- Name also: Lady’s Smock, Ladies Smock, Cuckooflower
- Family: Mustard Family – Brassicaceae (Cruciferae)
- Growing form: Perennial herb. Without runners.
- Height: 10–50 cm (4–20 in.). Stem cylindrical, full.
- Flower: Corolla regular (actinomorphic), white–reddish–light purple, approx. 1.5–2.5 cm (0.6–1 in.) wide; petals four, 8–15 mm (0.04–0.16 in.) long. Sepals 4. Stamens usually 6, of which 4 long and 2 short, anthers yellow. Gynoecium fused, a single carpel. Inflorescence a raceme, extending in fruiting stage.
- Leaves: In basal rosette and alternate on stem. Basal leaves long-stalked, stem leaves short-stalked. Blade pinnate, 1–7-paired, terminal leaflet. Basal leaves’ leaflets roundish, stem leaves’ leaflets usually narrowly elliptic (sometimes round).
- Fruit: Many-seeded, opening lengthwise, slim, flat usually 2–3 cm (0.8–1.2 in.) long siliqua.
- Habitat: Meadows beside rivers, lakes and the sea, shores, wet swamps, quagmires, ditches, puddles, young hillside meadows, damp meadows in fell tundra, snow-bed sites, shores, stream banks, commonly in water.
- Flowering time: June–August.
- Endangerment: Ssp. pratensis is near threatened.
Cuckoo flower is a perennial species, and no fewer than three sub-species can be found growing in Finland. The most widespread of these is ssp. paludosa, which can be found in southern and central Finland in waterside meadows which are prone to flooding, and also in many other kinds of damp places. Its rosette leaves have 3–8 pairs of same-sized leaflets that are usually clearly stalked, it has five or more stem leaves and its large flowers are pale. The type species (ssp. pratensis), classified as a near threatened species, grows in the south of Finland and is only common in the south-west, where it can also be found on dry, sloping meadows. Its rosette leaves have 1–7 pairs of almost stalkless leaflets with a clearly larger terminal leaflet, it has 2–4 stem leaves and its smallish flowers are usually quite dark. Ssp. polemonioides is small and looks like ssp. paludosa, but it has small, swollen leaflets and grows in eastern and northern parts of the country, although it is only common in northern Lapland. Sometimes it is regarded as a separate species.
It is easiest to see common cuckoo flower at the beginning of summer: when other wetland vegetation is still growing and is lacking any colour, genus Cardamine plants stand out a long way off, attracting butterflies, bumblebees, honeybees and flower flies. Cuckoo flower must be cross-fertilised as its own pollen will not do the job even in emergency situations. Despite its impressive inflorescence the plant spreads mainly through its leaves, which have bulbils that take root and make new plants when they break off and fall to the ground. Bulbils can even grow on basal leaves that have broken off and are lying on the ground.
Large bittercress (C. amara) is slightly more demanding than cuckoo flower, but they sometimes grow in the same places. When they are not in flower they can be differentiated by the fact that large bittercress’s basal and stem leaves look alike but cuckoo flower’s basal leaves have wider leaflets. When they are flowering they can be told apart by large bittercress’s petals, which are virtually always pure white or at least by the anthers: large bittercress’s are purple while cuckoo flower’s are yellow.