The onion (Allium cepa) (Latin ‘cepa’ = onion), also known as the bulb onion or common onion, is used as a vegetable and is the most widely cultivated species of the genus Allium.
This genus also contains several other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion (A. fistulosum), the Egyptian onion (A. ×proliferum), and the Canada onion(A. canadense).
The name “wild onion” is applied to a number of Allium species but A. cepa is exclusively known from cultivation and its wild original form is not known.
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.
The onion plant has a fan of hollow, bluish-green leaves and the bulb at the base of the plant begins to swell when a certain day-length is reached. In the autumn 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, theonion eelworm and various fungi that cause rotting.