Floridata Plant Encyclopedia

A Floridata Plant Profile 708 Allium cepa Cepa Group

Common Names: onion, bulbing onion Family: Amaryllidaceae (amaryllis Family)
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onion garden
Pull up onions when the tops fall over and begin to turn brown like these.


Bulbing onions have cylindrical, hollow leaves and an enlarged bulb that develops at ground level. The roots come off the bottom of the bulb. The flowers are produced in the second growing season (following a required "rest" period) in a rounded umbel (cluster with all flower stems originating from the same point) on a stalk 2-4 ft (0.6-1.2 m) tall. The umbels, about 2 in (5 cm) in diameter and consisting of many small purplish flowers, are quite showy.

There are two main kinds of onions, based on the daylength required for bulb formation. Short-day varieties start forming an enlarged bulb when days are 12 or 13 hours long; long-day varieties don't form a bulb until days are 14-16 hours long. For both types, bulb enlargement is arrested during hot, freezing or dry weather. Near the equator, where days are 11-13 hours long throughout the year, long-day onions will never form a bulb; and in Canada, where days are 14-20 hours long during the growing season, short-day onions will never form a bulb.

There are hundreds of onion cultivars, differing in day-length requirement, skin color (white, brown, yellow, red, or purple), size (1-6 in or 2.5-15.2 cm in diameter), shape (globe-shaped, flattened or spindle-shaped), pungency and sweetness. Both pungency and sweetness (which are not mutually exclusive) are determined to a considerable extent by the chemical characteristics of the soil in which the onion is grown. Popular long-day onions are 'Yellow Sweet Spanish', 'Wala Wala' and 'Early Yellow Globe'; popular short-day cultivars are 'Yellow Bermuda', 'Granex', and 'Texas Grano'.

Vidalia onions are sweet, non-pungent short-day onions (usually 'Granex', 'Texas Grano' or a similar hybrid) that are grown near the town of Vidalia in southeastern Georgia, and whose growers have purchased the exclusive right to use that name. Spring onions or green onions are immature bulbing onions that are used in place of bunching onions (A. fistulosum).


The onion, Allium cepa, is known only in cultivation, but probably was developed from a wild ancestor that grows in western Asia. Onion seeds have been found in Egyptian tombs dated to 3200 BC, and some authorities believe the onion may have been one of the first vegetables domesticated by humans.


Onions are grown as long season annuals in rich, well-drained soil. Most varieties take 90-150 days or more from seed to fully formed bulb. In northern areas, long-day varieties are planted after the last frost; they start bulb development in the long days of summer and will be ready for harvest before winter. In southern latitudes, short-day onions are planted in autumn or winter, to mature the following spring. In North Florida we start onions in autumn. They grow slowly through the winter, then speed up in early spring and are ready to harvest by April or May. Light: Onions will do best in full sun but can be grown in partial shade. Moisture: Regular garden watering is best. Onions may go dormant during extreme dry periods. Hardiness: USDA Zones 3 - 10. Prolonged freezing temperatures will top-kill onions, and even the bulbs will be killed if the soil freezes hard around them. In USDA zones 3-5, long-day varieties are planted as soon as the ground can be worked in spring, either from seedlings or "sets." In zones 8-10, short-day varieties are planted from seeds or seedlings in autumn. (Apparently they can't make sets for short-day onions.) In zones 6-7 it is possible to plant long-day onions in spring and short-day onions in autumn. (It may be necessary to mulch over-wintered onions before the ground freezes. Propagation: Onions are propagated from seed. It is absolutely imperative to select the proper day-length variety for your region. Long-day varieties planted in the south and short-day varieties planted in the north will produce fine green onions, but no bulbs. Many gardeners purchase pencil size seedlings, or onion "sets" which are pea-sized bulbs that were grown quickly and densely, then dried so that their development was stopped. Both sets and seedlings were started from seed at the nursery, but they give the home gardener a head start, and are the only way to go in areas with a short growing season. If you replant mature onion bulbs or leave them in the ground after the tops have dried and withered, they will stay dormant for a "rest" period, then start growing again. In their second growing season they will produce flowers and seeds, then die.


Harvest onions when the tops fall over and begin to wither and brown. Pull the plants and allow them to cure for a few days in dry shade before cutting off the tops and storing in a cool, dry place. In general, the more pungent varieties keep longer in storage.

Onions and their relatives have almost no odor at all until we cut into them. When the cell walls are broken, odorless compounds come into contact and react to form ammonia, pyruvic acid and various disulfides, the last of which are the main cause of the distinctive smells, and the compounds which form sulfuric acid in the eyes. Cooking drives off the odoriferous compounds and converts some of them to sugars.

If you have to peel a lot of onions, drop them in boiling water for a few seconds, then cool; the skins will slip right off. If you peel onions under running water, or if you chill them in the refrigerator first, there will be less crying. Rub your hands with salt or vinegar to remove the odor.

Here's a colorful collection of bulbing onions in red, yellow and white.


Plant taxonomists have divided the onion species, Allium cepa, into three groups: the Cepa Group includes the typical bulbing onions which have a single enlarged bulb, and are propagated from seeds; the Aggregatum Group includes shallots, multiplier onions (not to be confused with bunching onions, A. fistulosum), and potato onions, which do not produce seeds and are propagated from lateral bulbs off the main bulb; the Proliferum Group includes top-setting onions, walking onions, Egyptian onions and tree onions, which are propagated from bulbils (little "bulblets") borne in the inflorescence.

Some authorities place the onions, garlics, leeks and their relatives in a family of their own, the Alliaceae, and others put them in the lily family, the Liliaceae. There are about 400 species in the genus Allium, including some magnificent ornamentals. Other well known members of the genus include: garlic (Allium sativum), bunching onions (A. fistulosum), chives (A. schoenoprasum), garlic chives (A. tuberosum) and leeks and elephant garlic (A. ampeloprasum).

Steve Christman 6/10/00; updated 9/13/03, 1/25/04, 5/10/08

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Allium species profiled on Floridata:

Allium cepa Cepa Group

( onion, bulbing onion )

Allium fistulosum

( scallion, green onion, bunching onion, ciboule )

Allium giganteum

( giant onion )

Allium sativum

( garlic, softneck garlic, hardneck garlic, rocambole )

Allium schoenoprasum

( chive, cive )

Allium tuberosum

( garlic chives, Chinese leeks, Chinese chives, flat chives )

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