- Named also: Common Hedgemustard, English Watercress, Hairypod Hedgemustard
- Family: Mustard Family – Brassicaceae (Cruciferae)
- Growing form: Annual herb.
- Height: 30–60 cm (12–25 in.). Stem upper half strongly haired, usually short-haired, sometimes glabrous.
- Flower: Corolla regular (actinomorphic), yellow–pale yellow, approx. 0.5 cm (0.2 in.) wide; petals four, 2–4 mm (0.08–0.16 in.) long. Sepals 4. Stamens usually 6, of which 4 long and 2 short. Gynoecium fused, a single carpel. Inflorescence a raceme, extending in fruiting stage.
- Leaves: Alternate, stalked. Blade pinnately lobed, lobes usually short-haired (sometimes sparsely haired), terminal lobe large, triangular, with hastate base. Upper leaves commonly with only terminal lobe.
- Fruit: Many-seeded siliqua opening lengthwise, both sides 3-veined, usually hairy (sometimes glabrous), 1–1.5 cm (0.4–0.6 in.) long, pressing against stem. Stalk 1–2 mm (0.04–0.08 in.).
- Habitat: Yards, walls, gardens, beside streets, alleys, roadsides, waste ground, rubbish tips, shores, harbours, sometimes beside tracks, mills.
- Flowering time: July–September.
Hedge mustard only grows close to inhabited areas because there are not many places in Finland that fulfil its habitat requirements. The species clearly favours nitrogenous and alkaline soil. It commonly finds what it is looking for in parks, yards and roadsides, and it also thrives in cracks between stones, waste ground, newly-planted areas and on banks and compost heaps. In Finland it is most common in inhabited areas and the countryside around south-coast towns and southern Häme. The best place to search for it is around old inhabited areas, for although it grows in new suburbs it is mostly rare and scarce there. It has sometimes been carried far to archipelago bird-rocks by seagulls that have been grubbing around in city waste. Like many other plants that grow in old cultural places, it is harder for hedge mustard to find suitable habitats nowadays.
Hedge mustard may have been deliberately brought to Finland because according to its scientific name it has been used medicinally, at least to treat scurvy. Preachers, singers, actors and speech-givers have used a hedge mustard decoction to treat a husky voice and hoarse throat. Young leaves have also been used in salads and the seeds can be used like mustard. Most of the hedge mustard that grows in Finland are of the mutation var. officinale, which have hairy fruits, but the western European mutation var. leiocarpum, which has sparsely haired and glabrous fruits, also grows.
Hedge mustard takes on an appearance all of its own as flowering progresses: it is characterized by the rigid horizontal and vertical infructescences at the top of the plant and the stick-like siliquae that grow tightly along them. When the inflorescence wilts there is usually a small tassel left at the tip of the infructescence, and the plant can stay rigidly like this perhaps even all through the winter. Hedge mustard’s somewhat common relative tumble mustard also grows in Finland, and they can be told apart by the latter’s very long and spreading siliquae and its narrow-lobed upper leaves.