Plants perennial; rhizomatous, not cespitose. Culms 50-110 cm tall, 0.7-1.8 mm thick, ascending, geniculate, or decumbent, bushy and much branched above; internodes smooth and shiny for most of their length, glabrous throughout. Sheaths glabrous, margins hyaline; ligules 0.7-1.7 mm, membranous, truncate, lacerate-ciliolate; blades 4-18 cm long, 2-7 mm wide, flat, glabrous, smooth or scabridulous, those of the lateral branches similar to those of the primary culms. Panicles 2-15 cm long, 0.3-2 cm wide, sometimes dense; axillary panicles common, partly included in the sheaths; primary branches 0.3-4.2 cm, ascending, appressed, occasionally diverging up to 30° from the rachises; pedicels 0.4-5 mm, smooth or scabrous. Spikelets 2.2-4 mm, imbricate along the branches. Glumes subequal, 2-4 mm, 3/4 as long as to longer than the lemmas, 1-veined, unawned or awned, awns to 2 mm; upper glumes acuminate to acute; lemmas 2.2-4 mm, narrowly lanceolate, hairy on the calluses, lower 1/3 of the midveins, and margins, hairs 0.3-0.8 mm, apices scabridulous, acuminate, unawned or awned, awns 0.1-13 mm; paleas 2.3-4 mm, narrowly lanceolate, acuminate; anthers 0.3-0.6 mm, usually yellow, occasionally purplish. Caryopses 1.6-1.9 mm, fusiform, brown. 2n = 40.
Muhlenbergia frondosa grows in forest borders, thickets, clearings, alluvial plains, and disturbed sites within deciduous forests, at elevations of 20-1000 m. It grows only in southern Canada and the contiguous United States. The Oregon specimen supposedly documenting the occurrence of this species in Oregon has been revised to M. mexicana, in part because it was puberulent below the nodes. It remains probable that it was introduced to Oregon with cranberry plants. [The map was generated from a database developed by the editors of FNA 25. Peterson did not have the opportunity to review the Oregon specimen of M. frondosa before publication].
Plants with unawned or shortly (less than 4 mm) awned lemmas can be called M. frondosa (Poir.) Fernald forma frondosa, and those with lemma awns 4-13 mm long, M. frondosa forma commutata (Scribn.) Fernald.