- Name also: Charlock, Charlock Mustard
- Family: Mustard Family – Brassicaceae (Cruciferae)
- Growing form: Annual herb.
- Height: 20–60 cm (8–25 in.). Stem upper part branched, bristly, hairy–almost glabrous, sometimes violet.
- Flower: Corolla regular (actinomorphic), pale yellow, approx. 1.5 cm (0.6 in.) across; petals four, 9–13 mm (0.36–0.52 in.) long. Sepals 4, spreading, easily dropping. Stamens 6, of which 4 long and 2 short. Gynoecium fused, a single carpel. Inflorescence an elongating raceme in fruiting stage.
- Leaves: Alternate, lowest stalked, upper stalkless. Blade bright green, rough, lowest large toothed–lobed, upper lanceolate, with toothed margin.
- Fruit: Many-seeded, round, glabrous (sometimes sparsely stiff-haired), 3–7-veined, 3–4 cm (1.2–1.6 in.) long, ascending oblique, spreading siliqua, terminated by a straight, bristly, 10–15 mm (0.4–0.6 in.) long, seedless beak. Stalk 3–5 mm (0.12–0.2 in.). Seeds black.
- Habitat: Fields, fallow land, roadsides, railway yards, wasteland, heaps of earth, rubbish tips, gardens, lawns.
- Flowering time: June–September.
Wild mustard is most likely to be found on the fields of the Åland Islands, where it is an established alien after centuries of cultivation. It has also established itself in other places in mainland Finland and the south-west archipelago. In the north the species is a member of a large group of annuals that come and go, growing on new lawns, waste ground, rich earth and allotments. Most casual stands have begun with seeds that have arrived in silos of imported grain, and nowadays a growing number come from uncleaned lawn seed. Wild mustard has also arrived in Finland in sailing boats’ ballast soil, in Russian soldiers’ provisions and among different kinds of cargo – the species is a seasoned world traveler. Part of the new stands might originate from soil that has been transported from formerly cultivated fields, which means that it is Finnish in origin. Both glabrous-podded var. arvensis and hairy-podded var. orientalis also grow in Finland.
Wild mustard spreads proliferously and produces copious amounts of seeds, even tens of thousands in a large plant. The black seeds are suitable for making mustard, but the species is not an important ingredient of industrial mustard because there are better-quality options around. Mustard is made from genus Mustard member white mustard (S. alba) and genus Cabbage members black mustard (Brassica nigra) and brown mustard (B. juncea). All three can be found around Finnish harbours, railway yards and dumps. Wild mustard is most often confused with wild radish (Raphanus raphanistrum; also known as jointed charlock), which has more clearly pinnately lobed leaves, erect sepals, a very distinctive fruit, and jointed (torulose) siliquae like a pearl necklace.