Spikes:

Stiff, open or compact branching cluster at the top of the stem, occasionally also arising from the uppermost leaf axil, the main branches up to 4 inches long and more or less radiating in all directions from a central point, usually with short, divergent branchlets at each branch tip. At the tip of each branchlet is a hemispheric to round head of 4 to 110 stalkless spikelets (flower clusters). Spikelets are 2 to 5(8) mm long (average ~¼ inch), oval to egg-shaped, blunt at the tip, gray-green to yellowish-green when young turning dark rusty to blackish-brown or blackish-green, with the florets spirally arranged. Florets are perfect (both male and female parts) each with a 3-parted style and subtended by a single scale.

Often, bulblets form on one or more branches and sprout late in the season. At the base of the cluster are 3 or more leaf-like bracts, green at the base though may be blackish or spotted reddish-brown along the edge. Bract blades are shorter than to longer than the cluster branches. Bracts at the base of auxiliary branches are more scale-like.

Leaves and stems:

5 to 11 leaves are alternately arranged along the stem, 8 to 20 inches long and 7 to 20 mm (to ~¾ inch) wide. Lower leaf sheaths are green to light brown with many conspicuous cross-partitions that give them a polka-dotted appearance. Stems are single or a few from the base, erect, smooth and 3-sided in cross-section with rounded angles. Plants form loose clumps from short rhizomes.

Fruit:

Fruit develops in early to mid-summer, the mature achenes (seeds) dropping off individually and the scales soon after, leaving behind naked branchlets. Scales are 1.2 to 2.1 mm long, oval-elliptic, pointed at the tip with a midrib that often extends to an awn not more than .4 mm long. Scale color is variable: greenish when young turning blackish-brown to blackish-green, the midrib initially green turning light brown. Achenes are (.8)1 to 1.3 mm long, .4 to .6 mm wide, plumply to weakly 3-sided in cross-section, elliptic to urn-shaped in outline, pale brown to creamy colored at maturity. Surrounding the base are 5 or 6 barbed, pale bristles that are slightly shorter to slightly longer than the achene.

Notes:

Scirpus atrovirens is a common species of shores, marshes, swales, ditches and other open wet places throughout much of the state. It closely resembles 4 other Minnesota Scirpus species: Scirpus georgianus, Scirpus hattorianus, Scirpus microcarpus, and Scirpus pallidus. These share the common traits of a weakly 3-sided stem, 3 or more stem leaves, 3 or more leaf-like bracts that are shorter than to longer than the cluster branches, and a terminal cluster with fairly stiff erect to spreading branches that end in dense, hemispheric to round heads of several to many spikelets.

Scirpus atrovirens is the most common of the 4 and is distinguished by the consistently numerous and conspicuous cross-partitions on all sheaths (most evident on lower sheaths, even when sheaths/leaves dry up), floral scales usually dark brown at maturity with an awn not more than than .4mm long, 3-parted styles, 5 or 6 bristles more or less as long as the achene, and bulblets that sprout late in the season (usually present on some plants within a population, though may be absent).

The most readily distinguished of this group is S. microcarpus, which is rarely taller than 2 or 3 feet, has purplish-red sheaths, 2-parted styles, and at most 18 spikelets in a head. The other 3 can be very difficult to distinguish from each other and it may be necessary to examine several plants in a population to get a consensus on some traits. S. georgianus, which has only been recorded twice in Minnesota and not since 1983 (probably now extirpated), has at most 3 bristles all shorter than the achene. S. hattorianus is generally a more slender, spindlier plant with longer branchlets but smaller heads, slightly smaller floral scales with a minute point at the tip or the midrib not extending more than .2mm, 4 to 6 bristles all shorter than the achene, and inconspicuous cross-partitions on leaves and basal sheaths. S. pallidus often has larger heads in a more compact cluster, may or may not have conspicuous cross-partitions, lacks bulblets, and its floral scales are slightly larger and have an awn at least .4mm long (this may be the most consistent difference). Most references put some reliance on scale and achene colors, but I have found these too subjective and not consistently distinct. Scales on all can be streaked with black and mature achenes can all be creamy colored. And hybrids exist which complicates matters.

Compare these with other Bulrush species, which may differ by their round or more sharply 3-sided stems, erect bracts that appear to be a continuation of the stem, less leafy stems, nodding clusters, some or all spikelets single at branch tips, or other traits not as above.