Leaves simple, lamina circular or cordate, peltate, margins entire, lobed or dissected with crenate to toothed margins or palmately compound with 3–5 entire to lobed leaflets; petiolate, with scarious stipules.
Inflorescence usually a simple umbel, with or without bracts, or flowers in heads or spikes. Flowers bisexual or unisexual and plants dioecious; white, greenish or pinkish. Sepals minute or absent. Petals ± ovate, usually acute, the apex not inflexed. Stylopodium depressed.
| | Key to the species | |
| 1 | Leaves peltate, more or less circular; flowers either in clusters along a rachis or in compound umbels | 2 |
| Leaves not peltate; flowers in simple umbels or small heads | 3 |
| 2 | Flowers in clusters of 2–5 with 1–7 clusters along a simple sometimes branched rachis | Hydrocotyle verticillata |
| Flowers numerous in compound umbels
Back to 1 | Hydrocotyle bonariensis |
| 3 | Perennials with prostrate or ascending stems or stolons | 4 |
| Annuals with erect or ascending stems
Back to 1 | 9 |
| 4 | Leaves palmatisect into 3–5 segments | 5 |
| Leaves with lamina simple or sometimes palmatifid
Back to 3 | 6 |
| 5 | Leaf segments large, to 5 cm long | Hydrocotyle geraniifolia |
| Leaf segments small, usually less than 1 cm long, umbels few-flowered
Back to 4 | Hydrocotyle tripartita |
| 6 | Flowers conspicuously pedicellate in umbels | 7 |
| Flowers sessile or very shortly and inconspicuously pedicellate, in small, dense heads
Back to 4 | 8 |
| 7 | Leaves mostly less than 4 cm wide, both surfaces hirsute | Hydrocotyle laxiflora |
| Leaves mostly more than 5 cm, glabrous or with short more or less spreading hairs especially on lower veins
Back to 6 | Hydrocotyle pedicellosa |
| 8 | Flowers less than 12 per umbel; fruit c. 1 mm long and wide | Hydrocotyle sibthorpioides |
| Flowers c. 15 per umbel; fruit 1–1.7 mm long, 1.5–2 mm wide
Back to 6 | Hydrocotyle algida |
| 9 | Mericarps smooth or faintly wrinkled between dorsal and intermediate ribs | 10 |
| Mericarps with tubercles or many pits between dorsal and intermediate ribs
Back to 3 | Hydrocotyle trachycarpa |
| 10 | Plants usually grey-green or often purplish tinged; umbels usually shortly pedunculate, with 6–15 flowers | Hydrocotyle callicarpa |
| Plants usually bright green to yellowish green; umbels mostly sessile, leaf-opposed, with 3–6 flowers
Back to 9 | Hydrocotyle foveolata |
