Onion (Allium cepa) is a popular vegetable grown for its pungent bulbs and flavorful leaves. It is widely grown throughout the world. The bulb is composed of concentric, fleshy, enlarged leaf bases or scales. The outer leaf bases lose moisture and become scaly and the inner leaves generally thicken as bulbs develop. The green leaves above the bulb are hollow and arise sequentially from the meristem at the innermost point at the base of the bulb. The stem is very small and insignificant during vegetative growth.
After vernalization at temperatures below 10°C, the stem elongates rapidly, eventually producing compound umbels. Bolting has been reported to be related to the length of day. However, long days do not induce reproductive growth but tend to accelerate development of the seedstalk once it has been initiated by vernalization. Temperature has a major role in inducing bolting.The onion root system is fibrous, spreading just beneath the soil surface to a distance of 30 to 46 cm.
There are few laterals, and total root growth is sparse and not especially aggressive. Therefore, in monoculture, onions tolerate crowding, particularly in loose, friable soils such as peat and muck. Competition from aggressive root systems (as from weed growth) severely limits onion growth. Cultivars differ substantially with respect to the threshold daylength required for bulbing. Other factors such as temperature may interact with daylength to modify the bulbing response. In all cultivars, bulbing is accelerated with increasing temperature. Temperature extremes not only affect the rate of bulbing, but also affect the bulb shape. Thick and elongated necks are common in plants exposed to 6° or lower.
Onions can be grown successfully on any fertile, well-drained, non-crusting soil.
At one time, all onion cultivars were open-pollinated, and many of these cultivars are still offered by seed companies. The discovery of male sterility in onion led to a rapid change to F1 hybrids, possibly due to simplicity and low cost of seed production. Male sterility is a genic-cytoplasmic factor, and male fertility can be restored in plants carrying the sterility factor by introducing a single dominant allele. Any line carrying the sterile trait must be cross-pollinated, and seeds harvested from male sterile plants isolated with a normal pollen-bearing parent will be hybrid seed.
Hybrids have higher yield, larger and more uniform bulb sizes than open-pollinated cultivars. The bulb onion cultivars are grouped into short, intermediate, and long-day types. Short-day onions (12 to 13 hour threshold) are generally mild, soft fleshed, and suitable for storage. Long-day onions (over 14.5-hour threshold), if grown in the lower latitudes, will not form bulbs, and only green onions would be produced.
In contrast, short-day types grown in the higher latitudes will bulb very quickly and will be little more than sets in size.Short-day onions include the Bermudas and Grano-Granex types; long-day cultivars include yellow, white and red globes. The intermediate-day cultivars (13.5 to 14.5-hour threshold), are relatively soft-fleshed and used primarily for fresh trade. They are grown in areas of mild temperatures lying between 32 and 38° latitudes.
True pearl onions are classified as Allium ampeloprasum because they form just one storage leaf. In practice, short-day onion cultivars, such as Grano, Crystal Wax and others, when grown in northern latitudes, will develop pearl-size bulbs and be marketed as such. Most are used in pickling or in frozen mixtures of peas, broccoli and other vegetables.Green onions, scallions, multiplier, and bunching onions are all used in the immature stage.
Green onions generally are bulbing-type, white cultivars harvested at the miniature bulb stage. Scallions are white cultivars of A. cepa that do not form bulbs. Multiplier onions are cultivars of A. cepa of Group Aggregatum with white flesh and yellow or brown scales. These are distinguishable from the shallot by the latter’s red scales and more delicate flavor. The shallot can be used both in the immature stage and as a dry bulb. A cross of shallot with pink-root resistant A. fistulosum gave rise to the tetraploid cultivar Beltsville Bunching A. fistulosum includes the bunching onions, also called Japanese bunching onions or Welsh onions.