As the onion matures, food reserves begin to accumulate in the leaf bases and the bulb of the onion swells. From the underside of the plate, a bundle of fibrous roots extends for a short way into the soil. The base of each leaf is a flattened, usually white sheath that grows out of the basal plate of a bulb. They are at their broadest about a quarter of the way up, beyond which they taper towards a blunt tip. They are fleshy, hollow, and cylindrical, with one flattened side. The leaves are yellowish- to bluish green and grow alternately in a flattened, fan-shaped swathe. Modern varieties typically grow to a height of 15 to 45 cm (6 to 18 in). It is a biennial plant, but is usually grown as an annual. The onion plant has been grown and selectively bred in cultivation for at least 7,000 years. cepa, such as shallots and potato onions, produce multiple bulbs. 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. The crop is harvested and dried and the onions are ready for use or storage. 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 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. 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 onion is most frequently a biennial or a perennial plant, but is usually treated as an annual and harvested in its first growing season. Its ancestral wild original form is not known, although escapes from cultivation have become established in some regions. cepa is exclusively known from cultivation. The name 'wild onion' is applied to a number of Allium species, but A. ?proliferum), and the Canada onion (Allium canadense). This genus also contains several other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion (Allium fistulosum), the tree onion (A. The onion, also known as the bulb onion or common onion, is a vegetable and is the most widely cultivated species of the genus Allium.

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