Mat-forming graminoid herb with an extensive branched rhizome system. Rhizome segments with brown scaly leaves (prophylls) with pale veins raised on the surface, typical segment lengths of 1–5 cm between aerial shoots. Aerial shoots ascending from rhizome at short, regular intervals (see illustration in Elven et al. 2022: 274). Vegetative shoots very short, just a bunch of leaves at ground level. Reproductive shoots with culms, 0.5–10 cm long or more (and both extremes are common), slender or rather stout (0.6–1.3 mm broad), ascending and often curved (cf. the old name 'incurva'), with several, often brown prophylls (reduced basal leaves without or sometimes with very short blades) and with 4–8(10) ordinary, short leaves at the base of the culm. Culms indistinctly trigonous with low, smooth ribs, papillose between ribs.

Leaves folded or loosely involute (canaliculate), (1)2–9(12) cm long, 0.8–2 mm broad, usually curved or twisted, mid vein only slightly raised on lower surface and slightly impressed on upper surface, margins serrate, lower surface with low, indistinct papillae, upper surface with distinct papillae, both surfaces more or less bluish 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 of 3–5 spikes usually so densely clustered that they appear as a single globular spike, 0.5–1.2 × 0.5–1.2 cm, all spikes bisexual with female flowers at the base (basigynous). All bracts very short and scale-like, 1.5–3.0 × 1.3–2.0 mm, broadly ovate to oblong or suborbicular, obtuse or subacute, olive brown with paler brown mid vein and a very narrow, white hyaline margin. Scales of male and female flowers similar, 2.2–3.2 × 1.7–2.5 mm, broadly ovate or oblong, obtuse, dark brown with a broad (up to 0.5 mm) white hyaline margin. Perigynia (3.7)4.0–4.5 × 1.5–2.2 mm, with a lanceolate to narrowly ovate outline, broadly lens-shaped (lenticular with one nearly flat and one more convex side), with sharply angled margins, abruptly tapering to a short foot at the base, gradually narrowing into a ca. 1 mm long beak with truncate aperture, glossy and smooth but scabrous on the margins of the beak, with indistinct veins, golden brown, not papillose. Stamens 3. Stigmas 2.

Fruit a lenticular nut enclosed in the perigynium.

Sexual reproduction by seeds; efficient local vegetative reproduction by rhizome. The rhizome produces extensive, loose mats that may be subjected to disruption by ground disturbance (e.g., frost movements). Fruits mature regularly in Svalbard, but seeds germinated only to 2.7 % in an experiment (Alsos et al. 2013).

Fruits (inside perigynia) are easily dispersed by water (sea currents and downstream in rivers and broods) and by birds.

There is no other Carex species resembling C. maritima much in Svalbard. The faintly similar C. ursina is a densely tussocky plant with only one spike with blackish scales and pale green perigynia; the even more faintly similar C. glareosa and C. lachenalii are also tussocky but both have inflorescences with several, well separated spikes, in addition, they are acrogynous (female flowers at top of spikes). The hybrid or hybrid species C. lidii (assumedly C. maritima × parallela) is most similar to C. parallela in vegetative features (filiform leaves, dense swards) and also in inflorescence (one long, narrow apical spike and 1–3 smaller lateral ones, not at all a globular inflorescence).

Upper parts of seashores (upper geolittoral), sandy and gravelly plains with open vegetation cover, shallow mires, on well-drained to moist substrates. Most occurrences are on basic, often calcareous substrates.

In the middle and northern arctic tundra zones, bordering on the polar desert zone; in the transitional to clearly continental sections. In Svalbard, Carex maritima is restricted to N Spitsbergen and to Nordaustlandet. There is a single find near the mouth of Van Keulenfjorden (Nathorst Land); otherwise, all occurrences are from Isfjorden and Kongsfjorden northwards, on Nordaustlandet east to Louise Richardfjellet at Duvefjorden (Prins Oscar Land). The species is conspicuously absent from areas with less calcareous substrates, e.g., from the entire Adventdalen, Longyearbyen, and Colesbukta areas with many otherwise demanding plants and among the most intensively explored parts of Svalbard (i.e., the species has not been overlooked there).

Carex maritima is circumpolar in the arctic and boreal zones and quite common in most regions, mostly on or close to seashores but also with inland occurrences, especially in the Arctic and in the mountains in the boreal zone. The southern limit in Europe is in Iceland and S Scandinavia. It is bipolar, with a second range in southernmost South America (Moore & Chater 1971).

Attempts have been made to distinguish an arctic C. setina. Egorova (1965) accepted C. maritima ssp. setina; however, in her subsequent monographic study of Russian Carex (Egorova 1999) she accepted only C. maritima for Russia but discussed the variation. Reznicek (2002) recognized three species for North America: C. maritima and the two Cordilleran species C. incurviformis Mack. and C. perglobosa Mack. but not C. setina. Our inspection and comparison of materials from Svalbard and from mainland Fennoscandia (including the type specimen of C. maritima from Steigen, Nordland province) does not support a division into different taxa. The plants from the Arctic are smaller in all parts, but we have found no substantial difference. The variation is large inboth groups and extensively overlapping in all studied features.

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.

Egorova, T.V. 1965. Klassifikatsiya osok podroda Vignea Flori SSSR. – Novosti Sistematiki Vysshykh Rastenii 1965: 57–83.

Egorova, T.V. 1999. The sedges (Carex L.) of Russia and adjacent states. – St.-Petersburg State Chemical–Pharmaceutical Academy, St.-Petersburg, and Missouri Bot. Gard. Press, St. Louis.

Elven, R., Bjorå, C.S., Fremstad, E., Hegre, H. & Solstad, H. 2022. Norsk flora. 8. Ed. – Det Norske Samlaget, Oslo.

Moore, D.M. & Chater, A.O. 1971. Studies on bipolar disjunct species. I. Carex. – Botaniska Notiser 124: 317–334.

Reznicek, A.A. 2002. Carex Linnaeus sect. Foetidae (Tuckerman ex L. H. Bailey) Kükenthal. – In: Flora of North America Editorial Committee (eds.), Flora of North America north of Mexico. 23. Magnoliophyta: Commelinidae (in part): Cyperaceae: 309–311.