Solitary herb growing in small tussocks from a short, branched rhizome. All or nearly all leaves are basal. Culms 3–15 cm, erect, terete, smooth or slightly ridged.

Leaves 2–8 cm, distinctly shorter than culms, thin, 0.5–0.9 mm, filiform and canaliculate, terete, smooth or slightly ribbed, with a 0.5–1.5 cm long, pale brown sheath with no or indistinct auricles at the transition to the blade.

Inflorescence a head of (1)2(3) very short-pedicellate (0.5–0.7 mm) flowers, when 2 or more at distinctly different heights on the culm, usually separated by 1.5–2.5(3) mm, subtended by usually 2 bracts. The lower bract long and protruding up to 20 mm beyond the uppermost flower with a subulate blade. Other bract(s) up to 5–6 mm, ovate or oblong, obtuse or acute, brown in the middle with a broad, hyaline margin and a dark brown mid vein. The entire inflorescence turns blackish with age.

Flowers radially symmetric. Perianth of 6 (3 + 3) tepals. Tepals subequal, 3–4 mm, oblong, obtuse, reddish brown, as long as fruit or slightly shorter. Stamens 6, as long as tepals; anthers small and narrow, 0.4–0.6 mm long. Gynoecium of 3 carpels with 3 stigmas.

Fruit a one-roomed capsule, 3–4(5) mm, truncate to emarginate, with a short, ca. 0.5 mm, cylindrical style, shiny, dark olive brown, blackish in apex, with numerous seeds.

Sexual reproduction by seeds; no vegetative reproduction. Adapted to wind pollination but probably with much self pollination. Seed production is abundant in most years. Seeds germinated to ca. 75 % in an experiment (Alsos et al. 2013).

The seeds have no special adaptation to dispersal and are probably dispersed by water, wind (they are small and light) and birds.

Juncus biglumis has all flowers in a small cluster and can only be mistaken for J. albescens. They differ markedly in position of flowers and shape of fruits: Juncus albescens has its usually three flowers at the same level and an acute capsule; J. biglumis has its usually two flowers at different levels and a truncate to emarginate capsule with the style as an apical, narrow beak in a depression at top of the capsule. The inflorescence of J. biglumis remains black with age, whereas that of J. albescens turns whitish.

Juncus biglumis is found in a broad range of moist and wet site types with little competition: shallow mires and marshes, sediment flats, moist to wet soil polygons on frost patterned ground, shores of brooks, rivers, lakes and sea, and on any other ground with bare, moist substrate. It usually occurs in fine-grained substrates but quite indifferent as to soil reaction (pH), however, rare in the most acidic substrates in Svalbard.

Present in all zones and sections. Present on all major islands and on the majority of the smaller ones (e.g., Bjørnøya, Prins Karls Forland, Kong Karls Land). This is one of the hardiest and most widely distributed of Svalbard plants.

The global range is arctic–alpine circumpolar, very common in all arctic areas and reaching far south in some temperate mountain ranges, in Europe to the C European mountains.

This species seems to be fairly uniform almost everywhere. However, Schönswetter et al. (2006) have shown an intriguing genetic structure and the chromosome number variation is also enigmatic. Three or four chromosome numbers are reported and two major levels are documented from Scandinavia (2n = 60, 120). Scandinavian plants differ from the majority of arctic plants in size (smaller) and perhaps in some details. Initial molecular studies (AFLP; Schönswetter et al. 2006) show a large and strange geographical pattern, by the authors proposed as three clades, one restricted to the Taimyr Peninsula in N Siberia and with a fourfold larger genomic size than the two others. A few chromosome number reports suggest that the two other clades represent, respectively, the 2n = 60 and 2n = 120 plants, and that both co-occur in, e.g., E Greenland and Svalbard. The authors suggest that the three clades represent distinct gene pools and act as cryptic species. Evidently some problems to assess in future studies.

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.

Schönswetter, P., Suda, J., Popp, M., Weiss-Schneeweiss, H. & Brochmann, C. 2006. Circumpolar phylogeography of Juncus biglumis (Juncaceae) inferred from AFLP fingerprints, cpDNA sequences, nuclear DNA contents and chromosome numbers. – Molecular Phylogenetics and Evolution 42: 92–103.