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..
Scirpus Vahlii Lam. Tabl. Encycl. 1: 139. 1791.
F. congesta Torr. Ann. Lye. N. Y. 3: 345. 1836.
Annual, culms very slender, densely tufted, compressed, striate, erect or ascending, 1'-4' high, longer than or equalling the leaves. Leaves setaceous or almost filiform, rough, those of the involucre 3-5, erect, much exceeding the simple capitate cluster of 3-8 spikelets; spikelets oblong-cylindric, obtuse, 2"-4' long, about 4" thick, many-flowered; scales lanceolate, pale greenish-brown, acuminate; stamen 1; style 2-cleft, glabrous below; achene minute biconvex, yellowish-white, cancellate by longitudina and transverse ridges.
In moist soil, Missouri to Texas, east to North Carolina and Florida. California, Central and South America. Also in ballast about the eastern seaports. July-Oct.
Annual, tufted, glabrous, low, 6'-8' high or less. Culms very slender, compressed; basal leaves about i" wide, usually shorter than the culms, sometimes equalling them; involucral bracts 2 or 3, not longer than the inflorescence; umbel simple or somewhat compound; spikelets, or most of them, sessile, capitate, or some short-peduncled, or in capitate clusters at the base of the culms, ovoid or oval, 3" long or less; scales dull, green-brown, ovate, mucronulate; style-branches 3; style smooth; achenes rather larger than in F. autumnalis, distinctly reticulated, sometimes granular-tuberculate.
In moist soil, Maine to Ontario, Tennessee and Louisiana. July-Oct.
Scirpus autumnalis L. Mant. 2: 180. 1771. Fimbristylis autumnalis R. & S. Syst. 2: 97. 1817.
Annual, roots fibrous, culms very slender, densely tufted, flat, roughish on the edges or smooth, erect, ascending or spreading, 1-15' long, usually much exceeding the leaves. Leaves narrowly linear, flat, 1/2"-1" wide, long-acuminate, glabrous, those of the involucre 2-3, usually all shorter than the umbel; umbel compound or decompound (in dwarf forms sometimes reduced to a solitary spikelet), the primary rays 1/4'-1 1/2' long, the secondary filiform; spikelets linear-oblong, acute, 2"-5" long, 1/2" thick or less, several-many-flowered; scales ovate-lanceolate, appressed, subacute, strongly mucronate, greenish-brown, the midvein prominent; stamens 1-3; styles 3-cleft; achene obovoid, nearly white, 3-angled with a ridge on each angle, very finely reticulated and sometimes roughened.
In moist soil, Connecticut to Illinois, Florida and Texas. Also in tropical America. June-Sept.