- Name also: Common Mouse-ear Chickweed, Big Chickweed (ssp. vulgare)
- Family: Pink Family – Caryophyllaceae
- Growing form: Perennial, occasionally annual or biennial herb. Sparsely tufted. Sometimes rhizomatous.
- Height: 5–35 cm (2–14 in.). Stem ascending–erect, usually densely haired, sometimes also with glandular hairs, flowerless shoots at base.
- Flower: Corolla regular (actinomorphic), white, approx. 1 cm (0.4 in.) broad; petals five, 4.5–8 mm (0.18–0.32 in.) long, around same length as sepals, shallowly 2-lobed. Sepals 5, quite tapered, hairy, membranous margins. Stamens usually 10. Gynoecium syncarpous, with 5 styles. Inflorescence a 2-branched cyme, (3–)10–30-flowered.
- Leaves: Opposite, stalkless. Blade elliptically long–ovate, tip sharp or quite blunt, with entire margins and spreading hairs, usually greyish green.
- Fruit: Cylindrical, usually straight, brown, 10-valved, 8–15 mm (0.32–0.6 in.) long capsule.
- Habitat: Meadows, rocky ridges, wasteland, roadsides, gardens, yards, lawns, arable land, rich forests, forest clearings, hedgerows, springs, shores.
- Flowering time: June–August.
- Endangerment: Ssp. vulgare is endangered.
Common mouse-ear’s many mutations have given botanists a real headache, and many subspecies with different appearances, ways of growing and habitats have been described. Ssp. vulgare grows commonly in all of Finland, and probably arrived a long time ago from somewhere in the south. Nowadays it is an almost cosmopolitan follower of people, and it is no longer possible to say where it is native to. Apart from the type species, which thrives on culturally-influenced land and has an abundance of small flowers, ssp. fontanum grows as far south as Oulu and has a sparse inflorescence of large flowers. Its favoured habitats are stream-side meadows, hedgerows and springs.
There is also a critically endangered mutation of common mouse-ear, ssp. vulgare var. kajanense, which grows on calciferous serpentine and is smaller and less hairy than common mouse-ear. It has a handful of habitats in Kainuu, and has probably appeared in each place independently from the common mouse-ear population. Plants with the same mutations can also be found in Sweden and Norway, but nowhere else in the world – the mutation is therefore native to the Nordic countries. Unfortunately for the plant, there is a whole industry which is very interested in its favoured serpentine habitat, and the whole population is thus under threat. Because of the scarcity of stands, ssp. vulgare var. kajanense is threatened by many other human activities too, such as the spread of holiday homes and the impoverishment of the soil. It can be differentiated from its close relative field mouse-ear (C. arvense) on the basis of its corolla, which on the latter is clearly larger than the calyx, while common mouse-ear’s is the same length or only slightly longer at most.