The Onion Plant
Family: Liliaceae (Lily) or Amaryllidaceae (Amaryllis)
Species: Allium cepa.
The Plant
This is a genus of strong-smelling bulbous herbs of the either the Lily Family or Amaryllis Family. It includes more than 300 species of which about 70 are cultivated, some as ornamentals, most as vegetables. Onions, garlic, leeks, chives, and their relatives, are all members of this family.
The onion which we eat (allium cepa) is a hardy biennial grown as an annual from seed or from sets (small bulbs). It is usually grown for its firm, ripe bulbs, but also grown for its immature stems (shallots and green onions). These two types are known as "bulbous" and "bunching".
Onions are "tunicate" bulbs, which means that the scales are covered by a thin skin known as a tunic. These scales are also called "wrapping scales". The bulb is a modified shoot or flower bud that forms underground, though close to the surface of the soil. Its hollow, rounded, blue-green leaves average 18 inches in height.
The thick scales that protect the bud are actually swollen leaf bases. The scales are anchored to a tough basal plate (the flat end of the bulb) from which the roots will grow. The layers of scales store food to nourish the bulb when the plant's top growth dies back. The bulb actually contains nearly everything that the embryonic bud will need to grow and bloom, including a lot of water. When harvested, the yellow sweet spanish onion bulb is almost 95% water.
Some varieties of onions, the storage onions we are most familiar with, contain a high proportion of sulfur compounds. The sulfur is absorbed out of the soil and into the onion through its roots. When an onion is cut into, the sulfur compounds are released into the air. When it reaches the saline solution that your tears are made up of, it combines and becomes a mild form of sulfuric acid. This is what makes people cry.
Specialty sweet onions contain very small amounts of the sulfur-containing compounds, only about 50% of the typical levels found in other varieties. We taste the sugar instead of the sulfur and think they are much sweeter. Ironically, sugar levels in sweet onions are only slightly greater than those of some storage onions. The sulfur compounds serve an important purpose, however, increasing an onion's capacity for long-term storage. The specialty sweet onions will not store as long.
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Photoperiod is an important factor in bulb development. This term describes the effect of day length on bulb formation. Some onion varieties have a short day length response, forming bulbs when the days are 12 hours or less. Other varieties have a long day length response, forming bulbs when there are 15 or more hours of daylight. Short day length varieties are generally unsuitable for growing in all but the most southern parts of the U.S. Long day length and a few intermediate day length varieties are grown in the large production areas of Oregon and Idaho.
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The National Onion Association says there are two types of bulbous onions: fresh onions (spring and summer) and storage onions (fall and winter). Fresh onions are high in water and sugar content. They don't store well and ought to be used soon after harvesting. Storage onions are stronger in taste, usually smaller, are "hotter" due to their higher sulfur content, and keep well.
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Both fresh and storage onions come in yellow, red and white colors. This is significant because colored onions have a chemical in their scales, phenol, that inhibits smudge and neck rot. Sizes can vary from 1/8 inch pearl onions to 6 pound giants. Bulb shape varies widely. The botanist Henry Jones created nine bulb shape classifications: globe, flattened globe, high-globe, spindle, Spanish, flat, thick-flat, Granex and top.
There are hundreds of onion types: yellow, white, red, and even brown storage onions; specialty sweet onions like Maui, Vidalia, Walla Walla, Arizona, and Texas Sweets; spanish onions, boiling onions, pearl onions and creaming onions. Everyone has their favorites.
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