- Name also: Bog Yellow-cress, Marsh Yellow Cress
- Family: Mustard Family – Brassicaceae (Cruciferae)
- Growing form: Annual (occasionally biennial or perennial) herb.
- Height: 10–60 cm (4–25 in.). Stem upper part branching.
- Flower: Corolla regular (actinomorphic), pale yellow, under 0.5 cm (0.2 in.) wide; petals 4, approx. 2 mm (0.08 in.) long. Sepals 4, at least same size as corolla. Stamens 6, of which 2 short and 4 long. Gynoecium fused, a single carpel. Inflorescence a raceme, extending in fruiting stage.
- Leaves: Alternate, lowest long-stalked, stalk winged and with ciliate edges, uppermost short-stalked, stipulate. Blade pinnately lobed, lobes 2–8 pairs, with rounded–serrated margins, terminal lobe larger than others.
- Fruit: Many-seeded silicula, cylindrical, slightly curved (sometimes short, plum-shaped), 3–8 mm (0.12–0.32 in.) long, tipped with almost 1 mm (0.04 in.) long bristle. Stalk approx. 5 mm (0.2 in.), spreading.
- Habitat: Shores of lakes, rivers and seas, damp meadows, wet fields, banks, ditches, yards, gardens, waste ground, rubbish tips, railway yards.
- Flowering time: June–August.
Marsh yellowcress is native in Finland to open shingle next to rivers and lakes. It grows very commonly across the whole country apart from northernmost corners of Lapland and the south-western archipelago, where it is scarce. It has also made itself at home in environments that have been created by people, especially field ditches and fallow ground, and it sometimes grows as a weed in damp fields. In gardens it is a common nuisance and it can become large, branching and sturdy in loamy soil. It sometimes spreads from compost heaps and other earth stores to new lawns where it can produce abundant if short-lived stands.
Marsh yellowcress is usually short-lived, annual or at most overwintering. Biennial plants can be recognized by the remains of the previous year’s rosette around their base, and rare perennial individuals can have the previous year’s withered stem standing beside the new one. Marsh yellowcress varies not only with regard to life-span but also with its flowering time, appearance and need for dampness. It is a diverse species, and perhaps these forms represent stands that have adapted to different conditions.
Marsh yellowcress is pollinated by tiny flowerflies, which sometimes convey the pollen to the wrong flower, so in this way it can cross-breed with greater yellowcress (R. amphibia). These two species can be differentiated from each other by the latter’s unlobed upper leaves, and the degree to which the intermediate forms’ leaves are lobed depends on its parents. Creeping yellowcress (R. sylvestris) is quite common and can be told apart from marsh yellowcress by the large terminal lobe on its leaf-blade. Creeping yellowcress’s leaves are also exstipulate, its flowers are larger and its siliquae are linear and slightly arching. The species can grow together in places that have been disturbed by humans, but creeping yellowcress never spreads beyond such places.