Plants perennial; cespitose, older plants occasionally shortly rhizomatous. Culms 5-40 cm, slender, wiry, erect or slightly geniculate at the lower nodes; lower internodes glabrous, shorter than those above. Leaves mostly basal; sheaths glabrous, sometimes scabridulous, becoming flattened, persistent; ligules 0.2-0.5 mm, of hairs; blades 0.7-8 cm long, 0.5-1.5(2) mm wide, scabridulous, margins often with papillose-based hairs basally. Panicles 3-9 cm, with 2-7 branches; branches 7-25 mm, persistent, spreading, ascending, or appressed, straight to slightly arcuate, with 8-24(32) spikelets, axes terminating in a spikelet; disarticulation above the glumes. Spikelets appressed to pectinate, reddish-purple; with 1 bisexual floret and 1 rudimentary floret. Glumes bilobed; lower glumes 1.7-3.4 mm, slightly shorter than the upper glumes, veins excurrent to 0.6 mm; upper glumes 1.9-4 mm, glabrous or pubescent, hairs not papillose-based, veins excurrent to 1 mm; lower lemmas 1.2-2.2 mm, glabrous, sparsely appressed pubescentalong the veins or densely appressed pubescent for much of their length and on the margins, trilobed, lobes veined, tapering into 3 awns, awns 2.2-6.6 mm, central awns not flanked by membranous lobes; anthers 0.2-0.4 mm, yellow; rachilla internodes glabrous; upper florets glabrous, of 3-awned, awns equal, 2-7 mm. Caryopses 0.8-1.5 mm long, 0.3-0.6 mm wide. 2n = 20.
Bouteloua trifida grows on dry open plains, shrubby hills, and rocky slopes, at 2200-2500 m. Its range extends from the southwestern United States to central Mexico. It is a drought-resistant species that is sometimes mistaken for Aristida because of its delicate, cespitose growth habit and purplish, 3-awned spikelets. Juvenile plants may also be confused with B. barbata, but that species is annual, with the central awn flanked by two membranous lobes and the lowest paleas 4-lobed and 2-awned.
Perennials, Terrestrial, not aquatic, Rhizomes present, Rhizome short and compact, stems close, Stems nodes swollen or brittle, Stems erect or ascending, Stems geniculate, decumbent, or lax, sometimes rooting at nodes, Stems caespitose, tufted, or clustered, Stems terete, round in cross section, or polygonal, Stem internodes hollow, Stems with inflorescence less than 1 m tall, Stems, culms, or scapes exceeding basal leaves, Leaves mostly basal, below middle o f stem, Leaves conspicuously 2-ranked, distichous, Leaves sheathing at base, Leaf sheath mostly open, or loose, Leaf sheath smooth, glabrous, Leaf sheath hairy at summit, throat, or collar, Leaf sheath and blade differentiated, Leaf blades linear, Leaf blades very narrow or filiform, less than 2 mm wide, Leaf blades mostly flat, Leaf blade margins folded, involute, or conduplicate, Leaf blades mostly glabrous, Leaf blades more or less hairy, Leaf blades scabrous, roughened, or wrinkled, Ligule present, Ligule a fringe of hairs, Inflorescence terminal, Inflorescence with 2 or more spikes, fascicles, glomerules, heads, or clusters per culm, Inflorescence a panicle with narrowly racemose or spicate branches, Inflorescence with 2-10 branches, Inflorescence branches 1-sided, Inflorescence branches terminating in bristle or point, Flowers bisexual, Spikelets sessile or subsessile, Spikelets laterally compressed, Spikelet less than 3 mm wide, Spikelets with 1 fertile floret, Spikel ets solitary at rachis nodes, Spikelets all alike and fertille, Spikelets bisexual, Spikelets disarticulating above the glumes, glumes persistent, Spikelets secund, in rows on one side of rachis, Rachilla or pedicel glabrous, Glumes present, empty bracts, Glumes 2 clearly present, Glumes equal or subequal, Glumes distinctly unequal, Glumes equal to or longer than adjacent lemma, Glume equal to or longer than spikelet, Glumes 1 nerved, Lemma coriaceous, firmer or thicker in texture than the glumes, Lemma 3 nerved, Lemma glabrous, Lemma apex dentate, 3-5 fid, Lemma distinctly awned, more than 2-3 mm, Lemma with 3 awns, Lemma awn less than 1 cm long, Lemma margins thin, lying flat, Lemma straight, Palea present, well developed, Palea membranous, hyaline, Palea shorter than lemma, Palea 2 nerved or 2 keeled, Stamens 3, Styles 2-fid, deeply 2-branched, Stigmas 2, Fruit - caryopsis, Caryopsis ellipsoid, longitudinally grooved, hilum long-linear.