Spikes:

Separate staminate (male) and pistillate (female) spikes, with a single staminate spike 3/8 to 2 inches long at the tip of the stem. Below the staminate spike are 1 to 4 erect all-pistillate spikes, hemispheric to short cylindric in outline, on stalks up to ½ inch long. At the base of each pistillate spike is a leaf-like bract that is 2½ to 8 inches long and usually much over-tops the terminal spike. The pistillate spikes are typically close together, sometimes hard to distinguish from each other, but they do not typically crowd the terminal spike.

Leaves and stems:

Leaves are basal and alternate, mostly near the base, 3.5 to 8 mm wide, 3 to 10 inches long; the upper stem leaves may over-top the terminal spike. Stem leaf sheaths are U-shaped at the tip, papery translucent whitish-green, and loosely wrap the stem. The ligule (membrane where the leaf joins the sheath) is mostly longer than wide, rounded to pointed at the tip. Leaves are hairless but rough textured especially along the edges.

Bases are wrapped in a sheath that is purplish-red and not fibrous. Stems are slender, erect to ascending, 3-sided in cross-section, and slightly rough. Stems elongate up to 30 inches at maturity and are mostly longer than the leaves. Not all plants produce flowering stems. Plants are usually clump forming and may form small colonies from short rhizomes.

Fruit:

Fruit develops in late spring to mid-summer, the pistillate spikes forming clusters of seeds (achenes), each wrapped in a casing (perigynium), subtended by a scale. Perigynia are mostly ascending to widely spreading and are not tightly crowded on the spike. Each pistillate spike contains 1 to 12 fruits.

Pistillate scales are lance to egg-shaped, whitish turning light brown, a green midrib, pointed at the tip or the midrib extending to a short awn, the awns rough-textured, scales narrower and rather shorter than the perigynia. Perigynia are 10 to 16.5 mm long, 2.5 to 6.5 mm wide, green at maturity, hairless, strongly 13 to 23-veined, satiny-lustrous, lance to teardrop-shaped with a long taper to a straight beak that has 2 small teeth at the tip. Achenes are 3-sided, 3.5 to 5.7 mm long, 2.5 to 3.9 mm wide, widest above the middle, maturing to dark brown, with a long, persistent style that typically coils once in the lower half.

Notes:

Carex intumescens is a common sedge of open and forested wetlands throughout much of the state, except in the southwest quarter where it is absent.

Carex is a large genus, with over 600 species in North America and 150+ in Minnesota alone. They are grouped into sections, the species in each group having common traits. Carex intumescens is in the Lupulinae section; some of its common traits are: clump forming or not, rhizomatous but rarely forming colonies, hairless leaves, basal sheaths usually red-purple and not fibrous, sheaths and larger leaves with cross partitions between veins (septate-nodulose), 2 to 9 spikes, terminal spike all-staminate, lateral spikes stalked, erect to ascending, round to thick cylindrical, all-pistillate or with a few staminate flowers at the tip (androgynous), leaf-like bract subtending the lowest pistillate spike, perigynia  ascending to spreading, hairless, strongly veined, inflated, mostly teardrop to flask-shaped with a long taper to a toothed beaked, achenes 3-sided in cross-section with a long, persistent style. Species in this section have the largest perigynia of all Carex.

Carex intumescens is distinguished from all other Minnesota sedges by the combination of: 1 to 4 erect, stalked all-pistillate spikes usually close together near the terminal spike, 12 or fewer perigynia per spike, perigynia inflated, widest near the base, not tightly crowded and mostly ascending to spreading, achenes widest above the middle. By contrast with other members of the Lupulinae section: Carex grayi has only 1 or 2 spherical spikes each with up to 35 perigynia, the perigynia radiating in all directions from a central point; Carex lupulina with as many as 80 perigynia per spike, all ascending and crowded on the spike, and rhombic/diamond-shaped achenes. Some references note 2 varieties of C. intumescens, though these are not universally accepted: var. intumescens in lower elevations and the southern part of the range, with more inflated perigynia 5+ mm wide and achenes widest in the middle with a straight or arching style; var. fernaldii in higher elevations and the northern part of the range, with less inflated perigynia up to 5mm wide and achenes widest above the middle with a coiled style. By these accounts Minnesota should have textbook var. fernaldii.