- Name also: Arctic Mouse-ear Chickweed
- Latin synonym: Cerastium arcticum
- Family: Pink Family – Caryophyllaceae
- Growing form: Perennial herb. Densely tufted.
- Height: 5–10 cm (2–4 in.). Stem ascending, long-haired.
- Flower: Corolla regular (actinomorphic), white, approx. 1–2 cm (0.4–0.8 in.) wide; petals five, 8–13 mm (0.32-0.52 in.) long, 1.5–2 times as long as sepals, shallowly 2-lobed. Sepals 5, elliptic, sparsely glandular-haired, with narrowly membranous margins; calyx base round. Stamens usually 10. Gynoecium syncarpous, with 5 styles. Flowers usually 1–2, usually poorly developed; membranous margins on subtending bracts max. 0.2 mm (0.08 in.) wide.
- Leaves: Opposite, stalkless. Blade widely elliptic–lanceolate, with entire margin, sparsely long-haired–glabrous, with ciliate margins.
- Fruit: Cylindrical, virtually straight, 9–16 mm (0.35-0.65 in.) long capsule splitting into 10 lobes.
- Habitat: Snow-bed sites on fells, melt-water meadows, precipices, sandbanks.
- Flowering time: July–August.
- Endangerment: Near threatened, protected in all of Finland.
Arctic mouse-ear has been found flowering as far north as firm land exists. In Finland, arctic mouse-ear grows rarely on the north-west Enontekiö fells and is only common to any extent among the extreme conditions on Finland’s highest peak, Halti; compact stands also grow elsewhere, such as on Saana Fell. As can be easily deduced from the species’ habitat, it favours very cold climates. If one wants to find arctic mouse-ear one has to climb to the highest reaches of fell tundra and find damp gravels on the edges of snow-bed sites or beside meltwater streams. It demands a bit of effort to find the species because the small number of stands are usually scant, and the small plant can easily go unnoticed.
On the fell tundra and slightly below the tree line there grows a whole group of white-flowered mouse-ears and chickweeds (Stellaria). The flowers of both genera have five petals, but chickweeds’ are so deeply lobed that they appear to have ten. Of its close relatives, arctic mouse-ear is most easily confused with another large-flowered fell mouse-ear, alpine mouse-ear (C. alpinum), of whose three subspecies it most closely resembles ssp. lanatum. A definite classification can be made based on the subtle differences between their sepals and subtending bracts, even though a loupe is most definitely required. Arctic mouse-ear is usually first recognised by its tightly tufted way of growing and its dense leaves. Fell mouse-ears also cross with each other; arctic mouse-ear at least with alpine mouse-ear and common mouse-ear (ssp. fontanum).