Plants perennial. Culms 1-1.5 m. Sheaths prominently keeled,
margins villous; ligules 1-3 mm; blades 1-2 cm wide, flat, scabrous.
Panicles to 25 cm, loosely spicate; rachises readily visible, sparsely
villous; bristles 1(2), 15-30 mm, flexible, antrorsely scabrous. Spikelets
3-3.2 mm. Lower glumes about 1/3 as long as the spikelets, 3-veined; upper
glumes about 3/4 as long as the spikelets, 5-veined; lower lemmas equaling
the upper lemmas; lower paleas about 1/2 as long as the upper paleas, hyaline,
narrow; upper lemmas finely and transversely rugose; upper paleas
similar to the upper lemmas. 2n = unknown.
Setaria macrosperma grows on shell or coral islands, and occasionally in
old fields or hammocks. It is most frequent in Florida, but has been collected
in both South Carolina and Georgia. It also grows in the Bahamas and Mexico.