Spikes:

Open, nodding, branching cluster at the top of the stem with numerous spikelets (flower clusters) mostly single on slender stalks, some stalkless. Spikelets are 4 to 7 mm long (to ¼ inch), narrowly egg-shaped, blunt at the tip, grayish to blackish-brown, with florets spirally arranged. Florets are perfect (both male and female parts) each with a 3-parted style and subtended by a single scale.

At the base of the cluster, surrounding each of the lowest 3 to 5 main branches, are leaf-like bracts with blackish sheaths. Bract blades are shorter than to much longer than the cluster, the shorter bracts erect to ascending and the longer bracts arching to reflexed (bent downward). Bracts at the base of auxiliary branches are scale-like and also blackish.

Leaves and stems:

4 to 7 leaves are alternately arranged along the stem, 10 to 24 inches long and 3 to 5(6) mm wide. Lower leaf sheaths are brown. Stems are mostly erect, round in cross-section and smooth. Plants form dense clumps from short rhizomes and a mix of flowering and vegetative shoots. Old leaves persist forming tussocks.

Fruit:

Fruit develops in early summer, the mature achenes (seeds) dropping off individually and the scales dropping soon after. Scales are 1.3 to 1.8 mm long, oblong-elliptic, rounded at the tip sometimes with a minute point, usually blackish (at least at the tip end) with a green or pale midrib that becomes rather inconspicuous. Achenes are .7 to .9 mm long, to .5 mm wide, elliptic to urn-shaped in outline, 3-sided, straw-colored to whitish at maturity. Surrounding the base are 6 white bristles that are several times longer than the achene and much contorted. The bristles extend out beyond the scale and give the cluster a woolly appearance.

Notes:

Scirpus atrocinctus is a common wetland species found along lake and pond margins and in wet ditches, mostly in the northeast quadrant of the state but occasionally found farther south and west. It closely resembles 3 other Minnesota Scirpus species: Scirpus cyperinus, Scirpus pedicellatus, and Scirpus pendulus. All share the common traits of a leafy round stem, leaf-like bracts, a terminal cluster with nodding branches and numerous small spikelets, and achenes with contorted bristles much longer than the achene. Scirpus atrocinctus is distinguished by 4 to 7 leaves per stem, leaves not more than 6 mm (~¼ inch) wide, blackish sheaths at the base of the cluster, blackish floral scales with an inconspicuous midrib, spikelets all single and mostly stalked, and the long, contorted, white bristles extending beyond the scale giving the cluster a woolly appearance at maturity. It is noted that achenes may mature by early July where the others mature in later July and some into September, though S. atrocinctus may be fruiting into midsummer as well.

By comparison, S. cyperinus has spikelets that are clustered in groups of 3 to 7 or more, and mature heads are more dull rusty brown. S. pendulus has green to brown sheaths, scales with a prominent midrib extending to a short awn, and brown bristles on the achene that are more tightly scrunched and do not extend beyond the scale at maturity. S. pedicellatus is most similar and can be very difficult to distinguish, but is generally more robust, has about 8 leaves per stem, leaves up to 10 mm (~3/8 inch) wide, usually green to brown sheaths (sometimes blackish) and usually light brown scales (may be blackish at the tip). S. atrocinctus hybridizes with both S. cyperinus and S. pedicellatus and fertile intermediates may be found when both parents are present, the hybrids cross-breeding with themselves and their parents, known as a hybrid swarm. As if they weren't challenging enough already...

Compare these with other Bulrush species, which may differ by their 3-sided stems, erect bracts that appear to be a continuation of the stem, essentially leafless stems, globular clusters of numerous small spikelets, bristles about as long as or shorter than the achene, or other traits not as above.