Leaves whorled, opposite, alternate or irregularly arranged, submerged leaves often deeply pinnatifid forming regular narrow segments, emergent leaves entire or toothed.
Flowers mostly unisexual, rarely bisexual, usually solitary in the axils of the emergent leaves but sometimes in axillary clusters of 2 or 3; in monoecious plants the male flowers in the upper part of the flowering shoot. Male flowers mostly 4-merous; sepals usually present; petals hooded; stamens usually 8, sometimes 4. Ovary and styles vestigial or absent. Female flowers often lacking both petals and sepals; ovary usually of 4 carpels; styles free, one per carpel; stigmas usually fimbriate.
| | Key to the species | |
| 1 | Lower or submerged leaves always whorled | 2 |
| All leaves opposite, alternate or irregularly arranged | 12 |
| 2 | Emergent leaves pinnately divided | 3 |
| Emergent leaves entire or toothed
Back to 1 | 5 |
| 3 | Emergent leaves glaucous, in whorls of 4–6, mostly with 10–18 segments on each side of the leaf | Myriophyllum aquaticum |
| Emergent leaves green to reddish, usually in whorls of 2–4, with up to 6 segments on each side of the leaf
Back to 2 | 4 |
| 4 | Mericarps ovoid, smooth or sparsely papillose; leaves often crowded at the tips of the stems | Myriophyllum alpinum |
| Mericarps oblong, angular, distinctly warty; leaves not crowded
Back to 3 | Myriophyllum verrucosum |
| 5 | Emergent leaves ovate or oblong, entire or with a few short teeth towards the apex | 6 |
| Emergent leaves terete or lanceolate, entire or toothed
Back to 2 | 8 |
| 6 | Mericarps oblong to ovoid, smooth to warty | 7 |
| Mericarps cylindrical, spinose
Back to 5 | Myriophyllum caput-medusae |
| 7 | Mericarps ovoid, smooth or minutely papillose | Myriophyllum salsugineum |
| Mericarps oblong, distinctly warty
Back to 6 | Myriophyllum verrucosum |
| 8 | Emergent leaves terete, linear or lanceolate, entire or toothed | 9 |
| Emergent leaves lanceolate, shortly toothed in the upper half
Back to 5 | Myriophyllum latifolium |
| 9 | Flowers and fruits solitary; styles white to pink; leaves in whorls of 3–8, emergent leaves usually more or less entire | 10 |
| Flowers and fruits in axillary clusters; styles mauve to purple; leaves in whorls of 4–6, all emergent leaves toothed
Back to 8 | Myriophyllum papillosum |
| 10 | Emergent leaves in whorls of 3–6; flowering shoot more or less cylindrical with a truncate tip; stem glabrous | 11 |
| Emergent leaves in whorls of mostly 5–8; flowering shoot tapering to the tip; stem usually bearing scattered, small, crisped hairs
Back to 9 | Myriophyllum crispatum |
| 11 | Fruit yellow-brown, c. 1.5 mm long, mericarps cylindrical; emergent leaves in strict whorls of usually 5; stigmas white; aquatic plant | Myriophyllum variifolium |
| Fruit reddish purple, c. 1 mm long, mericarps ovoid; emergent leaves in whorls of mostly 3 or 4, sometimes irregularly so; stigmas pink (fading to white); aquatic or semiterrestrial plant
Back to 10 | Myriophyllum simulans |
| 12 | Leaves all, or predominantly, alternate or irregular | 13 |
| Leaves all, or predominantly, opposite
Back to 1 | 17 |
| 13 | Plants perennial, adventitious roots produced along the stems | 14 |
| Plants annual, more or less erect
Back to 12 | Myriophyllum glomeratum |
| 14 | Male flowers with sepals; aquatic or terrestrial plants | 15 |
| Male flowers lacking sepals; mat plants on wet ground
Back to 13 | 16 |
| 15 | Lowermost leaves pinnately divided with 8 or more segments on each side of the leaf; mericarps ovoid | Myriophyllum simulans |
| Lowermost leaves entire, trifid or pinnately divided with 2 or 3 segments on each side of the leaf; mericarps shortly cylindrical
Back to 14 | Myriophyllum gracile |
| 16 | Mericarps papillose; leaves always entire; plants strictly dioecious | Myriophyllum implicatum |
| Mericarps weakly ribbed by linking of papillae; leaves entire or pinnately lobed; plants monoecious or dioecious
Back to 14 | Myriophyllum striatum |
| 17 | Mericarps ovoid with an attenuate apex, smooth or warty; leaves linear to trifid, mostly 4–20 mm long | Myriophyllum pedunculatum |
| Mericarps cylindrical with a pale dorsal crest, smooth or papillate; leaves linear, rarely over 3 mm long
Back to 12 | Myriophyllum lophatum |
