| 1. Annual; stems generally all flower, ascending to erect |
| |
| 2. Calyx 8.512 mm; fruit 1422 mm; seeds ± 1 mm |
| C. dichotomum |
| 2' Calyx 3.57 mm; fruit 3.511 mm; seeds ± 0.41 mm |
| |
| 3. Sepals with hairs < tip, scarious margin of most outer sepals 0.20.6 mm wide; upper bracts scarious-margined; pedicels in fruit generally > sepals |
| C. fontanum subsp. vulgare |
| 3' Sepals with some long hairs 0.20.8 mm > tip; scarious margin of most outer sepals < 0.1 mm wide; all bracts herbaceous; pedicels in fruit (except lowest) < sepals |
| C. glomeratum |
| 1' Perennial; stems both vegetative (mat-forming) and flower (± erect) |
| |
| 4. Petals generally < to 0.5 mm > sepals |
| C. fontanum subsp. vulgare |
| 4' Petals ± 1.26 mm > sepals |
| |
| 5. Leaves clustered in flower stem axils (especially lower); petals 36 mm > sepals; fruit 916 mm; seeds ± 11.3 mm; < 2500 m |
| C. arvense |
| 5' Leaves not clustered in flower stem axils; petals 1.22.8 mm > sepals; fruit 5.58.5 mm; seeds 0.70.8 mm; 29004300 m |
| C. beeringianum var. capillare
|