Moist to very wet ground or shallow water of peatlands, river margins, marshes, ponds, swales, and ditches.

One of our most distinctive species in its very late fruiting, often extremely large stature in wet places, raised position of the grain on the tepals, and absence of a swelling at the joint in the pedicel. Even young flowering plants of R. crispus and related species have the joint evident. There is also a subtle difference in leaf venation, the main lateral veins in R. orbiculatus continuing prominent farther from the midrib than in R. crispus (and others), in which they soon become weak, branched, and anastomosed. The basal leaves may be over a meter long. We are reluctant to adopt the name R. britannica L. for this species, given the complexities noted by Fernald (1945) and the fact that the name has not been typified.