- Name also: Wavy Bitter-cress
- Family: Mustard Family – Brassicaceae (Cruciferae)
- Growing form: Perennial or sometimes biennial herb.
- Height: 10–40 cm (4–16 in.). Stem geniculately curved, often hairy at base.
- Flower: Corolla regular (actinomorphic), white, approx. 0.5 cm (0.2 in.) across; petals four, 2.5–3 mm (0.10–0.12 in.) long. Sepals 4. Stamens usually 6, of which 4 long and 2 short. Gynoecium fused, a single carpel. Inflorescence an elongating raceme in fruiting stage.
- Leaves: Basal rosette unclear and sparsely leaved, soon withering, stem leaves alternate, usually 5–10, rosette leaves larger, roughly same size. Blade pinnate in 3–6-pairs, with terminal leaflet, leaflets elliptic–kidney-shaped.
- Fruit: Many-seeded, opens lengthwise, slim, flat, 15–25 mm (0.6–1.0 in.) long, siliqua ascending oblique, tipped with clear, approx. 1 mm long bristle. Stalk 1/3–1/2 x siliqua length, spreading.
- Habitat: Springs, stream banks, damp broad-leaved forests, swamps, also as a weed in greenhouses, nursery gardens and planting beds.
- Flowering time: June–July.
- Endangerment: Endangered, protected in all of Finland, except the Åland Islands.
Wavy bittercress grows in Finland at the north-eastern limit of its habitat as a relic from the warmer period that followed the last Ice Age. It survives through the winter only in its favoured habitats. It gravitates towards springs where it has a sufficiently long growing season and the land doesn’t freeze throughout the year. Variations in the height of the surface water clear additional suitably bare areas for it to grow in, and it is also fond of ground that had been broken by fallen trees or grazing animals.
For a long time only one wavy bittercress stand was known about in Finland. This was on the Åland Islands, but it seems to have disappeared. Seven original wavy bittercress habitats are known in mainland Finland however, in Pöytyä, Asikkala, Valkeala and Luumäki, although two of these appear to have vanished. The species has become rarer in recent years in many of its habitats. Wavy bittercress is threatened by excavations and clearing work as it affects the dampness in the area. It is however a common greenhouse plant which can travel with young plants and garden waste and grow in surprising places, wherever there is enough dampness and shade.
Wavy bittercress can be difficult to tell apart from its large relatives large bittercress (C. amara) and cuckoo flower (C. pratensis; also known as lady’s smock), at least outwith flowering time. Large bittercress and cuckoo flower’s petals are clearly larger than wavy bittercress’s at 7–15 mm (0.28–0.6 in.), and wavy bittercress’s anthers are purple. Wavy bittercress also looks a lot like hairy bittercress (C. hirsuta), and the plants have sometimes been classified as subspecies of the same plant. Luckily the species’ different kinds of habitats reduce the risk of misidentification. The best identification markers are hairy bittercress’s distinct basal rosette and scant amount of stem leaves. Additionally, it usually has 4 stamens and its long, spreading stalks form a clear corner with the siliquae.