In North America both forms occur, but without the clear geographic limits offered by A. E. Porsild (1943). Carex nardina and C. hepburnii differ in the following characteristics: perigynia shape: ovate or spindle-shaped versus obovate or broadly elliptic; size: (3–)3.5–5 × 1.4–1.6 mm versus 3–5 × 1.5–2mm; beak formation and size: gradually formed, 0.5 mm, obscure to 0.4 mm; stipe formation and size: distinct, 0.5–1 mm verusus obscure (less than 0.2 mm); and range: Iceland, Svalbard, Norway, North America, Russian Far East versus Sweden and possibly North America. In time, perhaps, a clearer picture of the taxa in North America will emerge.
Carex nardina superficially resembles taxa in Carex sect. Filifoliae and can be confused with Kobresia myosuroides.