Soft rush grows in marshes, wet meadows, ditches, pond edges, swamps and damp open ground across much of North America and on every continent except Australia and Antarctica. Muskrats feed on its rootstalks, and the clumps give cover to wetland birds and small mammals. It is also studied for constructed wetlands because it takes up nutrients from wastewater. In Quebec the species is native, mainly as the eastern subspecies solutus, called jonc lâche. The Eurasian subspecies effusus is introduced here, and the similar Pylaie's rush (Juncus pylaei) is treated as a separate native species.
The genus name comes from iuncus, the Latin word for rush, and effusus means spread out, a reference to the loose flower cluster. In Europe the peeled pith was soaked in fat to make rushlights, a cheap replacement for candles, which explains the old name lamp rush. In Japan the same species, called igusa, is woven into the covers of tatami mats.