Plants tightly cespitose, not rhizomatous. Culms 15-75 cm tall. Sheaths smooth, mostly glabrous, margins ciliate; collars glabrous, with or with-out tufts of hair at the sides, hairs 0.7-2 mm; ligules 1-4 mm, glabrous or sparsely hairy, acute, sometimes ciliate; blades 1-2 mm wide, involute, 0.5-1 mm in diameter, abaxial surfaces scabridulous or smooth, adaxial surfaces densely hairy, hairs about 0.2 mm. Panicles 5-20 cm long, 0.5-2.8 cm wide, loosely contracted; branches strongly ascending, longest branches 0.5-2.5(5) cm. Spikelets evenly distributed along the branches; pedicels loosely appressed to the branches, paired, unequal, shorter pedicels in each pair usually less than 1/2 as long as the longer pedicels. Glumes slightly unequal, saccate below, tapering from about midlength, veins and sometimes also the intercostal regions puberulent, hairs to 0.1 mm, apices acute to acuminate; lower glumes 5.1-6.1 mm; upper glumes 4.3-5.2 mm; florets 2.8-4.2 mm, ovoid; calluses 0.2-0.4 mm, acute; lemmas indurate, dark gray-brown, smooth, densely pilose, hairs at midlength and at the apices similar, 2-3 mm, easily rubbed off, apices not lobed; awns 3-4.4 mm, rapidly deciduous, not geniculate, scabrous; paleas similar to the lemmas in length and texture, glabrous, apices pinched; anthers about 1 mm, dehiscent, penicillate. Caryopses 1-1.7 mm long, 0.8-1 mm in diameter, globose to obovoid. 2n = unknown.
Achnatherum arnowiae grows in pinyon-juniper, sagebrush, and mixed desert shrub communities in Utah, at 1400-2000 m. Welsh and Atwood (2003) state that specimens belonging to A. arnowiae are often filed as A. ×bloomeri, and they suggest that this species may also be hybrid in origin, possibly with Hesperostipa comata as one of its parents. Another possibility is that it is a derivative of A. hymenoides that is adapted to particular soil types.