Mat-forming graminoid herb growing in very loose, often extensive stands due to horizontal rhizomes with white, slender branches 1.5–2 mm thick, typically with rhizome branch lengths of 5–15 cm between aerial shoots. Aerial shoots ascending or erect from rhizome, without evident prophylls (leaves with reduced or missing blades). Culms 8–16(20) cm, erect, smooth and glabrous, without a sheath of withered leaves at base.

Leaves flat, abruptly narrowed in the apex, with a distinct keel formed by the mid vein on the lower surface, with numerous, indistinct lateral veins, smooth and glabrous, dark green. Basal leaves few (often only 2), up to 7–10(13) cm long, 1.2–1.7 mm broad, usually floating on the water. Culm leaves 2–3, the lowermost with a distinct blade up to 3–5 cm, the upper ones with very reduced blades, 0.3–1.5(2.0) cm or sometimes the uppermost (flag leaf) without any blade, flag leaf blade attached above middle of culm. Ligula 1.5–2.5 mm, acute.

The units of the inflorescence of Poaceae are the spikelets, nearly always numerous in a panicle or spike-like inflorescence. Spikelets are composed of 2 glumes (bracts for the spikelet) and one or more flowers (the term used below) or rather floral units often named ‘florets’ because we do not know what is the exact flower. A flower or floret is composed of a lemma with 1 mid vein (probably the floral bract), a palea with 2 mid veins (either 2 fused bracteoles or perhaps 2 fused perianth leaves), 3 small organs called ‘lodiculae’ and essential in the opening of the flower at anthesis (possibly transformed perianth leaves or transformed stamens), 3 stamens (mostly), and a gynoecium of 2 fused carpels with 2 feathery stigmas and one seed.

Inflorescence a raceme 3–6 cm long with 5–8 spikelets in an open row on one side. Spikelets on very slender peduncles, 1–3 mm, curved so that the spikelets are pendulous. Raceme ca. 1/3 of culm length, with smooth mid axis and peduncles. Spikelets 8–12 × 1.5–2.5 mm, narrow, parallel-sided, with 7–9(10) flowers, the lower flowers bisexual, the upper female (pistillate), and the 1–2 uppermost usually reduced and sterile. Bracts (glumes and lemmas) with rounded backs. Glumes ovate, acute to acuminate, with indistinct vein(s), violet at base and centre, hyaline otherwise, very small, the lower 1–2.5 mm, the upper 2–3.5 mm, often falling off during or after anthesis (caducous). Lemmas 3.5–5 × ca. 1.5 mm, obovate or broadly cuneate with truncate to rounded and fringed apex, dark violet with a broad, bronze yellow to white hyaline upper part, with ca. 7 raised veins darker than the surface, strigose on back. Paleas 3.5–5 mm, as long as lemmas, mostly hyaline, strigose, with two marked, dark violet, winged keels. Each keel carries a long, barbed, blackish violet awn arising from the lower parts of the keel and emerging out of the spikelet (a unique feature among Svalbard grasses). Anthers ca. 2 mm, usually well developed.

Fruit an achene (with one seed).

Sexual reproduction by seeds, at least potentially; extensive local vegetative reproduction by rhizome growth. Wind pollinated. Seeds may ripen in very good seasons but no Svalbard plants have been observed with shed fruits.

Dispersal of fruits (within florets) can possibly be effected by birds; however, the rarity and extreme concentrations of this plant in Svalbard suggest that efficient seed reproduction is very rare. Svalbard is probably a 'sink' for this widespread arctic species. Rhizome fragments may be dispersed by birds to nearby marshes and lakes, most evident in the Reinsdyrflya area.

There is nothing remotely resembling this little beauty of a grass in the Svalbard flora. At some distance, the closest similarity is with some sedge (Carex) with pendulous spikes.

Very wet marshes, growing in a permanently wet moss carpet or in shallow water, often with floating leaves along the shores of small lakes and tarns. Pleuropogon has a very restricted ecological span in its site types. It is probably independent as to soil reaction (pH), but the permafrost melt water of its marshes is never very poor in mineral nutrients.

Pleuropogon sabinei is known from only three localities or locality groups in Svalbard, all on Spitsbergen: the Bohemanflya peninsula in the outer parts of Isfjorden (Oscar II Land), possibly in two sites; Forkdalen west of Wijdefjorden (Andrée Land, here last seen in 1928); and on the south side of Reinsdyrflya peninsula facing Liefdefjorden (Haakon VII Land), in numerous stands. It is very rare in Svalbard and missing from the vast majority of assumed suitable wetlands. Its localities do not follow any zonal or sectional pattern, either. Bohemanflya and Forkdalen are in the middle arctic tundra zone, Reinsdyrflya in the northern arctic tundra zone. Bohemanflya and Reinsdyrflya are in the weakly continental section, Forkdalen in the clearly continental section. The pattern suggests three separate introductions to Svalbard. There is little reason to assume a previous larger range with extinction in the areas between the currently known sites.

This is a widespread arctic wetland species, quite common in the American and Asian Arctic but rare in the regions surrounding the North Atlantic.

Pleuropogon is a very small genus of five species, four of these restricted to the Pacific coast of North America, the northernmost being P. refractus reaching British Columbia (But 1977, 2007). It is fairly obvious that the species P. sabinei has its evolutionary origin in W North America and subsequently has spread in the arctic lowlands westwards and eastwards from Beringia. It is the only species of its tribe Meliceae in Svalbard, possibly distantly related to the sweetgrasses (Glyceria).

Pleuropogon sabinei has the chromosome numbers of 2n = 40 and 42, the latter possibly a hexaploid number with x = 7, the former a strange, possibly a tetraploid number with x = 10. This rare number, and the combination of the two different base numbers, is also known from many species of Glyceria.

But, P.P.H. 1977. Systematics of Pleuropogon R. Br. (Poaceae). – Ph.D. Thesis, Univ. California, Berkeley.

But, P.P.H. 2007. Pleuropogon R. Br. – In: Flora of North America Editorial Committee (eds.), Flora of North America north of Mexico 24. Magnoliophyta: Commelinidae (in part): Poaceae, part 1: 103–109.