Onions planted from seed are used as green onions within 40 to 50 days of planting. They are fully grown in 90 to 130 days, depending on the variety. Onions planted as soon as the soil can be worked in early spring are ready for the table by July. Successive plantings throughout the spring produce fresh onions all summer long. An abundant harvest of fully grown onions can be stored to last the winter.
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Onion types are referred to as “short day” or “long day” onions, depending on the length of daylight needed for the plant to start making bulbs. Short day onions are well-suited to southern climatic conditions where they have 10 to 12 hours of sunlight during the summer. Long day onion varieties need 14 to 16 hours of sunlight to produce bulbs, and they are more suited to northern gardens. Gardeners in mild climates plant both types of onion seed in the fall as well as spring.
Onion Growth Cycle
The "Texas Early Grano" onion is a variety that is fully grown 50 to 75 days after it is planted. Onions produce best in temperatures between 55 and 75 degrees Fahrenheit, allowing plants to develop foliage and roots before bulbing starts. The higher temperatures of late spring and summer then help bulbs to develop. Onions need a constant supply of moisture for the best growing results. The size of the onion bulb is determined by the number of green leaves it has. Each leaf indicates one ring of the bulb. They are fully grown when the leaves have fallen over.
Heirloom variety onions have the same growth cycle as hybrid varieties. They are types that were introduced in previous centuries and are not grown in modern large-scale agriculture. "Siskiyou," "Red Weathersfield" and "Rossa de Milano" are long to intermediate day heirloom onions that are fully grown in 110 days. Heirloom onion seeds are organically grown and non-genetically modified (GMO). Home gardeners are able to use saved heirloom onion seed from each year’s planting by allowing some onion plants to remain in the ground until they produce flowers and seeds. Hybrid onion variety seeds do not grow true to type the following year.
Fully grown onions are harvested when the tops fall over and dry out. Dig around the bulb carefully to loosen dirt without damaging the bulb. The roots are clipped and tops cut to within 1 inch of the onion top. They may be left on the ground to dry for a day or two before using or preparing for storage. Onions must be cured before storing for the winter. Place them in a well-ventilated warm area until the necks thoroughly dry out. They are now ready to store for the winter.