| | 1. Longest chaff scales scarious-winged throughout, wing widest near or below middle of scale; phyllaries 2–4, scarious throughout, ± persistent |
| |
| | 2. Wing of each chaff scale elliptic to ± obovate, base acute; disk ovary 0.2–0.6 mm, pappus bristles generally 6–12; receptacle ± club-shaped |
| S. citroleum |
| | 2' Wing of each chaff scale widely ovate, base round or cordate; disk ovary 0–0.2 mm, pappus bristles generally 1–5; receptacle cylindric |
| S. gnaphaloides |
| | 1' Longest chaff scales scarious-winged only toward tip, wing widest well above middle of scale; phyllaries 0 or vestigial, or 1–3, deciduous, not scarious throughout |
| |
| | 3. Receptacle club-shaped, length 2.8–3.5 × width; disk ovary 0.3–0.6 mm; outer fruit 0.6–0.8 mm; heads ± spheric, 3–4 mm wide; longest chaff scale < 3.4 mm; lowest leaves obtuse |
| S. sonorensis |
| | 3' Receptacle ± cylindric, length > 3.5 × width; disk ovary 0–0.3(0.4) mm; outer fruit, heads, chaff, and lowest leaves various |
| |
| | 4. Heads spheric, largest 5–9 mm wide; longest chaff scales 3.4–4.5 mm, body various; outermost chaff scales each enclosing a flower, woolly; fruit various |
| |
| | 5. Body of longest chaff scale hard, splitting lengthwise if forced; fruit compressed front-to-back; largest uppermost leaves generally 4–11 mm, ± elliptic to oblanceolate or obovate |
| S. intertexta |
| | 5' Body of longest chaff scale (except midvein) membranous, easily tearing as wool is pulled; fruit compressed side-to-side; largest uppermost leaves generally 11–18 mm, awl-like to lanceolate |
| S. micropoides |
| | 4' Heads ± ovoid, largest 1.5–4 mm wide; longest chaff scales < 3.4 mm, body hard; outermost chaff scales open, nearly glabrous; fruit compressed front-to-back |
| |
| | 6. Heads 1.5–2.5 mm wide; longest chaff scale 2–2.7 mm; fruit 0.7–1 mm; disk flowers 0.8–1.1 mm, generally 4-lobed; leaves narrowly obtuse |
| S. masonii |
| | 6' Heads 2.5–4 mm wide; longest chaff scale 2.8–3.3 mm; fruit 1.1–1.6 mm; disk flowers 1.2–2 mm, 5-lobed; leaves generally acute |
| S. psilocarphoides
|