Mat-forming herb with an extensively branched, horizontal system of rhizomes, with aerial shoots in dense rows along the long rhizome branches, often with less than 1 cm between shoots, and typically with 4–5 culms of current year, numerous dead culms from previous years, and 4–5 emerging shoots in preparation for next year's culms. Culms 15–20 cm, stout (1.3–2.0 mm broad), erect, terete, very finely ribbed, at base with 3–4 reddish brown prophylls (sheaths without blades), obtuse or subacute. Without leaves; culms with chlorophyll and assimilating.

See above.

Inflorescence a dense cyme with (2)3(4) flowers, apparently lateral on the culm but in reality apical with a 2–3 cm long, subulate bract in continuation of the culm. Other bracts much shorter (less than 1 cm), mostly ovate, obtuse and hyaline brown.

Flowers radially symmetric. Perianth of 6 (3 + 3) tepals. Tepals 4–6 mm, ca. 1–2 mm longer than the fruit, narrowly lanceolate, acuminate, with a broad, pale reddish brown mid vein, dark reddish brown besides the mid vein, and with a narrow, white to pale reddish brown hyaline margin. Inner tepals slightly shorter that outer ones and with a broader hyaline margin. Stamens 6, shorter than tepals; anthers narrow, 0.6–0.8 mm long. Gynoecium of 3 carpels with 3 stigmas.

Fruit a one-roomed capsule, 3.5–4.0 × 1.6–2.0 mm, oblong, obtuse, with insignificant style, shiny, dark reddish brown to blackish, with numerous seeds.

Sexual reproduction by seeds; efficient local vegetative reproduction by extensive growth and fragmentation of rhizome. Wind pollinated. Seeds are produced in Svalbard populations (emptied fruits observed).

Seed dispersal mainly by wind (very light seeds) and water, but the very restricted range of the species suggests that seed dispersal is inefficient in Svalbard. Local downstream dispersal along rivers by rhizome fragments is probable (I.G. Alsos & R. Elven observ. 2009).

Juncus arcticus may resemble a very coarse J. biglumis or J. castaneus, but these are both solitary herbs with small individuals, whereas J. arcticus has an extensive horizontal rhizome with numerous aerial shoots and forms large, open stands. The absence of leaves (except for prophylls) is a distinctive character of the group to which J. arcticus belong.

Only known from the banks and gravel bars of rivers, on frequently inundated fine-grained substrates with a basic soil reaction. The environment is always without closed vegetation cover and very little competition.

Probably thermophilous. In the middle arctic tundra zone and the weakly continental section. Only known from Spitsbergen from river banks in Nathorstdalen, Hugindalen and Kulmdalen, three valleys with a single estuary to Dicksonfjorden, which in its turn is a tributary fjord on the north side of Isfjorden (Dickson Land). The entire range is within 5 × 5 km. The plant was first noted in the 1920s but not confirmed again before 2009, when it was observed with large stands along the lower parts of the river in Nathorstdalen.

The species is arctic and boreal circumpolar in several races. Subspecies arcticus is known from NE Canada, Greenland, Iceland (with some complications vs. the closely related J. balticus Willd., see Hylander 1953a, 1953b), Svalbard, Fennoscandia, and N Russia eastwards to just across the Urals. In most arctic regions it is a very common and hardy plant, Svalbard being a striking exception. See Comments.

The Juncus arcticus group (including the mainly Atlantic J. balticus Willd. and the Pacific J. haenkei E.Mey.) is an uncommonly complicated group about which Brooks & Clemants (2000) wrote: "a wide-ranging and obviously polymorphic complex that has not read the literature. It is abundantly clear that the systematics of the group will not be solved on the basis of morphology alone and that resolution of the problem is ripe for molecular investigations". This does not concern the Svalbard plant very much as it corresponds closely with J. arcticus as described and with type from N Scandinavia, i.e., ssp. arcticus. It is replaced in the broadly Beringian regions by ssp. alaskanus Hultén, mainly distinguished by a much longer bract and a looser inflorescence with much more numerous flowers. For an extended discussion, see Elven et al. (2011).

The very restricted distribution in Svalbard is enigmatic. This is a hardy plant, common throughout the Arctic. It is highly improbable that it is a relic in Svalbard, i.e., that it previously had a larger range. If so, it is so hardy and adaptable that it should have survived in very many sites (the entire middle arctic tundra zone and parts of the northern arctic one is within its assumed potential range). It is much more probable that the occurrences in Nathorstdalen, Hugindalen and Kulmdalen are due to a single successful dispersal event followed by a subsequent local spread.

Brooks, R.E. & Clemants, S.E. 2000. Juncus Linnaeus. – In: Flora of North America Editorial Committee (eds.), Flora of North America north of Mexico. 22. Magnoliophyta: Alismatidae, Arecidae, Commelinidae (in part), and Zingiberidae: 211–255.

Elven, R., Murray, D.F., Razzhivin, V. & Yurtsev, B.A. (eds.) 2011. Annotated Checklist of the Panarctic Flora (PAF) Vascular plants. http://panarcticflora.org/

Hylander, N. 1953a. Taxa et nomina nova in opere meo: Nordisk kärlväxtflora I (1953) inclusa. – Botaniska Notiser 106: 352–359.

Hylander, N. 1953b. Nordisk Kärlväxtflora, I. – Almqvist & Wiksell, Stockholm.