A small annual or short-lived perennial which grows in shallow water or on wet soil. Typical sites include the margins of ponds and ditches, flushes, damp depressions, gateways and tracks in pastures and on heathland, and the sheltered backwaters of rivers. Unlike R. hederaceus, it is confined to acidic, mesotrophic or oligotrophic soils. 0-1005 m (Carnedd Llewelyn, Caerns.).
R. omiophyllus remains frequent in W. Britain and S. Ireland, where there are many more records than there were in the 1962 Atlas. The decline at the fringes of its range was already discernible in the 1962 Atlas, and is now much more marked.
Suboceanic Southern-temperate element.