A large and difficult family, with 16 genera and 272 species in Michigan. Identification of all but the most distinctive species usually requires mature fruit. The floral structure is simple, with usually bisexual flowers subtended by a single scale (not two, the lemma and palea, as in grasses), and then arranged into spikelets of varied morphology. An exception is Scleria, where flowers are unisexual. In Carex, the flowers are also unisexual, but the female are enclosed in a sac, flask, bottle, or tube-shaped, often flattened, structure called the perigynium, this then subtended by a single bract. See Rothrock (2009) for a fine illustrated treatment of the non-Carex genera, with a focus on Indiana.
1. Achenes enclosed in a closed sac (perigynium) subtended by a scale, the style protruding through the apex; flowers strictly unisexual (sedges with exclusively male flowers should be keyed here).
1. Achenes not enclosed in a closed sac, naked beside the subtending scale; at least some flowers bisexual (except in Scleria).
2. Achenes white, hard (bone-like), ± spherical; flowers all unisexual.
2. Achenes yellow, brown, or black, rarely whitish, not spherical; at least some flowers bisexual.
3. Scales of spikelets 2-ranked; spikelets ± flattened in cross-section and always more than one per inflorescence.
4. Stems usually ± angled, solid; inflorescences terminal; achenes without subtending bristles.
4. Stems round, hollow; inflorescences in the axils of stem leaves; achenes with subtending bristles.
3. Scales of spikelets spirally arranged (or if 2-ranked, the spikelet solitary); spikelets round or several-angled in cross-section, solitary or several to many per inflorescence.
5. Perianth bristles 6, 3 slender and 3 with an expanded, ± spongy, spoon-like portion at the tip.
5. Perianth bristles absent or 1 to many, all slender.
6. Spikelet or cluster of spikelets borne on one side of the stem at the base of a single ± erect to somewhat angled or curved involucral bract that appears to be a continuation of the stem.
7. Stems less than 0.5 mm thick; plants tiny, less than 10 cm tall.
7. Stems thicker than 0.5 mm; plants usually much taller than 10 cm.
6. Spikelet or spikelets terminating the stem or borne both terminally and laterally; if more than one spikelet, the inflorescence with (1–) 2 to several spreading to reflexed, leaf-like involucral bracts.
8. Spikelet solitary and terminal on the stem (very rarely a few smaller accessory spikelets occur at the base of the terminal spikelet in the bladeless genus Eleocharis).
9. Sheaths totally bladeless or at most with an apical tooth up to 1 mm long; achenes usually with an apical tubercle formed by the expanded and persistent base of the style.
9. Upper sheaths with short green blades 0.3–12 cm long; achenes blunt at apex, tubercle absent.
10. Achenes subtended by conspicuous silky, white or tawny, hair-like bristles many times as long as the achenes.
11. Bristles numerous, ca. (12–) 15–50 or more; rhizomes erect, very short.
Eriophorum (in part)
11. Bristles 6; rhizomes horizontal and short-creeping.
10. Achenes subtended by 1–8 bristles less than twice as long as the achenes, or bristles absent.
Trichophorum (in part)
8. Spikelets several to many on the stem, terminal or lateral.
12. Achenes subtended by (12–) 15–50 conspicuous, silky, white or tawny, hair-like bristles many times as long as the achenes.
Eriophorum (in part)
12. Achenes subtended by 1–8 bristles, or bristles absent.
13. Leaves flat or folded; with a definite, ± keeled midrib.
14. Achenes with a conspicuous tubercle formed by the expanded, persistent style base.
Rhynchospora (in part)
14. Achenes blunt at apex, without a tubercle; style base, if expanded, not persistent to maturity.
15. Widest leaves 4–15 mm wide; achenes subtended by 1–8 bristles.
16. Spikelets (10–) 15–36 mm long; achenes 3–5 mm long, including apiculus; anthers ca. (2.3–) 4–5 mm long; culms sharply 3-angled nearly or quite to the base; colonial from rhizomes with large corm-like thickenings.
16. Spikelets 2–10 (–12) mm long, achenes 0.9–1.2 mm long; anthers 0.5–1.3 mm long; culms terete, obtusely 3-angled, or sharply 3-angled only toward summit; cespitose or with rhizomes lacking corm-like enlargements.
15. Widest leaves 0.5–3 mm wide; achenes lacking bristles.
13. Leaves inrolled and wiry; rounded on the back and without a definite midrib.
17. Styles 2-cleft; achenes subtended by slender bristles.
Rhynchospora (in part)
17. Styles 3-cleft; achenes lacking bristles.
18. Achenes 2.2–3.5 mm long; perennials 4–11 dm tall; colonial from elongated rhizomes.
18. Achenes 0.7–0.8 mm long; tufted annuals 0.2–4 dm tall.