Sedges differ from grasses by having a 3-angled stem and structurally different flowers where the female flowers are enclosed in a sac like structure called the perigynium, which is subtended by a single scale. Foxtail Sedge is a perennial clump-forming sedge, growing to 30+ inches high, but on thin (to 4 mm), weak, unbranched stems (culms) with somewhat rough surfaces, with convex sides forming wings on the angles.

The leaf sheaths usually all have blades, the fronts smooth, whitish to green, red spotted; with the apex convex, translucent and colorless; thin, fragile, and are indistinctly veined and tear easily. Ligules of leaf blades are rounded, 5 mm long with up to .5 mm free. Basal sheaths of the previous year are usually persistent as long fibers.

The leaf blades are not more than 7 mm wide in this species and to 60 cm (24 inches) long, but often exceed the stem length, V to M shape when young, thin and soft, without hair, green in color.

The inflorescence is a dense elongated cylindric spike, up to 2 to 4 cm long and 1.5 cm wide, with as many as 8 to 12 branches, the lower spikes having a small separation from each other of only .5 mm whereas there are indistinguishable gaps between most of the upper branches. The spikes are androgynous, that is with staminate florets at the top and the pistillate florets below. The perigynia are spreading to ascending. (The bladder-like sacs that enclose the female flower and later the fruit are called perigynia, singular - perigynium). The bracts that form at the base of the lower spikes are thin and bristle-like, but visually apparent. These are usually without sheaths.

Each perigynium is veinless on the front side, weakly 3 to 5 veined, or entirely veinless, on the back side. The body is pale brown initially with coppery tones toward the tip. The body is up to 4 mm long by up to 1.7 mm wide and tapers to a beak that is up to 2 mm long (less than half the body length) and has minute teeth. The overall shape is lanceoloid, slightly inflated (rounded), flattened on one side, smooth surfaced, with a spongy base that is not distended, but forms a very short (to 15 mm) stalk (or stipe). The scales of the pistillate perigynia are smooth, membranous margins and a coppery color, oval-triangluar in shape, tapering to a narrow 2 mm awn. These are much shorter than the perigynia. There are two stigmas per pistillate floret.

Seed: Mature fruit is a brown flattened circular achene with a smooth surface, with a persistent style base. Each is about 1.5 mm long by 1.3 mm wide. Florets are wind pollinated.

Habitat: Foxtail Sedge grows from a short rhizomatous root system forming clumps in wet to mesic conditions such as in wet and open thickets, wet meadows, edges of riparian areas and wet woods and disturbed areas. Plants prefer full sun but will grow in partial sun to shade.

Names: The genus name, Carex, is from the Latin, being the old name for Sedges. The species, alopecoidea, is derived from the Greek alopekouros, meaning 'grass like a fox's tail', referring to the crowded spike clusters of the inflorescence. The author name for the plant classification - ‘Tuck.’ refers to Edward Tuckerman (1817-1886), American botanist and professor of Oriental History and then Botany at Amherst College. Best known for his study and publications on the North American Lichens, he was the first to publish (in 1843) the first serious systematic study of the genus Carex - Enumeratio Methodica Caricum Quarundam. Asa Gray had high praise for his work.

The various common names using the words 'fox sedge', which have also been applied to several other species, refers to the resemblance of the inflorescence to a foxes tail.

Comparisons: Foxtail Sedge is a member of the sedges in the Section Vulpinae, and has the distinguishing characteristics of fairly wide V-shaped leaf blades, clump-forming short rhizomatous roots, stem bases usually black or brown, raceme type inflorescences with multiple condensed spikes, spikes usually androgynous with lower spikes sometimes pistillate only, 2 stigmas per floret.

The most similar sedges like this, found in Minnesota, are C. stipata, the Awl-fruit Sedge, and C. conjuncta, the Soft Fox Sedge or Jointed Sedge, where the sheath fronts are rugose, not smooth, the achenes are ovate not circular, and the perigynia are prominently veined on the back. C. conjuncta, however, is found only in Dodge and Rice counties. Another similar sedge is C. crus-corvi, Raven's-foot Sedge, where the sheath fronts are smooth, the perigynia larger 6 to 8 mm - and they are prominently distended at the base. This sedge is also scarce - known only in Goodhue and Wabasha Counties. Another closely-resembling species, but not in this sedge section, is C. vulpinoidea, Brown Fox Sedge. This has wide distribution in Minnesota. The stems are not as tall but stiff and not winged, the sheath fronts are rugose and spotted red-brown or pale brown, the leaves are narrow but longer than the flowering stem, the inflorescence has the lower spikes distinctly separated and subtended by needle-like long bracts. The perigynia, which are much smaller, take on a brown color as they are green to pale brown with pale brown scales and they have much shorter beaks.