Andropogon

1. Sessile florets with body (5–) 6–9 mm long and awn tightly twisted basally; stalked floret rudimentary or well developed and staminate; base of at least some racemes usually exserted at maturity; ligule of major culm leaves ca. 0.9–2.5 mm long; stamens 3, the anthers (2–) 2.5–5 (–5.5) mm long.

A. gerardii

1. Sessile florets with body ca. 3–3.5 (–4) mm long and a straight awn; stalked floret usually represented by only a hairy pedicel; base of racemes ± included in spathe-like leaf base even at maturity; ligule of major culm leaves a minutely ciliate scale ca. 0.5–0.6 mm long; stamen 1, the anther not over 1 mm long.

A. virginicus

All species found in Andropogon

Andropogon gerardiiBIG BLUESTEM, TURKEY FOOT 
Andropogon virginicusBROOM-SEDGE 

Citation:

MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. June 23, 2017. http://michiganflora.net/genus.aspx?id=Andropogon.