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| 1 Leaves peltate, lacking a sinus extending to the attachment of the petiole. |
| ..2 Inflorescence umbellate; leaves 1-4 (-7) cm wide |
| ..2 Inflorescence verticillate or umbellate-verticillate (when first developing sometimes appearing merely umbellate); leaves 1-15 cm wide. |
....3 Inflorescence compound, the main inflorescence axis with nodes which produce verticils or umbels of pedicellate flowers, the inflorescence nodes also producing branches which themselves produce verticils or umbels of flowers; leaves (1-) 4-15 cm wide, the blade characteristically held in a vertical plane |
| ....3 Inflorescence verticillate, all the flowers borne sessile or on pedicels on the unbranched inflorescence axis; leaves 1-6 cm wide, the blae usually more-or-less in a horizontal plane. |
| ......4 Flowers and fruits pedicellate, the pedicels 1-10 mm long |
| ......4 Flowers and fruits sessile or subsessile |
| 1 Leaves not peltate, a sinus extending to the attachment of the petiole. |
| ........5 Central leaf lobe notably more distinct than the other lobes (the sinuses on either side extending 1/3 to 3/4 of the way to the petiolar attachment); stems and petioles fleshy |
| ........5 Central leaf lobe not more distinct than the other lobes (the sinuses on either side extending 1/10 to 1/4 the way to the petiolar attachment); stems and petioles filiform. |
| ..........6 Fruiting umbels on peduncles 1-3 mm long; leaves 10-50 mm wide; [native of bogs, spray cliffs, and other wetlands] |
| ..........6 Fruiting umbels on peduncles 9-24 mm long; leaves 5-30 mm wide; [alien of lawns and other disturbed habitats]. |
| ............ 7 Leaves 5-lobed, 15-30 mm wide |
| ............ 7 Leaves 7-lobed, 5-13 mm wide |