- Name also: Thale-cress, Wall Cress, Mouse-ear Cress
- Family: Mustard Family – Brassicaceae (Cruciferae)
- Growing form: Annual herb.
- Height: (5–)10–30 cm (2–12 in.). 1 or many-branched, stem unbranched–sparsely branched, fragile, at least lower part hairy.
- Flower: Corolla regular (actinomorphic), white, under 0.5 cm (0.2 in.) wide; petals four, 2–4 mm (0.08–0.16 in.) long. Sepals 4, commonly purplish, 2 outermost narrow-based. Stamens 6, of which 4 long and 2 short. Gynoecium fused, a single carpel. Inflorescence a raceme, extending in fruiting stage.
- Leaves: In basal rosette and alternate on stem, rosette leaves stalked, stem leaves almost stalkless. Blade elliptic–lanceolate, almost entire margins–sparsely toothed, surface uneven, both sides stellate-haired.
- Fruit: Many-seeded, arching, quite flat, usually 1–1.5 cm (0.4–0.6 in.) long, spreading siliqua, tipped with short bristle. Stalk approx. 1/2 length of siliqua, ascending oblique.
- Habitat: Rocky outcrops, meadows, river banks, railway embankments, banks, dry-stone walls, islets, sometimes fields, harbours and railway yards.
- Flowering time: (April–)May–June.
Thale cress is an annual but it usually germinates in the autumn already and overwinters as a leaf rosette. In April or May at the latest the rosette grows a flowering stem with flowers at the top, and the first fruits soon form. Thale cress’s white flowers are only a couple of millimetres in diameter, which is too small to attract insects, but seed-production is ensured by self-pollination. If drought does not seal the plant’s fate early it can go on to grow and branch until early summer as it ripens its seeds.
Thale cress is like the floral equivalent of the banana fly: both are cultivated in laboratories around the world and used in genetic research. It is easy to care for, fast-growing, small and very fertile so it is a wonderful model organism to explain many of the basic phenomena of the plant kingdom. It has a simple genome with only five pairs of chromosomes (e.g. a human has twenty-three pairs). This insignificant weed can be used to study useful phenomena in agriculture and economics more broadly. The basic genetic mechanisms of growth, flowering and seed production are the same throughout the plant kingdom, so research into thale cress can be applied to other useful plants.
Thale cress grows ferally in the southern half of Finland and rarely around the Bay of Bothnia in dry meadows, banks, stone walls and rocks. Its close relative Swedish cress (A. suecica) often grows in the same kind of places and they are easy to confuse: Swedish cress’s flowers are only slightly larger, the base of the petals is yellowish and the leaves are pinnately lobed or large-toothed at least at their base.