1. Lemmas not pilose at the base, glabrous (or with minute, even pubescence on back), awnless; culms loosely or densely tufted or matted, rhizomes, if present, thin and wiry, not densely clothed with overlapping scales.

2. Spikelets less than 2 mm long, mostly on pedicels more than twice as long, in an open panicle.

3. Panicles ca. 8–20 cm wide, nearly as wide as long when fully expanded.

M. asperifolia

3. Panicles 1–6 cm wide, clearly longer than wide.

M. uniflora

2. Spikelets ca. (2.2–) 2.4–3.5 mm long, mostly on pedicels less than twice as long, in a slender contracted panicle.

4. Ligules less than 0.5 mm long; lemmas with a little minute pubescence on back.

M. cuspidata

4. Ligules ca. (1.2–) 1.5–2.5 mm long; lemmas usually glabrous across back.

M. richardsonis

1. Lemmas pilose at base, glabrous or short-pubescent on back, awned or awnless; culms arising from conspicuous, elongate scaly rhizomes (except in M. schreberi).

5. Glumes minute, the larger one less than 0.5 mm long, the other vestigial or absent; culms often rooting at nodes of decumbent bases, but without elongate scaly rhizomes.

M. schreberi

5. Glumes at least half as long as body of lemma; culms from elongate scaly rhizomes.

6. Glumes (including prominent awn-tip) (3–) 3.5–6.5 (–7.5) mm long, mostly distinctly longer than the body of the lemma; lemma at most short-awned; anthers 0.5–1.3 mm long.

7. Internodes minutely puberulent or roughened over much of their surface (rarely nearly glabrous); ligule (excluding cilia) 0.5–0.7 mm long or shorter; anthers 0.8–1.3 mm long.

M. glomerata

7. Internodes of culm smooth and glabrous over most of their surface; ligule 0.7–1 mm long; anthers ca. 0.5–0.8 mm long.

M. racemosa

6. Glumes generally less than 3.6 mm long (rarely, especially on lower spikelets of panicle, up to 4 mm), mostly about equaling or shorter than the body of the lemma; lemma awnless to long-awned; anthers not over 0.5 mm long (except in M. tenuiflora with distinctive short, broad glumes).

8. Larger glumes 0.6–1 mm wide, less than 4 times as long, hence ovate and usually ± abruptly tapered at the tip; culms puberulent below the nodes.

9. Anthers not over 0.5 mm long; broader leaf blades 3–5 mm wide; ligules (1–) 1.3–2 mm long; sheaths all glabrous.

M. sylvatica (in part)

9. Anthers 1–1.5 mm long; broader leaf blades (4–) 6–13 mm wide; ligules 1 (–1.2) mm long or shorter; sheaths (at least some of them) usually ± pubescent.

M. tenuiflora

8. Larger glumes not over 0.6 mm wide, more than 4 times as long, hence narrowly lanceolate and usually ± attenuate at the tip; culms puberulent or glabrous below the nodes.

10. Culm smooth and glabrous throughout, sometimes decumbent at base and rooting at nodes, generally much branched and bushy above; inflorescences (except terminal one) with base often enclosed in upper leaf sheath.

M. frondosa

10. Culm puberulent below the nodes, ± erect, simple or branched; inflorescences all generally exserted.

11. Ligules ca. 1 (–1.3) mm long or shorter; some spikelets in panicle sessile or subsessile.

M. mexicana

11. Ligules, at least the longest, (1–) 1.3–2 mm long; spikelets all on distinct (though sometimes rather short) pedicels.

M. sylvatica (in part)