Wet marshy ground, shores, bogs and fens, interdunal swales, springy thickets; typically in tufts in very wet settings, even floating. 

The perigynia in this species are usually smaller than in C. prairea, although the extremes are about the same. In C. diandra they run (2–) 2.2–2.5 (–3) mm long and are definitely convex on the ventral face at maturity.