This annual plant probably originated in the Mediterranean, but is now worldwide. It is grown as a forage crop (i.e. consumed by livestock) in lowland areas, though in other places it is grown for its seed, which is ground to make mustard, or as a green manure crop. It is sometimes accidentally introduced with grain or bird seed. White mustard grows best in North and North East Scotland.
It closely resembles Charlock, or Field mustard, except that the leaves are pinnate with deeper lobes.
Another distinction from Charlock can be found in the siliques, or seed pods, which in White Mustard are hairy and end in a flattened curved beak, at least as long as the seeded portion of the siliqua. Each siliqua contains about 6 seeds, 1.5mm in diameter and usually beige in colour. These can be used whole (whole grain mustard), or crushed into a powder and mixed with other ingredients to make mustard paste. ‘White’ mustard seeds are considered milder in flavour than the black seeds of the Charlock. Even though the seeds of the Charlock are mildly toxic when taken in large amounts, they have been used in a culinary context.
The flowers are about 20mm across, yellow, with 4 cruciform petals, and grow in rounded racemes at the ends of the stems.