| | 1. Seed ± round, 1–1.5 mm wide, wing narrow, conspicuous; fruit 1.7–2 mm wide, hairs 0 |
| S. virginica |
| | 1' Seed oblong, < 1 mm wide, wing 0 or very narrow, inconspicuous; fruit < 1.5 mm wide, hairs sparse or 0 |
| |
| | 2. Fruit spreading to ± pendent, 1–1.5 cm, hairs sparse |
| S. deserti |
| | 2' Fruit ascending, 1.5–4 cm, hairs 0 |
| |
| | 3. Pedicel 6–12 mm, straight; basal leaves 0 or shed early; style not expanded toward tip |
| S. filifolia |
| | 3' Pedicel 2–3 mm, upcurved; basal leaves persistent; style expanded toward tip |
| S. rosulata
|