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Horticulture and Home Pest News
Horticulture & Home Pest News is filled with articles on current horticulture, plant care, pest management, and common household pests written by Iowa State University Extension specialists in the Departments of Entomology, Horticulture and Plant Pathology.

Onions

This article was published originally on 3/17/1995
Onions are part of the lily family, Amaryllidaceae, and belong to the genus Allium. Alliums have bulbous, onion-scented roots. Garlic, chives, shallots, and leeks also belong to this genus as do the ornamental alliums grown for their purple flowers. The common garden onions are termed Allium cepa by botanists. Allium meaning garlic and cepa meaning onion in Latin.

Onions are a more complicated class than other vegetables. There are five ways to classify them. Their use, flavor, color, shape of the bulb and day length. Use-wise onions can be divided into four basic types; storage, fresh, pearl or mini, and green. Onions grown for storage often have a darker color with thicker skins, a more pungent flavor, and are usable for many months of the year. Fresh onions have a flavor that is milder and sweeter with a lighter color and thin skin. The flavors of onions range from sweet to pungent. Flavor is often dependent upon growing conditions. Some soils seem to produce sweeter onions than others. Soils high in sulfur seem to produce more pungent onions. Onions come in the colors white, yellow or red. The shape is globe or round, flattened or torpedo shaped.

An interesting aspect of onions is the effect of day length on plant growth. Many plants are sensitive to the hours of daylight and darkness they receive. When onions are first planted, their growth is concentrated on root and top formation. Once a specific combination of daylight and darkness is reached, bulb formation begins. The tricky part is that each variety needs a particular combination. A short day onion is responsive to 11 or 12 hours of daylight, an intermediate day onion needs 12 to 14 hours of daylight, a long day onion requires 14 or more hours of light. For those of us in the northern part of the U.S., we need to select long day varieties.

Onions need full sun and well drained garden soils to do their best. All onion types can be started from seed. Seed can be sown indoors about 4 to 8 weeks before transplanting. Seed can also be sown directly into the garden as soon as the ground can be worked. Onions can also be grown from sets. Onion sets are small, dormant onion bulbs that are ready to be planted outdoors. They range in size from 1/4 to 1 inch in diameter (less than 1/2 inch is best) and should be planted about 2 to 3 inches deep and 1/2 inch apart. Their best use is for green onions. Harvesting the green onions is a good way of thinning the row so the remaining onions are 3 to 4 inches apart. Onion transplants can be purchased at garden centers and through mail order catalogs. Plant transplants 3 to 4 inches apart and about 1 inch deep. Onion plants can tolerate a light frost but will be damaged if temperatures fall below 20 F. Onions are shallow rooted. Cultivate carefully to control weeds and avoid damaging the root system. Onions, like other garden vegetables, require an inch of water a week during the growing season. The size of the onion bulb is dependent on the green growing portion of the plant. For each green leaf there will be a ring in the bulb. The larger the leaf above the soil, the larger the ring in the bulb. A perfectly grown onion has thirteen leaves and thirteen rings, never any more.

Onions will occasionally bolt or produce a flower stalk. Onions bolt as a reaction to cold weather stress. Temperatures under 45oF may cause the onion to bolt when the plant has five or more leaves. Some onions are more or less susceptible to bolting than others and the process is not completely understood. Unfortunately once the onion does bolt, the quality of the onion bulb deteriorates rapidly and it should be harvested and eaten as quickly as possible.

The National Garden Bureau is recognizing 1995 as the Year of the Onion. Onions are being studied with regards to their healthy benefits. It has been shown that Quercetin in onions is beneficial to the heart and circulatory system and that onions possess possible anti-cancer properties. A revised jingle might become more appropriate, "an onion a day keeps the doctor away".



This article originally appeared in the March 17, 1995 issue, p. 23.

Year of Publication: 
1995
Issue: 
IC-470(4) -- March 17, 1995