Aristida purpurea, purple threeawn

Image Source -

Intermountain Flora
Tufted perennials; culms 1.5-6.5 (10) dm tall, erect, sometimes branched at the lower nodes, glabrous; sheaths glabrous, except for pilose tufts of white hairs at the apical margins, the hairs 2-3 mm long, with shorter hairs sometimes extending in a fine line across the collar, often a few hairs extending down the upper margins forming a ciliate shoulder; lingules scarcely 0.5 mm long, consisting of a fringe of hairs, somewhat higher on the edges than in the center; blades involute or rarely flat, 1-2 mm broad and (4) 5-10 (20) cm long, often curved or flexuous on small plants and straight on large plants, glabrous or sometimes scaberulous to hirsute above; panicles (5) 6-25 (35) cm long, narrow to somewhat open, the branches usually appressed and closely clustered, sometimes looser and divergent by curvature; glumes linear-lanceolate, 1-nerved, acuminate, sometimes awn-tipped, the nerves often scabrous, the first glume 5-10 (14) mm long, the second (7) 10-18 (25) mm long, 1.5 to twice as long as the first; lemma (7) 9-15 (17) mm long (including the awn column), linear, scaberulous in lines above, faintly 3-nerved, often gradually narrowing into a thin, often twisted awn column (beak), the callus 0.5-1 mm long, densely short-pilose, sharp, the 3 awns subequal, (10) 12-50 (100) mm long, the lateral awns divergent, slightly flattened and scabrous below, becoming terete and glabrous above; palea much shorter than the lemma; lodicules large, 2-2.5 mm long; anthers 1-2.5 mm long; 2n = 22, 44, 66.

Cronquist, A., Holmgren, A.H., Holmgren, N.H., Reveal, J.L., and Holmgren, P.K. (1977) "Intermountain Flora" pg. 456, vol. 6, The New York Botanical Garden New York.