Spikes:

Separate staminate (male) and pistillate (female) spikes, with 3 to 5 staminate spikes crowded together at the tip of the stem, the terminal spike up to 2 inches long. Below the staminate spikes are 2 to 4 pistillate spikes, which are more widely spaced but typically held well above the leaves. Pistillate spikes are cylindric, up to 3 inches long, the uppermost stalkless or nearly so, the lower on short, erect stalks. Sometimes staminate flowers are at the tip of the spike (androgynous). At the base of each pistillate spike is a leaf-like bract, at least some of which over-top the terminal spikes.

Leaves and stems:

Leaves are basal and alternate, 8 to 20 mm wide, blue-green to light green, often longer than the flowering stem. Stem leaf sheaths are U to V-shaped, the front whitish to brown, splitting into fibers with age. The ligule (membrane where the leaf joins the sheath) is much longer than wide. Leaves are hairless though rough along the edges, M-shaped in cross-section when young.

Bases are wrapped in a sheath that is reddish to purple and fibrous, the fibers connected and forming a ladder pattern (ladder-fibrillose). Stems are stout, erect to ascending, 3-sided in cross-section, and mostly smooth but may be rough on the upper stem. Not all plants produce flowering stems and vegetative stands are common, forming colonies from long, creeping 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 ascending to spreading and mostly crowded. Each pistillate spike contains up to 150 fruits.

Pistillate scales are lance to egg-shaped, light brown to dark purplish brown with whitish edging and a green midrib, either tapering to a pointed tip or the midrib extending to a short awn, and are shorter than the perigynia. Perigynia are 4.7 to 7.8 mm long, 1.6 to 2.5 mm wide, hairless, green to olive at maturity, 14 to 28-veined, narrowly oval-elliptic with a long, straight taper to an obscure beak that has 2 small teeth at the tip. Achenes are 2 to 2.5 mm long, smooth, 3-sided with a persistent style, and mature to brown.

Notes:

Carex lacustris is a common sedge of lake shores, drainage ditches, woodland ponds, and open wetlands, often forming large stands.

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 lacustris is in the Paludosae section; some of its common traits are: rhizomatous and forming colonies, leaves usually hairless, leaf sheaths often splitting into thread-like fibers and forming a ladder shape, leaves and sheaths with cross partitions between veins (septate-nodulose), basal sheaths red to purple and usually fibrous, terminal spike staminate, leaf-like bract subtending the lowest pistillate spike, perigynia usually hairy, beaked and toothed, achenes 3-sided in cross-section.

C. lacustris is distinguished from other members of Paludosae by the combination of: strongly red and fibrous at the base, ligules longer than wide, sheaths fibrous, periginia with a straight taper to a minute beak with 2 small teeth at the tip. The shape of the perigynia is fairly unique and distinguishes it from other members of Paludosae, Vesicariae and Carex sections, which have more abruptly tapered or longer beaks, and/or longer teeth on the beak. Of note is that the common name cited at a number of online references is “hairy sedge”, which speaks to the usefulness of common names since there isn't a hair to be seen on this species.