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By Tim Coolong
UK Vegetable Specialist
Although we’re in the midst of winter, it is never too soon to think about next growing season. This is particularly true if you want to grow onions.
Onions are a good crop for Kentucky farmers. Typically in late winter, it takes eight to 10 weeks to produce a reasonably sized transplant. If you want to plant in late March or the beginning of April, you should have seeded your transplants in late January.
If you have not already seeded, it’s not too late for an onion crop this year. You can purchase transplants. Best production is obtained when cool temperatures (55 F to 75 degrees F) prevail over an extended period of time, permitting considerable foliage and root development before bulbing starts.
An important aspect of onion development is the length of day or photoperiod. Day length, along with temperature, controls when the onions form bulbs. Onions can typically be categorized into short, intermediate and long day types. Be sure to buy intermediate day-long or day-neutral onion varieties. Short-day varieties need 11 to 12 hours of sunlight per day to bulb, intermediate-day varieties bulb with 12 to 13 hours of sunlight, and long-day varieties bulb with 13 to 14 hours.
Intermediate or day-neutral onions will grow anywhere.
The types of onions grown during the winter in the Deep South, such as Vidalias, are short-day types. Unfortunately, Kentucky is far enough north that if planted here short-day onions will bulb very early in the spring when plants are still small. The result will be small golf-ball or tennis-ball size bulbs.
Instead, try to buy intermediate-day types such as the popular Candy onion. These will begin to bulb in mid-May and should mature by early- to mid-July.
Many growers report bulbs the size of softballs when growing Candy onions. If you decide to plant a long-day variety, such as Walla Walla or Sweet Spanish, your plants will begin to bulb in June and continue until early August when they will be mature.
Although long-day types yield well in Kentucky, high summer temperatures may lead to an increased disease risk.
Onions grow best in a loose, well-drained soil of high fertility and plenty of organic matter. Avoid heavier soils such as clay and silt loams unless 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 of a 1-2-2 ratio (5-10-10, for example) are good for onion production.
As the onion plant’s root system is very limited, high soil fertility is essential for good production.
After the plants are well-established, mulch will conserve soil moisture, prevent soil compaction and help suppress weed growth.
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.