Extensively mat-forming graminoid herb with a horizontal, branched rhizome system. Rhizome branch lengths 1–5 cm. Vegetative shoots 3–5(20) cm tall, ascending from rhizome. Culms (1)2–4(13) cm, smooth, usually shorter than the vegetative shoots.

Leaves 3–5(8) cm long, 2–3 mm broad at the base, ending in a pale brown, narrow tip, involute (V-shaped in cross section), densely papillose on both surfaces, margins scabrous, olive green to rusty red.

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 2–4 spikes, 1 terminal male, 1–3 lateral female or mainly female. Bracts with very short, reddish-brown sheaths not enlarged into distinct auricles, with extended blades. Bract of the lowermost spike exceeding the spike; bract of the next spike shorter or longer than the spike. Male spikes (8)9–10(11) × 2–3 mm on 5–10 mm long peduncles, erect. Female spikes 7–9(10) × 2–3.5(4) mm on 0–5 mm long peduncles, with (2)4–10(12) flowers, erect, rarely a few male flowers are found at the top of the female spikes (i.e., basigynous). Scales obtuse to acute, rusty brown with a narrow green to pale brown mid vein. Scales of male spikes more elongated than of female spikes, usually acute, and with a hyaline margin. Perigynia 2.2–3.0 × 1.2–1.8 mm, lenticular, with ca. 0.1 mm indistinct beak, papillose, smooth at margin, pale green with reddish spots but turning rusty red or brown during maturing. Stigmas 2.

Lenticular nut within the perigynium.

Sexual reproduction by seeds; efficient local vegetative reproduction by rhizome. Local vegetative growth by runners is very efficient and many extensive stands may consist of only one clone. We assume that cross pollination predominates in C. subspathacea, as also assumed by Brochmann & Steen (1999). However, recruitment of new plants from seeds in the dense mats is probably a very rare event compared with vegetative growth. Seeds germinate to 50 % in an experiment (Alsos et al. 2013), an unusually high level of germination in Carex.

Fruits (within perigynia) have no special adaptation to dispersal but are generally easily dispersed by wind, water and birds, and for seashore species especially by ocean currents. There may also be some short-range dispersal of detached shoots that easily root.

The section Phacocystis includes three species in Svalbard: Carex bigelowii, C. concolor (C. aquatilis ssp. stans) and C. subspathacea. Carex subspathacea differs from the two others in very short culms and leafy shoots that are clearly longer than the culms, in fewer flowers in each spike, and in the reddish brown scales and perigynia (at least in mature stage).

Carex subspathacea is one of the most widespread of all arctic salt marsh plants, and this is also one of its two main site types in Svalbard. It is a constituent of almost every salt marsh visited (Brattbakk 1979). If there is any zonation on the salt marsh, the C. subspathacea community occupies the belt above the Puccinellia phryganodes community and below a Carex ursina and/or C. glareosa community.

The other main site type is valley mires and wetlands, on often water-soaked fine-grained sediments together with Dupontia and sometimes Arctophila (and, of course, mosses). Such inland occurrences of C. subspathacea are unknown from the more southern parts of its range, in the boreal and southern arctic zones, where it is an obligate salt marsh plant. It is also a characteristic feature that the morphology of the species changes when it moves into the mires: it becomes much more tall-grown. Svalbard C. subspathacea may occasionally be up to 10 times taller than the maximum size of plants from Scandinavia. Even if it occurs inland in many valleys, these inland occurrences are always in the vicinity of some coastal occurrences. Its presence in the valleys may have two explanations: either an inland migration from salt marshes (perhaps with the help of birds), or they may be remnants from earlier postglacial times when the sea level was higher.

Salt marshes are rare in the Arctic due to the scouring of ice floes in winter (if the sea is not fully frozen), spring and throughout much of the summer. This means that estuaries and sheltered lagoons behind gravel bars are the main sites of this species on the seashores.

The middle and northern arctic tundra zones and the transitional to clearly continental sections. Carex subspathacea occurs in all parts of Svalbard where there are salt marshes: Bjørnøya, Spitsbergen from Sørkapp Land to the northernmost parts, W Edgeøya, and N Nordaustlandet.

The general distribution is circumpolar in the arctic and boreal zones, in Europe southwards to C Norway.

Carex subspathacea belongs to the taxonomically quite complicated section Phacocystis where all species seem to be able to hybridize wherever and whenever they meet. Hybrids are often partially fertile, and several assumedly hybridogeneous species (stabilized offspring from initial hybrids) have been described (see, e.g., Cayouette & Morisset 1985; Volkova et al. 2008; Korpelainen et al. 2010). The section Phacocystis includes two species groups differing in both morphology and ecology: one group in inland heaths, mires, swamps and shores with the Svalbard species C. bigelowii and C. concolor, and another group on seashore marshes with the single Svalbard species C. subspathacea. At least five hybridogeneous species involving seashore species as parents have been proposed in the northern regions (C. halophila F.Nyl., C. ramenskii Kom., C. recta Boott, C. salina Wahlenb., C. vacillans Drejer), the majority of these in the North Atlantic regions, and one or two are already convincingly proved to be functional species with a hybrid background (see references above). Three others have recently been studied, two involving C. subspathacea as one of the parental species (Nowak et al. 2020). Several of the hybrid hypotheses include C. aquatilis s. lat. as a mire and wetland parent and one of the seashore species as another parent. These hybrids have sometimes been mistaken for C. bigelowii. Torstein Engelskjøn and others have re-identified some of these as tall-grown mire C. subspathacea and a few from Forkdalen by Widjefjorden as hybrids between C. subspathacea and C. concolor (with its only known stand in Svalbard in Forkdalen).

The morphological differences between the very short-grown salt marsh plants and the tall-grown valley wetland plants of C. subspathacea are intriguing. Canadian botanists (e.g., S.G. Aiken and collaborators in Ottawa) have suggested that the reason for those species being so short-grown in salt marshes is millennia of grazing by geese. This explanation leaves something to explain as the valley marshes are just as much grazing ground for the geese as are the salt marshes.

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.

Brochmann, C. & Steen, S.W. 1999. Sex and genes in the flora of Svalbard - implications for conservation biology and climate change. – Det Norske Videnskaps-Akademi. I. Matematisk Naturvitenskapelig Klasse, Skrifter, Ny serie 38: 33–72. Doi

Cayouette, J. & Morisset, P. 1985. Chromosome studies on natural hybrids between maritime species of Carex (sections Phacocystis & Cryptocarpae) in northeastern North America, and their taxonomic implications. – Canadian Journal of Botany 63: 1957–1982.

Korpelainen, H., Virtanen, V., Kostamo, K. & Väre, H. 2010. Hybridization and introgression in Carex aquatilis and C. paleacea. – Plant Systematics & Evolution 287: 141–151.

Nowak, M.D., Pedersen, A.T.M., Brysting, A.K., Schrøder-Bielsen, A., Elven, R. & Bjorå, C.S. 2020. Testing hypotheses of hybrid origin for two seashore species of Carex section Phacocystis (Cyperaceae). – Botanical Journal of the Linnean Society 194: 100–117.

Volkova, P.A., Shipunov, A.B., Elven, R. & Brochmann, C. 2008. Seashore sedges of the Russian Kola Peninsula: How many species? – Flora 203: 523–533.