Purple spikerush

Scirpus atropurpureus Retzius, Observ. Bot. 5: 14. 1789

Plants tufted, without creeping rhizomes. Culms 2–12(–19) cm × 0.2–0.4 mm. Leaves: distal leaf sheaths firm, distally tightly sheathing, apex acute. Spikelets ovoid to ellipsoid, 2–6(–8) × 1–2.5 mm, apex acute; proximal scale with or without flower, not amplexicaulous; floral scales to 100, 15–19 per mm of rachilla, often loosely appressed, dark red-brown to stramineous, ovate to elliptic, 0.6–1.3 × 0.3–0.7 mm, membranous, apex rounded to acute. Flowers: perianth bristles (0–)4–6, typically 4, colorless to whitish, vestigial to 1/2 as long as achene, smooth or spinuliferous; styles 2-fid. Achenes black, obovoid, biconvex, 0.3–0.5 × 0.3–0.4 mm, apex often constricted proximal to tubercle, smooth at 40X. Tubercles stramineous to whitish, umbonate to subconic, 0.1–0.2 × 0.1–0.2 mm, apex acute. 2n = 20.

Fruiting summer–fall (Jun–Sep). Canal banks, hammocks, irrigation ditches, lake and pond margins, maritime shores, rice fields; 0–1800 m; B.C.; Ala., Calif., Fla., Ga., Iowa, Kans., La., Mich., Mo., Nebr., N.Mex., N.C., Okla., S.C., Tex., Wash.; Mexico; West Indies; Central America; South America; Europe (naturalized); Asia; Africa; Pacific Islands.

Eleocharis atropurpurea has been reported from Colorado, Montana, and Virginia; I have not seen voucher specimens.