Flower:

Separate staminate (male) and pistillate (female) spikes, with a single staminate spike up to 10 mm (3/8 inch) long at the tip of the stem. Just below the staminate spike are 1 or 2 pistillate spikes (occasionally none or 3), up to 7 mm (~¼ inch) long, stalkless or nearly so. There are also 1 to 3, short-stalked, all-pistillate spikes at the base of the stem.

At the base of lowest non-basal pistillate spike is a slender, bristle-like bract that does not overtop the terminal spike. Bracts of other pistillate spikes are shorter or obscure.

Leaves and stems:

Leaves are basal and alternate, all near the base, .5 to 2.5 mm wide, erect to spreading, mostly longer than the flowering stem, with some old leaves persisting to the next season. The ligule (membrane where the leaf joins the sheath) is wider than long. Bases are wrapped in a reddish brown to purple sheath that becomes fibrous. Stems are erect to spreading, slender, 3-sided, rough along the angles. Stems may elongate up to 6 inches at maturity and remain shorter than the leaves. Plants usually form dense clumps from short, stout rhizomes.

Fruit:

Fruit develops in mid to late spring, the pistillate spikes forming clusters of seeds (achenes), each wrapped in a casing (perigynium), subtended by a scale. Each pistillate spike contains 4 to 12 fruits, the perigynia erect to ascending, overlapping and crowded on the spike.

Pistillate scales are 2.8 to 3.9 mm long, egg-shaped to nearly triangular, pale to dark reddish-brown usually with narrow whitish edging, tapering to a pointed tip, lacking awns, and are about as long as the perigynia. Perigynia are 2.2 to 3.2 mm long, 1 to 1.4 mm wide, yellowish green to light brown at maturity, 2-ribbed, moderately hairy on the upper half, the body round to elliptic in outline and round in cross-section, the base initially spongy but shriveling to a stalk-like structure (stipe), and abruptly tapering to a straight or slightly bent green beak .4 to 1 mm long that has 2 teeth at the tip. Achenes are 1.4 to 1.7 mm long, 1 to 1.3 mm wide, oval to urn-shaped in outline, weakly 3-sided in cross-section, and mature to brown.

Notes:

Carex umbellata is an occasional to frequent sedge primarily found in open, sandy or rocky soils and Jack pine stands and reaches the western edge of its US 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 umbellata is in the Acrocystis section (formerly Montanae); some of its common traits are: mostly dry habitat, hairless leaves, basal sheaths typically fibrous, small spikes often tightly clustered, terminal spike staminate, perigynia typically hairy, perigynia with 2 small teeth at the tip of the beak, achenes 3-sided to round in cross-section.

Carex umbellata is distinguished by the combination of: clump-forming, red basal sheaths that become fibrous, single terminal staminate spike, pistillate spike at the plant base, the lowest non-basal pistillate spike with a bristle-like bract that does not overtop the terminal spike, pistillate scales as long as the perigynia, hairy perigynia up to 3.2 mm long with a round to elliptic body and straight to slightly bent beak usually less than 1 mm long.

Carex umbellata is one of a group of 4 members of Acrocystis that have pistillate spikes at the base, the others are Carex rossii, Carex deflexa, and Carex tonsa. Of these 4, C. tonsa and C. umbellata both have slender, bristle-like bracts at the base of the lowest non-basal spike that do not overtop the terminal spike plus pistillate scales about as long as the perigynia, where the other two have leaf-like bracts that overtop the terminal spike and scales shorter than the perigynia. C. tonsa perigynia are larger than C. umbellata, 3.1 to 4.7 mm long with a longer beak 1 to 2 mm long.

Carex abdita is considered a synonym, though some references treat it as a separate species with characteristics similar to those described above and C. umbellata having larger perigynia and beaks, more like C. tonsa var. rugosperma. Of note is the C. tonsa county distribution map, showing it present in a number of counties but with no records in the Bell Herbarium—we have no explanation for that, except perhaps some specimens labeled as C. umbellata might actually be C. tonsa, or perhaps the missing C. tonsa records are all in another herbarium. The C. tonsa-umbellata complex can be confusing and is apparently under review. Maybe a revision will make everything more clear.