1. Inflorescences relatively open panicles, narrowly elliptic to ovate or broadly triangular in outline, the branches ascending to spreading at maturity, mostly free from the subtending leaf sheaths at maturity … ⇒ 2,3,4
…
1. Inflorescences spikelike panicles, linear to narrowly elliptic in outline, the branches appressed to the main axis or nearly so, sometimes all or mostly enclosed within the subtending leaf sheaths at maturity … ⇒ 5
   5. Glumes noticeably unequal in length, the lower glume 1/2-2/3 as long as the upper glume … ⇒ 6
      6. Spikelets 4.0-7.5 mm long … ⇒ 7
         7. Lemma and palea glabrous but usually roughened along the midnerve … ⇒ 3. S. COMPOSITUS
         7. Lemma and palea hairy (look carefully with magnification to see short, straight, light-colored hairs
appressed to the surface between the nerves) … ⇒ 2. S. CLANDESTINUS
         6. Spikelets 1.5-2.8 mm long … ⇒ S. CRYPTANDRUS, S. INDICUS
   5. Glumes nearly the same length, the lower glume more than 3/4 as long as the upper glume … ⇒ 9
      9. Lemma and palea glabrous … ⇒ 10
         10. Lemma 1-nerved, white or evenly tinged with purple at maturity; lowermost leaf sheaths glabrous
or with a few long hairs at the tip … ⇒ 7. S. NEGLECTUS
         10. Lemma 3-nerved, the lateral nerves sometimes faint, pale yellow and usually irregularly mottled
with purple at maturity; lowermost leaf sheaths sparsely to densely pubescent
with long pustular-based hairs on the margins, tip, and surface … ⇒ 8. S. OZARKANUS
      9. Lemma and palea hairy (look carefully with magnification to see short, straight, light-colored hairs
appressed to the surface between the nerves) … ⇒ 11
         11. Plants perennial; spikelets 4.5-7.5 mm long … ⇒ 2. S. CLANDESTINUS
         11. Plants annual; spikelets 1.9-5.0 mm long … ⇒ 12
            12. Glumes slightly longer than the floret; lowermost leaf sheaths hairy on the surface, noticeably inflated,
appearing mostly 1.5-3.0 mm wide in profile (do not unfold) … ⇒ 8. S. OZARKANUS
            12. Glumes usually shorter than the floret; lowermost leaf sheaths often glabrous on the surface,
only slightly inflated, appearing 0.8-1.7 mm wide in profile (do not unfold) … ⇒ 10. S. VAGINIFLORUS

1 Plants annuals or short-lived perennials flowering in the first year (2)
    Plants perennial (6)

…

5 Lemmas strigose; spikelets 2.3-6 mm long; mature fruits (1.1)1.8-2.7 mm long ⇒ 3. S. vaginiflorus
    Lemmas glabrous; spikelets 1.6-3 mm long; mature fruits 1.2-1.8 mm long ⇒ 4. S. neglectus
6 Plants with rhizomes (7) … ⇒ (10)
   Plants without rhizomes (11) … ⇒ (34)

…

10 Fruits 1-2 mm long; pericarp gelatinous, slipping from the seed when wet; panicles 5-30 cm long, 0.4-1.6 cm wide; lemmas glabrous, smooth ⇒ 6. S. compositus
   Fruits (1.5)2.4-3.5 mm long; pericarp loose but neither gelatinous nor slipping from the seed when wet; panicles
5-11 cm long, 0.04-0.3 cm wide; lemmas minutely pubescent or scabridulous ⇒ 7. S. clandestinus

…

34 Lemmas minutely pubescent or scabridulous, chartaceous and opaque; pericarps loose but neither gelatinous nor slipping off the seeds when wet; fruits (1.5)2.4-3.5 mm long ⇒ 7. S. clandestinus
   Lemmas usually glabrous and smooth, membranous to chartaceous and hyaline; pericarps gelatinous, slipping off
the seeds when wet; fruits 1-2 mm long ⇒ 6. S. compositus

Jones et al. present a checklist of all Texas vascular plants. For the S. compositus complex they consider S. clandestinus a variety of S. compositus, for which they recognize other varieties compositus and macer. S. compositus var. drummondii is downgraded to synonym for S. compositus var. compositus.

S. vaginiflorus is considered to have two varieties: var. neglectus (= S. neglectus) and var. vaginiflorus.

They present no evidence to support these views. Their proposal of S. clandestinus as a variety of S. compositus was accepted by Diggs et al. 1999 justified by the following, p. 1327:

We are following Wipff and Jones (1995) for nomenclature of this taxon. While it has often been recognized at the specific level (e.g., Kartesz 1994), because var. clandestinus differs morphologically from var. compositus in only minor ways, Wipff and Jones (1995) argued that it is most appropriately recognized at the varietal level.

The total ‘argument’ in Wipff & Jones 1995 (Phytologia 78, p. 244) consisted of less than 7 lines of printed text:

Sporobolus
clandestinus (J. Biehler) A. Hitchcock and S. compositus (J. Poiret) E. Merrill are
very similar morphologically. Riggins (1977) separated the two taxa by the following
characters: Sporobolus clandestinus has sparsely, appressed, pubescent lemmas and a
pericarp loose when moist, whereas, S. compositus has glabrous lemmas and a
pericarp gelatinous when moist. We believe that this kind of morphological variation
is best recognized at the variety level; thus making the above combination necessary.