Plants annual or perennial; sometimes rhizomatous, sometimes cespitose. Culms 5-150 cm, glabrous or pubescent, basal branching extravaginal. Sheaths open the entire length or fused at the base; auricles absent; ligules membranous, often erose to lacerate, sometimes ciliolate; blades rolled in the bud. Inflorescences terminal panicles, open and diffuse to dense and spikelike; branches antrorse-scabrous. Spikelets (2.4)4-9 mm, usually subsessile to pedicellate, rarely sessile, laterally compressed, with 2-5 florets; reduced florets (if present) distal; rachillas hairy, internodes evident, prolonged beyond the distal bisexual florets; disarticulation usually above the glumes and between the florets, subsequently below the glumes, in some species initially below the glumes. Glumes subequal or unequal, keels scabrous, apices usually acute and unawned, often apiculate; lower glumes 1(3)-veined; upper glumes 3(5)-veined, lateral veins less than 1/2 the glume length; calluses hairy; lemmas 3-7-veined, margins hyaline, unawned or awned from above the middle with a single awn, apices usually bifid, sometimes entire; paleas subequal, equal to, or longer than the lemmas, membranous, 2-veined, veins usually extended as bristlelike tips; lodicules 2, shallowly and usually slenderly lobed to fimbriate; anthers 3; ovaries glabrous or pubescent; styles 2. Caryopses elongate-fusiform, compressed, brown; embryos elliptic, to 1/3 the length of the caryopses; endosperm milky. x = 7. Name from the Latin tres, three, and seta, bristle, alluding to the three-awned appearance of the lemmas of the type species, Trisetum flavescens.
Spikelets 2(3-5)-fld, disarticulating above the glumes and between the lemmas; rachilla prolonged behind the second palea, often hairy; glumes thin or membranous, with broad hyaline margins, about as long as the spikelet, the first 1-3-veined, the second 3-5-veined; lemmas thin, elongate, rounded on the back or distally keeled, obscurely veined, short-bearded at base, entire and awnless, or with a straight or bent awn arising from above the middle of the back; seeds with minute embryo and liquid endosperm; ligules membranous, erose; tufted perennials (rarely annual) with spiciform or subcapitate to loose and elongate panicles. 50+, cosmopolitan.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.