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The most widely grown species of the genus Allium is the vegetable known as an onion (Allium cepa L., from the Latin word cepa, which means “onion”). Other names for onions include bulb onions and common onions. Until 2011, the shallot, a botanical variety of the onion, was considered a separate species. 21 It has garlic, scallion, leek, and chive as close relatives.

The Japanese bunching onion Allium fistulosum, the tree onion Allium proliferum, and the Canada onion Allium canadense are all members of this genus and are cultivated for their edible qualities. A number of Allium species are referred to as “wild onions,” but only A. cepa is known from cultivation. Despite the fact that escapes from cultivation have become established in some regions, its original wild form is unknown. The onion is most often a biennial or a lasting plant, however is typically treated as a yearly and reaped in its most memorable developing season.

When a certain day length is reached, the onion plant’s bulb at the base begins to swell, and it has fan-shaped, hollow, bluish-green leaves. The bulbs are made up of underground stems that are compressed and shortened. These underground stems are surrounded by fleshy, modified scale (leaves) that wrap around a central bud at the stem’s tip.

The outer layers of the bulb become more dry and brittle as the foliage dies down in the autumn (or spring, in the case of overwintering onions). The crop has been dried and harvested, and the onions can now be used or stored. The onion fly, the onion eelworm, and a variety of fungi that can cause rotting are just a few of the diseases and pests that can harm the crop. A few assortments of A. cepa, like shallots and potato onions, produce various bulbs.