Solitary graminoid herb growing in dense or loose tussocks with branching outside leaf sheaths (extravaginal). Almost all leaves basal. Culms several per tussock, up to 25 cm, moderately stout and stiff, (0.4)0.5–1.0(1.2) mm broad, spreading or ascending, weakly trigonous with strong ribs, smooth but slightly scabrous beneath the inflorescence, distinctly papillose on and especially between ribs, with 2–3 prophylls (basal leaves without or only with a reduced blade) and 3–4 leaves on the lower part of the culm, increasing in length upwards on culm.

Leaves 3–10 cm long, much shorter than culms, narrow, 0.8–2.0 mm broad, flat or folded, with mid vein sharply raised on the lower surface and impressed on the upper surface, margins minutely serrate in the upper part, moderately papillose, yellowish or greyish green.

The flower in Carex is unisexual (either male or female), without perianth, and supported by a scale (the bract of the single flower). The male flower consists of 3 stamens. The female flower consists of a gynoecium of 2 or 3 fused carpels, with a single style and 2 or 3 stigmas, and with a single seed. The gynoecium is surrounded by a perigynium, a container with a narrow apical opening through which the style and stigmas emerge. The perigynia (and nuts) are either lenticular (when two carpels/stigmas) or trigonous (when three). The inflorescences are spikes, one or more per culm. If two or more spikes, all except for the uppermost are supported by more or less leaf-like bracts. Spikes may be unisexual or bisexual, and bisexual spikes may have the female flowers at base (basigynous) or at top (acrogynous). Flowers are wind pollinated and usually cross pollinated because the male flowers reach anthesis before the female flowers (protandry). Cross pollination predominates among sedges investigated in alpine Norway (Berggren & Haugset unpubl.), either due to the protandry or to genetic incompatibility. Seeds are spread inside their perigynia.

Inflorescence a dense cluster, 1.0–1.5 × 0.5–0.9 cm, of (2)3(4) spikes, all bisexual with female flowers at top (acrogynous). All spikes with scaly bracts 1–3 × 0.8–2.5 mm, very broadly ovate, subacute, brown, with narrow (ca. 0.2 mm) white hyaline margin, and with a darker or paler mid vein that on the lowermost bract sometimes continues as a subulate blade up to 5 mm, with serrate or stiffly hairy margin. Scales of male and female flowers similar, 1.0–1.5 × 0.8–1.2 mm, broadly ovate, obtuse to subacute, brown with a slightly paler mid vein and with a 0.1–0.2 mm broad, white hyaline margin. Perigynia lenticular, with one nearly flat and one convex surface, elliptic, 1.6–2.2 mm, tapering at base into a short foot and at top into a distinct, nearly cylindrical beak 0.2–0.4 mm, truncate at the aperture, with more or less indistinct, darker veins, brown, indistinctly papillose. Stigmas 2.

Fruit a lenticular nut enclosed in the perigynium.

Sexual reproduction by seeds; no vegetative reproduction. Fruits mature regularly. Seeds germinate to 14 % in an experiment (Alsos et al. 2013).

Dispersal of fruits (inside perigynia) is probably quite limited, but we assume downstream by water and perhaps a little by wind and birds.

The two Svalbard species most closely related to C. lachenalii (in section Glareosae) are C. glareosa and C. marina. Carex lachenalii differs from C. glareosa in much stouter culms and broader leaves (more than 0.8 mm), in culms erect or ascending, in less distinct papillae on culms and leaves, and in a more distinct beak 0.2–0.4 mm. It differs from C. marina especially in this beak (C. marina has no beak whatsoever) but also in oblong, brown spikes (C. marina has nearly globular or club-shaped, pale spikes).

Carex lachenalii occurs in dry meadows and grassy slopes, snowbeds, brook ravines and landslide areas. Mostly on moderately well-drained substrates but tolerates occasional submergence. It seems to prefer substrates with a circumneutral to acidic soil reaction (pH), never found on purely calcareous substrates. It also seems to prefer a stable snow cover.

Middle and northern arctic tundra zones, weakly continental to weakly oceanic sections. Carex lachenalii is found on Bjørnøya (rare, only in the north), Prins Karls Forland (one site), and on Spitsbergen where it is rather common on the Tertiary sandstones on the peninsula between Van Mijenfjorden and Isfjorden (Nordenskiöld Land). Elsewhere it is rare and with a discontinuous range but with reports from Sørkapp Land in the south to northern Andrée Land on the north coast. It is conspicuously missing from some areas with extensive calcareous bedrock, e.g., the entire Bünsow Land peninsula in Isfjorden and all of middle and inner Wijdefjorden.

Carex lachenalii is circumpolar, arctic–alpine, and common in all northern regions, in Europe south to the C European mountains. It is also represented in the southern hemisphere by ssp. parkeri (Petrie) Toivonen.

Alsos, I.G., Müller, E. & Eidesen, P.B. 2013. Germinating seeds or bulbils in 87 of 113 tested Arctic species indicate potential for ex situ seed bank storage. – Polar Biology 36: 819–830. Doi 10.1007/s00300-013-1307-7.