Description from Caro and Sanchez (1971).
Plants perennial, erect. Culms 55-90 cm tall, with cleistogenes in the axils of the basal intravaginal innovation leaves. Leaf sheaths open to the base, pubescent on the margins towards the apex; ligules about 0.3 mm long, papery but with hairs about 1-1.5 mm long on the back; blades to 30 cm long and 1-3 mm in width, convolute-filiform or flat, stiff, more or less straight, pubescent on the margins near the ligule, the lower surface smooth below, scabrlous near the tip, the upper surface scabrous, somewhat sharp, the tip somewhat sharp. Panicle 15-35 cm long, lax, with few to many spikelets, sometimes partially included in the uppermost leaf sheath. Glumes subequal, 7-11 mm long, lanceolate-acuminate, 3-veind, scabrous over the midvein and sometimes over the latera veins, the midvein extending into an awnlike tip, the lateral veins much shorter. Anthecium 5.5-6.5 mm long; callus 0.5-0.8 mm, obconic, obtuse, with white straight hairs; lemma fusiform, laterally compressed, 5-veined, pubescent over the midvein and between the lateral veins near the margins, sometimes glabrous between the midvein and the adjacent lateral veins, the lemma apex narrow and with a ring of hairs about 1 mm long, without a crown; awn 1-2 cm long, centric, capillary, persistent, twice geniculate, scabrous, the basal segment 2-4 mm, the mid-segment 2-4 mm, and the bristle 6-12 mm; palea much shorter than the lemm, coriaceous, 2-veined, the sides papery to hyaline, pubescent over the veins and between and with short hairs at the tip; lodicules 2-3, if 3, the third unequal in length, all lodicules membranous, hyaline, glabrous, without veins; anthers 3, about 3 mm long, ciliate at the tips. Caryopsis obovoid, about 3 mm long and 1.3 mm in diameter, with a persistent, excentric style base; hilum linear, a little shorter than the caryopsis.
Distribution: Argentina (Buenos Aires, Mendoza); Chile.
The inclusion of A. brachychaetum and A. caudatum in Achnatherum needs to be investigated.