Marshes, wet shores, ditches and swales, tamarack swamps, fens; often in water (sometimes as deep as 1.8 m); the Eurasian subspecies is now a common invader of roadsides, ditches, and also native wetlands and lake shores.

A cosmopolitan species, found around the world, represented in Michigan by two subspecies. The native subsp. americanus Saltonst., P. M. Peterson & Soreng has shiny, reddish to purplish lower stem internodes (usually exposed because the sheaths are readily deciduous), and the membranous part of the ligule (excluding the apical fringe) ca. 0.4-1 mm long. The introduced subsp. australis has duller yellow or yellowish-brown lower stem internodes, the sheaths mostly persistent after the culm senesces, and the membranous part of the ligule 0.1-0.4 mm long. Though showing some overlap, glume length also differs with the lower glume of the alien subspecies mostly 2.6-4.2 (-4.8) mm long while the native has lower glumes usually 4-7 mm long.

The native subspecies occurs throughout Michigan, while the introduced subspecies, first collected in Michigan in 1979, is concentrated in developed areas of southern Michigan, becoming less common northward and apparently still uncommon in the Upper Peninsula. McNabb & Batterson (1991) noted the spread of this entity, before it was known that there were two subspecies in Michigan, and suggested that it started to occur along roadsides about 1975. Though still much undercollected, as of 2015, the alien subspecies was documented by herbarium specimens from Delta, Iron, Kalamazoo, Lapeer, Mason, Mecosta, Ontonagon, Tuscola, Washtenaw, and Wayne Cos.

The two subspecies differ in growth form; the native subsp. americanus often has rather scattered stems in a colony, whereas the introduced subsp. australis generally forms very dense stands, choking out most other species. The introduced subspecies is aggressively invasive and frequent now in ditches, urban wetlands, and other disturbed, often saline, habitats, though it is also a major problem on Great Lakes shoreline wetlands. The native subspecies occurs in natural communities; fens, sedge meadows, lake and river shores, etc. Catling, Mitrow, & Black (2007) provide very helpful commentary on this species in eastern Ontario, including more detailed discussion of characters. The two are amply distinct in Michigan, but apparently the morphological gap is bridged by other entities to our south. Vegetative reproduction can be very rapid in both subspecies, with aggressive rhizomes, and also long stolons, the latter noted as long as 13 m, as observed by L. H. Harvey on the flats at Cecil Bay, Emmet Co. Both entities can have red color on the stolons.