Scirpus atrovirens Willd.

Scirpus atrovirens is variable and often confused with all of its close relatives: S. georgianus, S. flaccidifolius, S. hattorianus, and S. pallidus. The septa in the blades and sheaths of the proximal leaves tend to be more prominent in S. atrovirens than in any of the related species, so that the leaves and sheaths appear distinctly nodulose in dried specimens. The scales have relatively little black pigmentation, so that the heads appear brownish, not blackish as in S. pallidus, S. hattorianus, and S. flaccidifolius.

Scirpus atrovirens occasionally hybridizes with S. georgianus, S. hattorianus, S. pallidus, and S. polyphyllus.

Plants cespitose; rhizomes short, tough, fibrous. Culms: fertile ones upright or nearly so; nodes without axillary bulblets. Leaves 6–11 per culm; sheaths of proximal leaves light brown; proximal sheaths and blades with septa many, ± conspicuous; blades 20–54 cm × 7–17 mm. Inflorescences terminal, rarely also with 1 lateral inflorescence from distal leaf axil; rays ascending or divergent (commonly both in the same inflorescence), proximal branches smooth, distal branches scabrellous to scabrous, rays often bearing axillary bulblets; bases of involucral bracts green, margins usually speckled with red-brown, rarely solid black, not glutinous. Spikelets in dense clusters of 4–110 (largest cluster with 17–25+), spikelets sessile, ovoid to narrowly ovoid, 2–5(–8) × 1–2.5 mm; scales dark brown with pale midribs, elliptic or broadly elliptic (rarely almost circular), 1.2–2.1 mm, apex mucronate, mucro 0.1–0.3(–0.4) mm. Flowers: perianth bristles persistent, (5–)6, rather stout to somewhat slender, straight or curved, shorter than to slightly exceeding achene, with retrorse, thin-walled, round-tipped barbs in distal 0.3–0.6, enclosed within scales; styles 3-fid. Achenes light brown, elliptic or obovate in outline, plumply trigonous or plano-convex, (0.8–)1–1.3 × 0.4–0.6 mm. 2n = 50–60.

Man., Nfld. and Labr. (Nfld.), Ont., Que.; Ala., Ariz., Conn., Del., D.C., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Mo., Mont., Nebr., N.H., N.J., N.Y., N.Dak., Ohio, Okla., Pa., R.I., S.Dak., Tenn., Tex., Vt., Va., W.Va., Wis.

Fruiting late spring–early summer (late Jun–Jul).

Marshes, moist meadows; 0–900m.

Scirpus atrovirens var. pycnocephalus Fernald

Scirpus atrovirens Wilid. Enum. 79. 1809
? Scirpus reticulatus Lam. Tab. Encyc. 1: 142. 1791.
Holoschoenus atrovirens Link, Hort. Berol. 1: 293. 1827.
Scirpus sylvaticus var. atrovirens A. Gray, Man. ed. 2, 500. 1856.
Scirpus sylvaticus var. sychnocephalas S. N. Cowles, Am. Nat. 3: 101. 1869.
Scirpus georgianus Harper, Bull. Torrey Club 27: 331. 1900.
Scirpus atrovirens var. pycnocephalus Fernald, Rhodora 8: 163. 1906.
Scirpus atrovirens i. sychnocephalus Blake, Rhodora 15: 161. 1913.
Scirpus atrovirens var. georgianus Fernald, Rhodora 23: 134. 1921.
Scirpus atrovirens var. georgianus subvar. viviparus Farwell, Am. Midi. Nat. 11: 72. 1928.
Scirpus atrovirens var. georgianus f. viviparus M.-Victorin, Proc. & Trans. Roy. Soc. Canada III.
23': 267. 1929. Scirpus atrovirens i. proliferus F. J. Herm. Rhodora 40: 77. 1938. Scirpus atrovirens var. Jlaccidifoliiis Fernald, Rhodora 40: 396. 1938.
Scirpus atrovirens var. georgianus f. cephalanthus Fernald, Rhodora 45: 296. 194 v
Scirpus atrovirens var. georgianus f. angustispicatus Fernald, Rhodora 45: 296. 1943.
Fibrous-rooted perennial; culms 0.8-1.5 m. high, stout, 1-1.5 cm. thick at the base, 2-3 mm. broad at the summit, sharply trigonous, smooth, green; sheaths long, green, drying brown, often nodulose, the blades pale green, 7-15 mm. broad, to 5 dm. long, equaling or exceeding the culm, scabrous on the margins and midrib, tapered to the blunt tip; umbel terminal, compound, the primary rays terete, minutely scabrous; spikelets in glomerules at the end of shorter, secondary rays, the glomerules usually few but occasionally very numerous and forming a densely packed head as much as 12 cm. long; spikelets dull greenish-brown or rufescent, narrowly ovoid to cylindric, 3.5-8 (rarely 10) mm. long, in glomerules of 10-30, often viviparous; scales 1.5-2 mm. long, loosely spreading, the margins greenish-black, the midrib tapered to a scabrous, mucronate tip; bristles 6 or fewer, sparsely to strongly retrorsely barbed, white, nearly straight, about equaling the achene or rarely absent; style trifid, reddish; achene ca. 1 mm. long, white, obovoid-oblong, trigonous, conspicuously pointed.
Type locality: "In America boreali."
Distribution: Meadows and bogs; Newfoundland to Saskatchewan, south to Georgia and Texas.

Scirpus atrovirens, known as dark-green bulrush, is a perennial sedge native to wetlands of eastern Canada and the United States. It is sometimes called dark green bulsedge, black bulrush, or green bulrush.

Scirpus atrovirens grows in a wide variety of wetland habitats, typically in sunny areas rather than shady. It can be found in wet meadows and shrubby thickets, openings in swamps, marshes, shorelines, as well as roadside ditches. The plant can grow up to five feet tall, and thrives in hardiness zones 3–9.

It is closely related to Scirpus hattorianus and Scirpus georgianus, with mature fruits necessary for accurate identification.

Scirpus atrovirens là loài thực vật có hoa trong họ Cói. Loài này được Willd. miêu tả khoa học đầu tiên năm 1809.