Plants perennial; usually cespitose, occasionally stoloniferous. Culms 6-65 cm, erect. Leaves mostly basal; sheaths smooth, glabrous, striate, margins hyaline, collars with tufts of 1-3 mm hairs; blades usually folded, pilose basally, margins white, cartilaginous, apices acute but not sharp. Inflorescences terminal, simple panicles (racemes in depauperate specimens), exserted well above the leaves. Spikelets laterally compressed, with 3-20 florets, distal florets staminate or sterile; disarticulation above the glumes and between the florets. Glumes thin, membranous, 1-veined, acute to acuminate; calluses with hairs; lemmas rounded on the back, 3-veined, veins conspicuously pilose, at least basally, apices toothed or obtusely 2-lobed, midveins often extended into awns, awns to 4 mm, lateral veins sometimes extended as small mucros; paleas shorter than the lemmas, keels ciliate, intercostal regions pilose basally; lodicules 2, adnate to the bases of the paleas; anthers 1 or 3. Caryopses glossy, translucent; embryos more than 1/2 as long as the caryopses. x = 8. Name from the Greek erion, wool, and neuron, nerve, a reference to the hairy veins of the lemmas.

Erioneuron is an American genus of three species. Its seedlings appear to have a shaggy, white-villous indumentum, but this is composed of a myriad of small, water-soluble crystals.

Stoloniferous plants are unusual in the region covered by the Flora, but they are quite common in populations of Erioneuron nealley and E. avenaceum from central Mexico.

1.   Erioneuron pilosum (Buckley) Nash
Hairy Tridens, Hairy Woolygrass

Culms (6)10-30(40) cm tall, (0.3)0.6-1(2.5) mm thick, glabrous or hispidulous. Ligules 2-3.5 mm; blades (1)3-6(9) cm long, (0.5)1-1.5(2.5) mm wide, both surfaces sparsely pilose or glabrous, grayish-green. Panicles 1-4(6) cm; branches with 3-9 shortly pedicellate spikelets. Spikelets 6-12(15) mm, with (5)6-12(20) florets. Glumes exceeded by the lowest florets, pale; lower glumes 4-7 mm; upper glumes 4-7 mm; lemmas 3-6 mm, green or purplish-green when young, becoming stramineous at maturity, awned, awns 0.5-2.5 mm, apices acute, entire or bidentate, teeth 0.3-0.5 mm; anthers usually 3, 0.3-1 mm. Caryopses 1-1.5 mm. 2n = 16.

Erioneuron pilosum grows on dry, rocky hills and mesas, often in oak and pinyon-juniper woodlands. In North America, it is represented by E. pilosum var. pilosum. This variety differs from the other two varieties, both of which are restricted to Argentina, in its longer, less equal glumes and shorter awns.

2.   Erioneuron avenaceum (Kunth) Tateoka
Large-Flowered Tridens, Shortleaf Woolygrass

Culms (7)10-30(40) cm tall, (0.4)0.7-1 mm thick, glabrous; nodes glabrous or villous. Ligules to 0.5 mm; blades (1.5)3-5(8) cm long, (0.5)1-1.5(2.5) mm wide, both surfaces sparsely pilose. Panicles 2-8(10) cm; branches with 2-10(16) shortly pedicellate spikelets. Spikelets 6-8(10) mm, purplish, with (4)6-12(20) florets; lower glumes 4-7 mm; upper glumes 6-9 mm, equaling or exceeding the lowest florets; lemmas 4-7 mm, purplish-green, awned from between the lobes, awns 2-4 mm, apices bilobed, lobes 1-2 mm, obtuse to acute; anthers 0.4-1 mm or (when monandrous) to 1.3 mm. Caryopses 1-1.4 mm. 2n = 16, 32.

Erioneuron avenaceumis common in rocky areas from the southwestern United States to central Mexico; it also grows in Bolivia and Argentina. North American plants belong to E. avenaceum (Kunth) Tateoka var. avenaceum. Stoloniferous plants occur in the Flora region, but they are most common in central Mexico.

3.   Erioneuron nealleyi (Vasey) Tateoka
Nealley's Erioneuron, Nealley's Woolygrass

Culms (15)30-65 cm tall, 0.8-2 mm thick, glabrous or hispidulous; nodes glabrous or densely villous. Ligules 0.2-0.6 mm; blades 5-10 cm long, 2-2.5 mm wide, flat in moist conditions, both surfaces pilose to villous, green. Panicles compact (rarely open), 5-10 cm long, 1-3 cm wide, usually 2-4(6) times longer than wide, occasionally interrupted in the lower 1/2; branches with 5-17 shortly pedicellate spikelets. Spikelets 7-11 mm, purplish to pale, with 3-15 florets; lower glumes 5-7 mm; upper glumes 6-9 mm, generally equaling or exceeding the lowest florets; lemmas 4-6 mm, awned from between the lobes, awns 1-3.5 mm, apices bilobed, lobes 1.5-2.5 mm, rounded to truncate, lateral veins forming a mucro to 1 mm; anthers 1, 1-1.5 mm. Caryopses 1.3-1.5 mm. 2n = 16.

Erioneuron nealleyi is found on rocky slopes in the southwestern United States and central Mexico. Stoloniferous plants are known only from central Mexico.