Scirpus acutus of Michigan Flora.

On wet shores or more often in shallow water of ponds, lakes, rivers, and ditches; in fens and "fenny" lakes and sometimes relic in open tamarack swamps; on sand, gravel, marl, or peat. Usually in water up to about 1.5 m deep, but plants with culms as long as 4.2 m have been collected in even deeper water. May form pure stands, as in S. tabernaemontani, or grow with other marsh species, such as Phragmites australis, Typha spp., S. pungens, and S. tabernaemontani.

The pedicels are occasionally nearly or quite suppressed, the spikelets therefore ± densely crowded. Dried specimens are especially difficult at times to distinguish from S. tabernaemontani. Even more than usual, one must weigh both choices in the key with the expectation that a plant will fit neither perfectly. There seems to be a tendency for the anthers of S. tabernaemontani to be slightly shorter [1–1.6 (–2.1) mm long] than in S. acutus [(1–) 1.5–2.5 mm]. The flattened pedicels in S. tabernaemontani are scabrous on the two edges and smooth on the sides; in S. acutus, they are sometimes puberulent and/or red-flecked like the scales.

Where Schoenoplectus acutus and S. tabernaemontani grow near each other, intermediate hybrids may rarely occur. These have been collected in Chebygan and Muskegon Cos., and are probably overlooked.