Syn. Pl. Glumac. 2: 79. 1855.
Culms 2–40 cm × 0.5–1.5 (–2) mm. Leaves: apex of distal leaf-sheath obtuse to acute, tooth to 0.3 mm. Spikelets lanceoloid to subcylindric or ovoid, 5–10 (–20) × 2–3 (–4) mm, apex acute (to rounded); proximal scale empty, encircling ca. 2/3 of culm; floral scales 25–100 (–200), 8–12 per mm of rachilla, orangebrown to stramineous, 2 (–2.5) × 1–1.3 mm, midribs mostly keeled, ovate, apex narrowly rounded to subacute. Flowers: perianth bristles present or often absent, 5–8, brown, stout, rudimentary to slightly exceeding tubercle; stamens (2–) 3; anthers brown to yellow, 0.3–0.7 (–1) mm; styles 2–3-fid. Achenes 0.9–1.1 (–1.5) × 0.7–1.1 mm. Tubercles depressed, subdeltoid, 0.1–0.3 (–0.4) × 0.6–0.9 (–1) mm, 1/10–2/5 as high as wide, 1/4 or less as high as achene, 9/10 as wide as achene, very rarely 2/5 as high as achene (in E. engelmannii var. robusta). 2n = 10.
Phenology: Fruiting spring–fall.
Habitat: Fresh shores, marshes, disturbed places
Elevation: 30–2400 m
Alta., B.C., Man., Ont., Ala., Ariz., Ark., Calif., Colo., Conn., Del., D.C., Idaho, Ill., Ind., Iowa, Kans., Ky., Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., N.H., N.J., N.Mex., N.Y., N.Dak., Ohio, Okla., Oreg., Pa., R.I., Tenn., Tex., Va., Wash., W.Va., Wis.
Robust plants known only from Missouri and described as Eleocharis engelmannii var. robusta have well-developed caudices, culms to 2 mm wide (pressed), spikelets to 2 cm, floral scales to 2.5 mm and slightly cartilaginous, styles mostly trifid, achenes to 1.5 × 1 mm, and tubercles 0.3–0.5 × 0.7–1 mm and to 2/5 as high as achene. Some specimens are intermediate between E. engelmannii and E. obtusa, or rarely with E. lanceolata. I have not seen specimens to verify literature reports of E. engelmannii from Manitoba, Connecticut, Delaware, Idaho, New Hampshire, North Carolina, Oregon, Rhode Island, South Carolina, South Dakota, or Vermont.