| | 1. Annual; stolons and tubers 0; lowest inflorescence node with 2 bisexual flowers |
| S. montevidensis subsp. calycina |
| | 1' Perennial from stolons and tubers; lowest inflorescence node with 3 pistillate flowers |
| |
| | 2. Emergent leaf blades linear and 3-angled to lanceolate; pedicels of pistillate flowers recurved, thickened in fruit |
| S. sanfordii |
| | 2' Emergent leaf blades generally sagittate; pedicels of pistillate flowers ascending in fruit, not thickened |
| |
| | 3. Lower lobes of emergent leaf blades generally 2 × terminal lobe; tubers spheric, tan |
| S. longiloba |
| | 3' Lower lobes of emergent leaf blades < or = terminal lobe; tubers oblong, ± white or bluish |
| |
| | 4. Fruit beak ascending to ± erect, < 0.5 mm; lower lobes of emergent leaf blades < terminal lobe |
| S. cuneata |
| | 4' Fruit beak spreading, 1–2 mm; lower lobes of emergent leaf blade ± = terminal lobe |
| S. latifolia
|