Eleocharis tenuis is a perennial spike-rush, colonial from slender, purplish rhizomes, stems slightly tufted or scattered, wiry, slender, 4-8 ribbed, dark red at base, 5-90 cm tall;
Leaves bladeless, sheath tight, obliquely truncate, orifice dark.
Flower spikelet egg-shaped or oval, wider than stem, dark reddish brown, solitary at top of stem, 0.5-1 cm long, scales dark, margins membranous.
Fruit a dry achene, about 0.1 cm long, 3-sided, surface pitted, capped by prominent tubercle, narrower than achene and separated from top by a groove; May-June.
Wetland status: FACW+ (USDA, NRCS 2010).
Frequency in NYC: Infrequent.
Habitat: Open pond edges, vernal ponds, swamps; intolerant of salt or shade; soil pH 6.2-7, moderately tolerant of fire, and anaerobic soil, intolerant of drought, shade, and salt (USDA, NRCS 2010).