Sporobolus is one of the genera of grasses in which the fruit differs from a true grain or caryopsis in that the pericarp is not adnate to the seed coat. Sporobolus compositus, S. clandestinus, S. cryptandrus, S. neglectus, and S. vaginiflorus all often have the entire panicle concealed within the uppermost sheath, the plants thus appearing vegetative.
1. Summit of leaf sheath with conspicuous dense beard of long, spreading straight hairs on the outside; spikelets (panicle often hidden in the sheath) ca. 1.5–2.5 mm long.
1. Summit of leaf sheath glabrous or at most with a few, sometimes curled hairs (except inside); spikelets 1.5–6.5 mm long (panicle concealed or not)
2. Spikelets mostly 1.5–3 mm long.
3. Glumes unequal, the shorter 0.5–0.9 mm, panicle strongly exserted.
3. Glumes ± equal, 1.5–2.5 mm long; panicle largely or entirely included in the sheath.
2. Spikelets 3.5–6.5 mm long.
4. Lemma and palea pubescent with short appressed hairs, especially near the base.
5. Plants perennial with thick bases and short rhizomes, ca. 40–100 cm tall.
5. Plants annual, fibrous rooted and lacking rhizomes, (8–) 10–45 (–65) cm tall.
4. Lemma and palea glabrous.
6. Panicle partly or mostly included in upper leaf sheath, narrow and contracted; glumes both shorter than lemma.
6. Panicle exserted, ± open with spreading branches at maturity; larger glume equaling or exceeding lemma.