| | 1. Petals white; cauline leaves 1-pinnate; plant in aquatic to very wet places, rooting at nodes or not |
| |
| | 2. Lateral leaflets prominently dentate; terminal leaflet of leaves on fertile stem ± = lateral; lower pedicels often bracted; leaflets 7–13; pedicel junctions flat |
| R. gambelii |
| | 2' Lateral leaflets ± entire or wavy-margined; terminal leaflet of leaves on fertile stem > lateral; pedicels not bracted; leaflets generally 3–7; pedicel junctions not flat |
| R. nasturtium-aquaticum |
| | 1' Petals yellow; cauline leaves simple, entire to deeply pinnately lobed, rarely 1-pinnate; plant often in wet but generally not aquatic places, rarely, if ever, rooting at nodes |
| |
| | 3. Perennial, often from creeping underground roots, rhizomes, or coarse lateral root system |
| |
| | 4. Cauline leaves entire to dentate, not lobed, sessile, clasping stem; seeds generally 0 |
| R. austriaca |
| | 4' Cauline leaves deeply dentate to deeply pinnately lobed, rarely 1-pinnate, sessile to short-petioled; seeds present |
| |
| | 5. Fruit narrowly oblong; sepals not persistent in fruit; pedicel ascending to recurved, 5–11(15) mm |
| R. sinuata |
| | 5' Fruit ± round to widely oblong; sepals persistent in fruit; pedicel erect to ascending, 3–8 mm |
| |
| | 6. Fruit hairs dense; fruit stigma ± expanded; axillary inflorescences elongate; sepals, style hairy |
| R. columbiae |
| | 6' Fruit hairs ± 0; fruit stigma ± unexpanded; axillary inflorescences ± umbel-like to ± elongate; sepals, style glabrous |
| R. subumbellata |
| | 3' Annual, biennial, rarely ± short-lived perennial herb, generally from taproot |
| |
| | 7. Fruit ± as long as wide, 1–2.5 mm |
| R. sphaerocarpa |
| | 7' Fruit generally > 2 × longer than wide, 1.5–15 mm |
| |
| | 8. Fruit linear to oblong, not narrowed near middle, < 1.5 mm wide |
| R. curvisiliqua |
| | 8' Fruit oblong to ± round, often ± narrowed near middle, generally > 1.5 mm wide |
| |
| | 9. Plant 4–10(14) dm, ± erect, with 1 dominant stem from base |
| R. palustris |
| | 10. Fruit ovate to round, 2–6 mm; leaf hairy both surfaces; stem hairs ± dense |
| var. hispida |
| | 10' Fruit oblong, 7–15 mm; leaf hairs 0; stem hairs 0 or below, sparse |
| var. occidentalis |
| | 9' Plant 1–5 dm, prostrate to erect, often with several–many stems from base |
| |
| | 11. Fruit rough, minutely papillate, acute at tip, not narrowed near middle |
| R. tenerrima |
| | 11' Fruit smooth, glabrous, acute to ± obtuse at tip, generally narrowed near middle |
| R. curvipes |
| | 12. Fruit ovate to lanceolate, generally 2 × longer than wide, tip acute to ± obtuse |
| var. curvipes |
| | 12' Fruit oblong, 2–3.5 × longer than wide, tip ± truncate to widely obtuse |
| var. truncata
|