Rhizomes 1-3 mm thick, nodes with whorls of roots. Culms 5-80 cm, erect, glabrous. Ligules of lower leaves 0.4-3 mm; ligules of flag leaves 1-4(5.5) mm, usually lacerate; blades 1-13 cm long, 1-4 mm wide. Panicles 2.5-18 cm long, 1-6 cm wide. Spikelets 4-8.5(9) mm; rachilla internodes 1-1.5 mm. Glumes 4-8.5(9) mm; lemmas 3-6.5 mm; paleas 2.8-6 mm; anthers 1.5-3.5 mm. Caryopses 1.5-3 mm. 2n = 42, 44, 66, 84, 88, 132.
Dupontia fisheri grows in wet meadows, wet tundra, marshes, and along streams and the edges of lagoons, ponds, and lake shores, in sand, silt, clay, and moss, rarely in bogs. It is sometimes dominant, or co-dominant, with Alopecurus borealis.
Two subspecies are sometimes recognized in North America. Dupontia fisheri R.
Br. subsp. fisheri supposedly
differs from subsp. psilosantha (Rupr.) Hultén in being less
than 40 cm tall, having erect panicle branches, 2-4 florets per spikelet,
pubescent, obtuse lemmas, and 2n = 84, 88, or 132. Dupontiafisheri subsp. psilosantha is
taller, has reflexed panicle branches, 1-2 florets per spikelet, more or
less glabrous, acute lemmas, and 2n = 42 or 44. Plants referable
to subsp. psilosantha are restricted to coastal marshes, rarely
penetrating inland along riparian habitats, from James Bay to the lower
arctic archipelago. Plants referable to subsp. fisheri are less
halophytic and more northerly in their distribution, being found in a variety
of inland marshes and wet tundra habitats from northern Alaska to Ellesmere
Island. Intermediates are readily found (e.g., hexaploid, 2n = 66
hybrids from Alaska) and the correlations among chromosome number, morphology,
ecology, and distribution are relatively weak in North America and Greenland.
For these reasons, no subspecific taxa are recognized in this treatment.