene 3-angled, broadly obovoid, obtuse, nearly white, very much shorter than the scale.
Cyperus dentatus Torr. Fl. U. S. 1: 61. 1824.
Perennial by scaly rootstocks which sometimes bear small tubers, culms rather stiff, 8'-2o' tall, longer than or equalled by the leaves. Leaves keeled, 1"-2" wide, those of the involucre 3-4, one or two of them usually exceeding the inflorescence; umbel several-rayed, somewhat compound; longer rays l'-3' long; spikelets linear, very flat, many-flowered, mostly blunt, 5"-10" long, nearly 2" wide; scales light reddish-brown, ovate-lanceolate, thin, keeled, 5-7-nerved, mucronate, separating from the rachis when mature, their tips spreading, causing the spikelet to appear toothed; stamens 3; style 3-cleft, the branches exserted; achene 3-angled, obtuse, mucronate, light brown, much shorter than the scale.
In sandy swamps and on shores, Maine to northern New York, West Virginia and South Carolina. Scales often modified into tufts of small leaves. Aug.-Oct.
Cyperus rotundus L. Sp. PI. 45. 1753.
Cyperus Hydra Michx. Fl. Bor. Am. 1: 27. 1803.
Perennial by scaly tuber-bearing rootstocks, culm rather stout, 6'-2o' high, usually longer than the leaves. Leaves 1 1/2" - 3" wide, those of the involucre 3-5, the longer equalling or exceeding the inflorescence; umbel compound or nearly simple, 3-8-rayed, the longer rays 2'-4 1/2' long; spikelets linear, closely clustered, few in each cluster, acute, 4"-10" long, i"-i 1/2" wide; scales dark purple-brown or with green margins and center, ovate, acute, closely appressed when mature, about 3-nerved on the keel; stamens 3; style 3-cleft, its branches exserted; achene 3-angled, about one-half as long as the scale.
In fields, Virginia to Florida, Missouri, Kansas and Texas. Adventive in southern New York and in ballast deposits at eastern seaports. Tropical America, and the Old World. A troublesome weed in the South. Coco-grass, Round-root. July-Sept.