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.

Trisetum, a genus of approximately 75 species, occurs primarily in temperate, subarctic, and alpine regions. Eight species are native to the Flora region; two have been introduced, one of which is not known to have persisted. Trisetum usually differs from Sphenopholis, with which it occasionally hybridizes, in having longer awns that are inserted lower on the lemmas and spikelets that disarticulate above the glumes. It differs from Deschampsiain its more acute, bifid lemmas and in having the awns inserted at or above the midpoint of the lemmas.

Trisetum spicatum is important as forage on native rangelands. Like other species of the genus, it is a significant component of natural food pyramids, especially in arctic and alpine regions and in mountain parks. Trisetum flavescens was introduced from Europe as a pasture grass; T. interruptum is often weedy.

As of May 2007, Trisetum projectum is now considered to be a separate species, instead of being included as a synonym for Trisetum canescens.

SELECTED REFERENCES Dixon, J.M. 1995. Trisetum flavescens (L.) Beauv. (T. pratense Pers., Avena flavescens L.). J. Ecol. 83:895-909; Finot, V.L., P.M. Peterson, R.J. Soreng, and F.O. Zuloaga. 2004. A revision of Trisetum, Peyritschia, and Sphenopholis (Poaceae: Pooideae: Aveninae) in Mexico and Central America. Ann. Missouri Bot. Gard. 91:1-30; Finot, V.L., P.M. Peterson, R.J. Soreng, and F.O. Zuloaga. 2005. A revision of Trisetum and Graphephorum (Poaceae: Pooideae: Aveninae) in North America north of Mexico. Sida 21:1419-1454; Hultén, E. 1959. The Trisetum spicatum complex. Svensk. Bot. Tidskr. 53:203-228; Louis-Marie, Father, O.C.1928. The genus Trisetum in America. Rhodora 30:209-228, 231-245; Shelly, J.S. 1987 [published 1988]. Rediscovery and preliminary studies of Trisetum orthochaetum, Missoula County, Montana. Proc. Montana Acad. Sci. 47:3-4 [abstract].