Carex haydenii is a species of seasonally saturated soils in open habitats. It appears to be declining in abundance with changes in land use.
Carex haydenii is often confused with C. stricta; it can be identified by the inflated perigynia that are rounded apically, acute scales, and glabrous leaves and sheaths. Only basal sheaths of C. haydenii are ladder-fibrillose. Although often sympatric with C. stricta and C. nigra, no hybrids appear to be formed.
Plants tufted, the slender stems arising laterally, scabrous on the angles, 5-12 dm, usually much overtopping the lvs; lowest lvs reduced to dark reddish-brown, bladeless sheaths; main lvs 2-5 mm wide, inversely W-shaped in x-section, the sheaths glabrous, dorsally green, with a concave, red-dotted mouth; lowest bract lf-like, usually a little shorter than the infl, the others much reduced; staminate spikes 1-3, the terminal one 2-5 cm, the others much smaller; pistillate spikes 2 or 3, sessile or nearly so, erect, separate or somewhat overlapping, 1-5 cm, linear-cylindric, the uppermost with some distal staminate fls; pistillate scales with green midvein and dark brown sides, narrower than the perigynia, long-acuminate and projecting 0.5-1.5 mm beyond them; perigynia 1.5-2.6 mm, olive-green with red dots, biconvex, rounded-obovoid and slightly inflated, rounded above to a minute beak, with 1 or 2 faint nerves on each side; achene lenticular, loosely enveloped in the lower half of the perigynium; 2n=54. Marshes, wet meadows, and wet, open woods; N.B. to Minn. and S.D., s. to N.J., Pa., Ind., and Mo.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.