Annual herb with a single root and shoots often extensively branched at base and at higher leaf axils. Stems very soft, green, indistinctly angled, with articulate hairs in a stripe on one side, alternating sides at nodes.
Leaves opposite; lower leaves with petioles up to 2 cm, with a stripe of hairs; upper leaves sessile. Blades to 3 × 2 cm, broadly ovate, acute or obtuse, with more or less distinct veins, soft, glabrous or sparsely pubescent.
Inflorescence an apical, dichasial cyme with large bracts similar to stem leaves.
Pedicels up to 15 mm, with a stripe of hairs on one side. Flowers radially symmetric with 5 free sepals and petals. Sepals 4–6 × 2–3 mm, lanceolate, subacute or obtuse, green with an indistinct midvein and a white hyaline margin, dull and with articulate hairs on the backs (dorsally), glabrous and shiny on the inner surface (ventrally). Petals slightly shorter than sepals, 4–5 mm, cleft into two narrow lobes 0.6–0.7 mm broad, translucent white. Stamens 3–5; anthers red. Gynoecium of 3 carpels with 3 styles, with one room.
Capsule opening by 6 valves, with numerous, reddish brown seeds ca. 1 mm in diameter.
Sexual reproduction by seeds; no vegetative reproduction. Self pollinated. Mature seeds have been observed a few times in Svalbard but no reproducing population has been observed (yet).
Seeds are papillose and attach very easily to animals (and people).
The deeply cleft petals are unique in Stellaria among Svalbard plants. Stellaria media differs from the two native species (S. humifusa and S. longipes s. lat.) in, among several other features, being annual and in having broad, petiolate leaves.
Ruderal ground in settlements and by cabins, best developed on ground with some fertilization.
Introduced. Stellaria media has been recorded from 11 separate localities from Bjørnøya north to Virgohamna on Danskeøya (Albert I Land) in N Spitsbergen. Most records have been made from Ny-Ålesund (Oscar II Land), with records from five different years, flowering in three of these and fruiting in two, with fully mature seeds in 1965. It has also been recorded two or more years in the three major settlements of Longyearbyen, Barentsburg (Nordenskiöld Land) and Pyramiden (Dickson Land), also here flowering or fruiting.