Growing form: Annual or biennial herb. Root thin (ssp. campestris, ssp. oleifera) or swollen, wide and flat, yellow–white (ssp. rapa).
Height: 40–100 cm (16–40 in.). Stem glabrous–sparsely haired.
Flower: Corolla regular (actinomorphic), yellow, approx. 1 cm (0.4 in.) wide, petals four, 6–10 mm (0.24–0.4 in.) long. Sepals 4. Stamens 6, of which 4 long and 2 short. Gynoecium fused, a single carpel. Inflorescence a raceme, extending in fruiting stage. Open flowers reach higher up than the buds.
Leaves: Alternate, lowest stalked, stalk winged, hairy, upper leaves stalkless, amplexicaul. Blade glabrous, underside hairy along veins, basal leaves shallowly lobed–with toothed margins, bright green, stem leaves with entire margins, bluish green.
Fruit: Many-seeded, opening lengthwise, 5–6 cm (2–2.5 in.) long, spreading siliqua, tip usually with 1–1.5 cm (0.4–0.6 in.) long seedless beak. Stalk approx. 15 mm (0.6 in.).
Habitat: Fields, vegetable gardens, mills, roadsides, loading areas, rubbish tips.
Flowering time: June–July(–August).