Plants perennial. Culms 60-100 cm tall, 2.5-5 mm thick, often decumbent at the base. Sheaths glabrous, smooth, not keeled, midvein prominent; ligules 0.5-0.8 mm, not translucent, truncate to slightly rounded; blades 15-40 cm long, 2-8 mm wide, abaxial surfaces smooth, adaxial surfaces scabridulous. Panicles 5-15 cm long, 2.5-6 cm wide, ovoid, erect, dense; branches 2.5-8 cm, strongly ascending, with 10-30 spikelets; pedicels 1-14 mm. Spikelets 4-7 mm long, 2.5-4 mm wide, somewhat laterally compressed, oval in side view, with 4-7 florets. Glumes keeled, 1-veined, veins not extending to the apical margins, apical margins hyaline, acute, entire or often splitting with age; lower glumes 1.6-2.5 mm, lanceolate to narrowly ovate or obovate; upper glumes 1.7-3.5 mm, ovate-elliptic to obovate, obtuse to rounded; rachilla internodes 0.2-0.4 mm; lemmas 3-3.9 mm, 5-9-veined, veins not raised, apices rounded, somewhat prow-shaped; paleas subequal to the lemmas, lengths 2-2.8 times widths, keels well-developed, not winged, tips pointing towards each other, narrowly notched between the keels; anthers 2, 0.6-0.8 mm. Caryopses 1.5-1.8 mm. 2n = 40.
Glyceria obtusa is a distinctive species that grows in wet woods, swamps, and shallow waters, primarily on the eastern seaboard of North America, from Nova Scotia and New Brunswick to South Carolina.
Culms 6-10 dm, often decumbent at base; main lvs 3-8 mm wide; ligule under 1 mm; panicle dense and compact, ovoid or oblong, 0.6-1.2 dm, usually with 3-8 strictly ascending branches from each node, the 2 lowest internodes each less than 2(-2.5) cm; spikelets 4-7 mm, 4-7-fld; glumes broad and scarious, the first 1.5-2 mm, the second 2-2.7 mm; lemmas broadly elliptic to obovate, 3-3.7 mm, scarious at the margins and the blunt summit, about as long as the palea, the veins visible but not raised; stamens 2; 2n=40. Swamps, wet woods, and shallow water; N.S. to N.C., chiefly near the coast, and extending inland to the Catskills and e. Pa.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.