The onion plant has a fan of hollow, bluish-green leaves and its bulb at the base of the plant begins to swell when a certain day-length is reached.
The bulbs are composed of shortened, compressed, underground stems surrounded by fleshy modified scale (leaves) that envelop a central bud at the tip of the stem.
In the autumn (or in spring, in the case of overwintering onions), the foliage dies down and the outer layers of the bulb become dry and brittle.
The crop is harvested and dried and the onions are ready for use or storage.
The crop is prone to attack by a number of pests and diseases, particularly the onion fly, the onion eelworm, and various fungi cause rotting.
Some varieties of A. cepa, such as shallots and potato onions, produce multiple bulbs.
Onions are cultivated and used around the world.
As a food item, they are usually served cooked, as a vegetable or part of a prepared savoury dish, but can also be eaten raw or used to make pickles or chutneys.
They are pungent when chopped and contain certain chemical substances which irritate the eyes.