Perennial, short-lived (probably less than 5 years).
Solitary herb with a central root and one main rosette with numerous prostrate, non-rooting vegetative branches and numerous prostrate or ascending flowering stems. Almost no old leaves retained past the growing season. Flowering stems 1–3(5) cm with 1(2) pairs of stem leaves beneath the inflorescence.
Leaves opposite, (3)4–7 mm, linear or very narrowly lanceolate, with hyaline sheaths with auricles, mid vein not prominent, acuminate with a hyaline tip, glabrous. Stem leaves with short, hyaline sheaths connected between the two leaves.
Flowering shoots with single terminal flowers or sometimes with 2–5 or more flowers in a dichasial cyme on extended branches.
Pedicels 5–10(20) mm long, glabrous. Flowers radially symmetric with 4–5 free sepals and petals. Sepals 1.3–2.2 × 1.2–1.4 mm, nearly orbicular or very broadly oblong or ovate, boat-shaped (concave), rounded, obtuse or with a small point, dark green or tinged purplish with a narrow hyaline margin. Petals 1.2–1.8 mm long, as long as or slightly shorter than sepals, obovate or spathulate, entire, white. Stamens 8–10, sometimes fewer. Gynoecium of 4 or 5 carpels with 4 or 5 stigmas.
Fruit a one-roomed capsule that opens apically with 4 or 5 teeth, with numerous seeds.
Sexual reproduction by seeds; no vegetative reproduction. Flowers very small and not adapted to insect pollination; self pollination assumed to prevail. The plant flowers and fruits regularly in Svalbard, sometimes twice in one season, and regularly produces large amounts of mature seeds. Seeds germinate to 97 % in an experiment (Alsos et al. 2013).
No special adaptations to seed dispersal.
Sagina nivalis can be confused with S. caespitosa and with species of Sabulina, all with small, white flowers. Sagina has 4–5 styles and capsule teeth, whereas Sabulina has 3 styles and capsule teeth. This is not always easily observed but the shapes of buds, sepals, and capsules are usually different. Whereas Sagina has nearly globular buds, short and very concave sepals, and ovoid capsules, Sabulina has elongated buds, lanceolate and more flat sepals, and usually more elongated capsules.
Sagina nivalis and S. caespitosa can be distinguished by the following characters: Sagina nivalis grows as one central rosette with lateral, non-rooting branches, has petals equal to or shorter than sepals, procumbent to (rarely) ascending flowering stems, and no dense layer of old, retained leaves; S. caespitosa grows in dense tussocks or cushions of several rosettes from a branching rhizome, has petals longer than sepals, erect flowering stems, and a dense layer of withered leaves retained on its basal parts.
All kinds of bare patches in early snowbeds, open heaths, on periodically inundated river terraces and gravel bars, shores of lakes and ponds, and seashores. Many sites are moist but the species tolerates some drought, especially in the fruiting season. Substrates are mainly loam, sand, or fine gravel. Probably indifferent as to soil reaction (pH).
Common in all zones and sections. Present and probably common on all major islands, including Bjørnøya, and also present on several smaller islands.
The global distribution is circumpolar and arctic–alpine, common in the majority of arctic regions and in Europe reaching south to S Norway.
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.