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The common onion (Allium cepa), one of the most widely grown onions, is grown from either seed, plants or sets for use as both green onions and dry bulbs. The home gardener will usually have more success with sets. Any standard onion variety or hybrid can be used for green bunching onions if harvested at the proper stage of maturity. Onions can be used as green onions within 30 days if grown from plants or sets, or 40 to 50 days if grown from seed. For dry onions from sets or plants, 100 or more days are required from planting, depending on the variety grown. The potato, or multiplier, onion and the Egyptian onion are grown from vegetative parts rather than seed. The underground portion of the potato onion (Allium cepa var. solanium) is a compound bulb formed from the segregation of a large mother bulb. Each bulb in the compound bulb produces 6 to 12 plants. The principal use of these onions is the production of early green bunching onions. The Egyptian onion (Allium cepa var. viviparum) produces clusters of small bulbs called bulbils at the top of the seed stalk in late summer. The bulbils are used to produce very early green onions. Both potato and Egyptian onions are planted in the fall, overwintered with some mulch protection, and brought into production in the early spring. These onions are referred to as "winter onions." Climatic Requirements: The onion is adapted to a wide range of temperatures and is frost-tolerant. Best production is obtained when cool temperatures (55 to 75 degrees F) prevail over an extended period of time, permitting considerable foliage and root development before bulbing starts. After bulbing begins, high temperature and low relative humidity extending into the harvest and curing period are desirable. A constant supply of adequate moisture is necessary for best results. For onions started from plants, a light mulch will help conserve moisture for uniform growth. An important aspect of onion development is the length of day (photoperiod). Photoperiod, along with temperature, controls when the onions form bulbs. Some onion varieties are short-day and form bulbs when the days are 12 hours or less in length. Other varieties are long-day plants, forming bulbs when there are 15 or more hours of daylight. The specific photoperiod of some onion varieties makes them unsuitable for northern climates, as they will begin to bulb when the plants are too small. The influence of day length requires that Sweet Spanish and Bermuda onions be grown from plants rather than seed in the Northeast. Southern types such as Vidalia do poorly in New England. Unfavorable growing conditions may result in onions bolting or sending up flower stalks. If flower stalks should develop, carefully cut them from the plant immediately or bulbing will be reduced. Soil Requirements: Onions grow best in a loose, well-drained soil with high fertility and plenty of organic matter. Avoid heavier soils such as clay and silt loams, unless they are modified with organic matter to improve aeration and drainage. Onions are sensitive to highly acid soils and grow best when the pH is between 6.2 and 6.8. Fertilizers: As with most vegetables, lime and fertilizers are best applied using the results of a soil test as a guide. Fertilizers of a 1-2-2 ratio (5-10-10, for example) are generally good for onion production. As the onion plant's root system is very limited, high soil fertility is essential for good production. Establishing the Planting: Onions should be planted early in the spring as soon as the soil can be worked. Onion seed is sown 1/2 inch deep, while sets are planted one to two inches deep. A three-inch plant spacing is desirable. Rows should be 12 to 18 inches or more apart depending on the method of cultivation. For wide row planting, plants or sets are placed on 3-inch centers. Onions are ideal for wide row planting, but keep in mind that weeding must then be done by hand. Problems: After the plants are well-established, a mulch will conserve soil moisture, prevent soil compaction and help suppress weed growth. In windy areas, small plants must be protected with a windbreak of some type to prevent serious damage or loss of plants. Weeds, insects and diseases must be controlled. Thrips, onion maggots, downy mildew, neck rot, pink root and smut can all harm onion crops. Harvesting, Curing and Storing: Harvest onions when the tops have fallen over and dried. On sunny, breezy days, onions may be pulled and left in the garden for a day or two to dry before they are taken to a curing area. Curing must take place for the onions to be stored for any length of time. Cure onions by placing them in a warm, well-ventilated area until the necks are thoroughly dry. With warm temperatures, good air circulation and low humidity, curing should be completed within two weeks after harvest. Onions are best stored in a cool, moderately dry area in ventilated containers.
Adapted from Marianne Riofrio and E. C. Wittmeyer, Ohio State University Extension, 2000
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