C. scirpoidea grows in the mountains in turfy to rocky soils in heath, meadows, and tundra. The soil pH of sites where C. scirpoidea grew ranged from 3.8 to 7.5 with an average pH of 5.6.
This species is observed in the park at elevations from 281 m to 1548 m with an average site elevation of 1005 m. The slope angle for sites where C. scirpoidea grew ranged from flat to 40% with an average of 15%.
Carex scirpoidea is an incompletely circumpolar species with a widespread distribution. In North America, this species ranges from Alaska east to Greenland and south to the northern lower 48 states and western U.S. (except for Arizona and New Mexico). In Alaska, this species occurs in suitable habitat statewide except is absent in the Aleutian chain. In Denali, this species occurs widely and commonly in the mountains on both sides of the Alaska Range.
C. scirpoidea is dioecious (atypical for a sedge). Plants are wind-pollinated. Seeds are disseminated by wind or gravity.
Carex scirpoidea is perennial and flowers in early to mid-summer.
Carex scirpoidea is a dioecious, single-spiked perennial sedge that can be rhizomatous or tufted. This species is common and occurs widely across Denali from lowlands to the alpine in a variety of habitats including woodlands, shrublands, meadows, rocky sites and alpine tundra on both sides of the Alaska Range. Plants are typically found in rocky soils, growing from a thick rhizome. Stems of Carex scirpoidea are sharply triangular, stiffly erect, and 5-35 cm tall. Leaves are 3-4mm wide, scabrous and shorter than the stems. Plants are dioecious, male and female flowers are produced on separate plants. Female spikes are linear in shape and 2-3 cm long; male spikes are shorter and obovate in shape with exserted stamens. Pistillate scales are oblong-ovate in shape, purple-black, ciliate, shorter than the perigynia, with scarious margins. Perigynia are brown or green, beaked, and distinctly hairy, 2.5-3 mm long. Fruits are three-angled achenes with 3 styles. The only other sedge in Denali that is sometimes dioecious with single spikes is the rare Carex gynocrates. That species is smaller, less densely tufted and has narrow leaves, the spikes are shorter, the scales are brown, not black, and the perigynia are not hairy.