Spikes:

Separate staminate (male) and pistillate (female) spikes, with a single staminate spike up to 3 inches long at the tip of the stem; rarely 2 or 3 staminate spikes are present, the secondary spikes smaller and crowding the terminal spike. Below the staminate spike are 2 to 5 erect to ascending, all-pistillate spikes, broadly cylindric, each up to 3 inches long, the lowest spike on a stalk up to 7 inches long. The uppermost pistillate spikes are crowded together just below the terminal spike and are shorter stalked to nearly stalkless. At the base of each pistillate spike is a leaf-like bract that is 5 to 20 inches long and usually much over-tops the terminal spike.

Leaves and stems:

Leaves are basal and alternate, mostly near the base, 4 to 8.5 mm wide, 6 to 24 inches long; the upper stem leaves often over-topping the terminal spike. Stem leaf sheaths are U to V-shaped at the tip, papery translucent whitish-green, and loosely wrap the stem. The ligule (membrane where the leaf joins the sheath) has a broad band of loose tissue around the edge, is pointed at the tip, and mostly longer than wide. Leaves are hairless but rough textured especially along the edges. Leaves of vegetative shoots may be wider than those of flowering shoots.

Bases are wrapped in a sheath that is purplish-red to brown and may become fibrous. Stems are stout, mostly erect, 3-sided in cross-section, and smooth. Stems can elongate up to about 3 feet at maturity but are often over-topped by the leaves. Not all plants produce flowering stems. Plants are loosely clump forming or not, have short rhizomes but are not especially colonial.

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 ascending to slightly spreading and tightly crowded on the spike. Each pistillate spike contains 15 to 80 fruits, occasionally as few as 8.

Pistillate scales are lance to egg-shaped, white to straw-colored with a green midrib, pointed at the tip or the midrib extending to an awn up to 6 mm long, the awns rough-textured, scales narrower and rather shorter than the perigynia. Perigynia are 11 to 19 mm long, 3 to 6 mm wide, green to yellowish at maturity, hairless, strongly 13 to 22-veined, somewhat shiny, the body oval and inflated but usually flattish on the inner side, broadest near the base, tapering to a straight beak that is about as long as the body and has 2 teeth at the tip. Achenes are 3-sided, 3 to 4 mm long, 1.7 to 2.6 mm wide, maturing to dark brown, rhombic/diamond-shaped in outline, widest at the middle and flat to concave on the sides, with a long, persistent style that is contorted near the base.

Notes:

Carex lupulina is a common sedge of open and forested wetlands and reaches the northwest edge of its range in Minnesota.

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 lupulina 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 lupulina is distinguished from all other Minnesota sedges by the combination of: 2 to 5 erect, stalked all-pistillate spikes, the uppermost close together near the terminal spike, spikes thickly cylindric, 15 to 80 perigynia per spike, perigynia 11 to 19 mm long, inflated, widest near the base, tightly crowded and all ascending to slightly spreading, achenes rhombic/diamond-shaped and widest at 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 intumescens with 12 or fewer perigynia per spike, perigynia ascending to widely spreading and not crowded on the spike, and achenes broadest above the middle. C. lupulina spikes generally look like a very robust C. intumescens. C. lupulina also resembles Carex retrorsa, which has much smaller perigynia (6 to 10 mm long) and the perigynia towards the base of the spike are angled downward (reflexed), where C. lupulina perigynia are twice the size and all ascending.