Solitary or diffusely mat-forming herb with a very short, simple, subterranean stem (caudex), ending in a single leaf rosette. Each plant has several slender, leafless, above-ground runners (stolons) up to 10–20 cm long, ending in small leaf rosettes. In the majority of populations, the plants have flowering stems up to 7–8 cm (rarely more), with 3–6 stem leaves; in a minority of populations, the plants have almost no stem, flowering directly from the rosette. Numerous short, multicellular, glandular hairs are found on all above-ground plant parts, up to and including the calyx.

Leaves alternate. Basal leaves 0.7–1.2 × 0.3–0.4 cm, oblong or obovate, without a distinct petiole, with several seemingly parallel main veins, margins with stout, white, unicellular glandular hairs. Stem leaves similar but smaller, lanceolate or oblanceolate, and sessile; upper stem leaves densely covered by articulate glandular hairs.

Flowers terminal on stems, single or 2–3.

Flowers radially symmetric with 5 free sepals and petals. Sepals 4–5 × 2.5–3.5 mm, broadly oblong or ovate, obtuse or rounded, densely ciliate along margins with 0.2–0.3 mm unicellular glandular hairs, on the lamina with shorter, articulate glandular hairs, dark green or reddish. Petals 6–10 x 3–5 mm, non-overlapping, oblong, bright yellow with darker veins. Stamens 10, yellow. Ovary semi-inferior, of two carpels with two rooms, split apically.

Fruit a capsule with numerous seeds.

Sexual reproduction by seeds; local vegetative reproduction by the efficient runners, forming small, often clearly demarcated stands by clonal growth. The minute rosettes at the end of runners are also easily detached and may spread the plant further. A clone usually consists of a smaller number of flowering plants and a larger number of rosettes in diverse size and age stages. Flowering is regular in most stands. Flowers are fairly spectacular and are adapted to insect pollination. Fruit-set probably regular in Svalbard but no ripe seeds were found in 2008 (Alsos et al. 2013).

Capsules have apical opening which ensures that the seeds only are dispersed at a minimum wind speed. Seed dispersal is often after the first snow fall, which increases the dispersal distance as the seeds are blown across a smooth surface (Savile 1972). Seeds are also dispersed by water and animals, e.g. geese that selectively feed on seed capsules (Prop et al. 1984).

The three yellow-flowered Saxifragas of Svalbard are rather different. Saxifraga platysepala is distinguished from the two others by its long, above-ground runners (stolons) ending in small rosettes (the "Spider Saxifrage") and by being glandular pubescent. Saxifraga aizoides and S. hirculus are distinguished by the former having ciliate leaves, small and often several flowers per stem, and by sepals appressed or patent in late flower and fruit stages; the latter having entire leaves, large and mostly single flowers, and by sepals deflexed in late flower and fruit stages.

Nearly always found on firm, moist or at least temporarily moist substrates with a sparse vegetation cover. A dense moss or lichen layer prevents rooting of the stolon rosettes, and the species is therefore almost never found in vegetation with a dense cover of cryptogams; neither is it found in dense heath, snowbed or mire vegetation. The substrate is usually fine or mixed fine/coarse (silt, sand, gravel). The plant seems to have a certain preference for basic soils but is also frequent on circumneutral soils.

Frequent in the middle and northern arctic tundra zones and transgressing into the polar desert zone. Present in all sections but rare in the clearly continental section. Present on all major islands in the Spitsbergen group (Spitsbergen, Prins Karls Forland, Edgeøya, Barentsøya, Nordaustlandet) but absent from the areas with the most acidic substrates (e.g., the northwestern and southern parts of Spitsbergen).

The general range is circumpolar in the arctic zones. See Comments for ranges of related species.

Saxifraga platysepala belongs to a large species group – the S. flagellaris group – centered on the Caucasus, C Asia and the Himalayas and surroundings. The arctic plants belong to two species, the only ones extending the range of this species group to the Arctic (Elven et al. 2011): the tetraploid (2n = 32) S. platysepala is circumpolar; the diploid (2n = 16) S. setigera Pursh is restricted to the Beringian regions. Both these two species differ in several morphological features from each other and from S. flagellaris s. str., which is restricted to the Caucasus. However, the name S. flagellaris has very often been applied to the arctic plants.

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.

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

Prop, J., van Erden, M.R. & Drent, R.H. 1984. Reproductive success of Barnacle Goose Branta leucopsis in relation to food exploitation on the breeding grounds, western Spitsbergen. – Norsk Polarinstitutts Skrifter 181: 87–117.

Savile, D.B.O. 1972. Arctic adaptations in plants. – Canada Department of Agriculture Research Branch Monograph 6. 81 pp.