Spikes:

Separate staminate (male) and pistillate (female) spikes, with usually a single staminate spike up to 1½ inches long at the tip of the stem, occasionally with a much smaller one just below it. Below the staminate spikes are 1 or 2 pistillate spikes, oval to short-cylindric in outline, up to ¾ inch long, erect to ascending, and stalkless or nearly so. At the base of the lowest pistillate spike is a slender, leaf-like bract that is about as long as the inflorescence (group of spikes). Pistillate spikes are widely separated from each other.

Leaves and stems:

Leaves are basal and alternate, mostly near the base, .5 to 2.5 mm wide, arching at maturity and usually not longer than flowering stem, though upper stem leaves may over-top the terminal spike. The edges are rolled in (involute), the leaf forming a slender, wiry tube barely 1 mm in diameter. Stem leaf sheaths are U-shaped at the tip, membranous whitish-green, and tightly wrap the stem. The ligule (membrane where the leaf joins the sheath) is thick, membranous, and usually wider than long. Leaves are hairless though may be slightly rough. The remains of leaves typically persist to the next season.

Bases are wrapped in a sheath that is purplish-brown to purplish-red and not fibrous. Stems are mostly erect, weakly 3-sided to nearly round in cross-section, and smooth but slightly rough near the flowering spikes. Stems elongate up to 3 feet at maturity and are mostly longer than the leaves. Plants can create large colonies from 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 mostly ascending and not tightly crowded on the spike. Each pistillate spike contains 3 to 15 fruits.

Pistillate scales are lance to egg-shaped, brown with a narrow translucent band around the edge and a green midrib, pointed at the tip, awnless, toothless but easily tearing and becoming jagged around the edges as they dry, are shorter than and about as broad as the perigynia, and become widely spreading at maturity. Perigynia are 4 to 6.7 mm long, 2.5 to 3.4 mm wide, yellowish-green at maturity, leathery, smooth and shiny, 7 to 15-veined, egg to teardrop shaped tapering to a short, straight beak that has 2 small teeth at the tip. Achenes are 3-sided, 2.8 to 3 mm long, up to 1.5 mm wide, maturing to brown, with a persistent style.

Notes:

Carex oligosperma is a common sedge of bogs, peaty wetlands and shores of acid lakes, and may form large colonies of fine, arching foliage.

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 oligosperma is in the Vesicariae section; some of its common traits are: clump forming or not, rhizomatous, hairless leaves, basal sheaths brown or red-purple, sheaths often splitting into fibers and forming a ladder shape, sheaths with cross partitions between veins (septate-nodulose), 2 to 10 spikes, terminal spike all-staminate, leaf-like bract subtending the lowest pistillate spike, perigynia mostly ascending to spreading, hairless, mostly egg to teardrop shaped, beaked and toothed, at least slightly inflated, achenes 3-sided in cross-section with a persistent style.

Carex oligosperma is distinguished from all other Minnesota sedges by the combination of: wiry leaves rolled in along the edges (involute), (usually) a single terminal staminate spike, 1 or 2 stout pistillate spikes up to ¾ inch long and stalkless or nearly so, pistillate spikes each containing 3 to 15 fruits, perigynia up to 6.7 mm long, inflated, shiny, with conspicuous veins, tapering to a toothed beak, achenes 3-sided with a persistent style. When flowering stems are absent, C. oligosperma can be confused with Carex lasiocarpa, which also creates large colonies of wiry leaves but has basal sheaths that are distinctly ladder-fibrous and strongly red-purple, where C. oligosperma basal sheaths are not fibrous and only slightly red or purple tinged (if at all). Of note is that most references do describe C. oligosperma basal sheaths as red-purple, but we did not observe this in the field.