- Name also: Great Yellowcress, Great Yellow-cress, Greater Yellow-cress
- Family: Mustard Family – Brassicaceae (Cruciferae)
- Growing form: Perennial herb.
- Height: 40–80 cm (16–32 in.). Stem branched.
- Flower: Corolla regular (actinomorphic), yellow, approx. 1 cm (0.4 in.) across; petals four, 4–5 mm (0.16–0.2 in.) long. Sepals 4. Stamens 6, of which 2 short and 4 long. Gynoecium fused, a single carpel. Inflorescence an elongating raceme in fruiting stage.
- Leaves: Alternate, lowest stalked, upper stalkless. Basal leaves’ blade ciliately pinnately lobed, sunken lobes long and narrow, upper leaves’ blade entire, lanceolate, with toothed margin.
- Fruit: Many-seeded, broadly elliptic, round, 3–4 mm (0.12–0.16 in.) long silicula, terminated by a 1–2 mm (0.04–0.08 in.) long bristle. Stalk 8–10 mm (0.32–0.4 in.), slightly descending oblique. Seeds often remain undeveloped.
- Habitat: Lake shores, flood-influenced meadows, places with quite rough sand, small ponds, mud holes, waterside alder meadows which are prone to flooding.
- Flowering time: June–July.
Greater Yellowcress only grows in Finland in the Satakunta region, in southern Häme along the banks of the Kokemäki River until close to Pori, and in the Pyhäjärvi and Vanajavesi regions. In its compact and distinctive habitats it is however quite common. The species has made itself at home with people and many areas of the world, and in Finland it seems to be indigenous. Due to increasing water traffic it might be expected to spread to new areas, also in Finland.
Greater yellowcress grows on both sides of the watermark. (Specific epithet amphibia, in English amfibious, describes a plant able to live on land or in water.) It is perhaps easiest to differentiate from other shore plants by its light green stem. The best place to search for the plant is from low shore meadows, stream mouths, ditch banks and clay hollows in flood-influenced meadows. Trying to find greater yellowcress certainly demands character: most people are not so attracted to squishy mud, especially if it is combined with filthy water. Greater yellowcress however likes waste water and other urban delights, and one of its favourite habitats is near the mouths of drains.
Yellowcresses are quite a difficult group as they cross-breed and create hybrids. Greater yellowcress breeds with Finland’s other original yellowcress creeping yellowcress (R. sylvestris; also known as keek, yellow fieldcress) in the species’ common habitats: the lobing on the hybrids’ leaves is between the parent plants’. Of our yellowcresses, greater yellowcress bears most resemblance to Austrian yellowcress (R. austriaca; also known as Austrian fieldcress), which is rare in Finland, growing mainly beside railways. On both species the upper stem leaves are unlobed and the siliqulae are short and almost round.