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Onion is a cool-season crop that will withstand moderate freezes. It may be grown by seeding directly in the field or by setting transplants. Onions grown from sets do not make the best bulbs, are rather costly, and are not recommended for commercial plantings.
Any fertile, well-drained, loamy soil, fairly high in organic matter with a pH of 6.0 to 6.5 is suitable for onion production.
Varieties are classified into groups according to when they will bulb. Regardless of when they are planted, varieties won't bulb until the days are long enough for them to do so. Shortday varieties that may be harvested in May or June include Granex-33, 1015Y, and any variety on the approved Vidalia variety list.
How To Plant
Method 1. Sow seed in early October about 1/2 to 3/4 inch deep, with about 15 to 25 seeds per foot of row (thinning necessary). This will require 3 to 4 pounds of seed per acre. Plant two rows spaced 12-14 inches apart on 38-inch beds (depending on equipment).
Method 2. Set transplants in December. Plants should be about 6 inches high and about the thickness of a lead pencil at time of transplanting. Set plants at a uniform depth, with the bottom of the plant about 1½ inches below the soil surface. Set three plants per foot of row. Plant two rows 12 to 13 inches apart on a 38-inch bed (depending on equipment).
Bolting, or seed stalk formation, occurs only from fall-planted onions grown through the winter for spring harvest. The size of the overwintering plant and the exposure to cold temperatures are the most critical factors in determining whether the plant will bolt. Early plantings in late August and September are more likely to bolt than are the later plantings in October and November. An extended warm period following planting produces a larger overwintering plant (more than ¼-inchshank diameter), which results in a high percentage of bolting when exposed to extended temperatures below 50 °F.
Apply 200 pounds of 0-46-0 fertilizer per acre to fall-seeded onions. Apply as a band 2 to 4 inches directly below the seed or transplant. When active growth begins in the spring, sidedress with 30 to 40 pounds of nitrogen and 75 pounds of potassium. Make two additional nitrogen sidedressings at the same rate at about 3-week intervals, or apply all P and K from soil test recommendation preplant. Sidedress with N when actively growing in the spring. Avoid N application within 3 weeks of harvest.
The seedbed is usually irrigated immediately after planting, and then as necessary to maintain a moist condition until emergence. Irrigate seedlings as soon as possible after transplanting. Irrigate 1 inch every week during bulbing if necessary, but do not irrigate when plants start to mature.
Weeds can seriously reduce yield. For preemergence weed control in onions, use 5 to 6 pounds of Prefar or 11 to 12 pints of oxyfluorofen (Goal) on broadcast basis. Prefar performs best when incorporated 2 to 3 inches deep with the soil before planting. Do not incorporate Goal. Apply it to the soil surface after planting and just before planting transplants. You can also apply Goal postemergent to onions during the growing season to extend preemergent weed control to harvest. Follow labels carefully when using any herbicide.
Diseases of onions include pink root, botrytis blast, downy mildew, purple blotch, white rot, and neck rot. Pink root is caused by a soil-borne fungus. Prevent problems by buying disease-free seed or transplants, growing in disease-free fields, and when possible, planting resistant varieties.
Leaf diseases include botrytis blast (numerous white specks), downy mildew (pale green, oval sunken spots), and purple blotch (purplish lesions with yellow margins).
White rot can be identified as a white fungal growth on the base of the onion. Apply control before seeding.
Neck rot occurs during or following harvest. The disease can be recognized by the grayish mold on the surface of the infected area. Bulbs that are well dried are less likely to succumb to the disease. They should be dried at 90 to 120 degrees Fahrenheit for 2 to 3 days and then stored at 32 °F. There should be adequate ventilation to maintain a dry atmosphere during storage.
Consult “Southeast U.S. 2010 Vegetable Crop Handbook” for more detailed information on pest control.
Onion thrips and onion maggots are important, widespread pests. Onion thrips are similar to other thrips. Onion maggots are larvae of a small fly that lays its eggs near the base of the plant. The small maggots burrow into the stem and bulb and kill the plants. Control both with pyrethroid sprays according to manufacturer's instructions.
Harvest when 75 percent or more of the tops fall over. To hasten drying, some growers use a subsurface knife to cut the roots a few inches below the bulb. The bulbs are then allowed to dry for a few days.
After this preliminary curing, the tops are cut 1 to 1½ inches from the bulb. This may be done by hand with shears or by a topping machine. Roots are trimmed off and the bulb placed in field sacks.
During extremely hot weather, it is desirable to place the onions into sacks almost immediately after pulling. These are then stacked in the field for curing. Such a practice avoids sunburn. The curing process is usually completed by keeping the bulbs for several weeks in slatted crates or trays. These are stacked in open sheds or in any place where there is free circulation of air.
Do not store onions permanently until thoroughly cured. Remove loose material such as dirt. Sort out all soft and immature specimens and bulbs with thick necks. These will not keep in storage.
Onions are graded according to size and quality. A high-quality pack is obtained by eliminating immature, decayed, sunburned, and mechanically injured bulbs, double bulbs, and bulbs that have started a second growth.
Buyers usually specify minimum sizes of the onions they will buy. This minimum is usually 2 inches in diameter, with bulbs greater than 3 inches bringing a much better price. The famed "Vidalia" onion is a Granex grown in low-sulfur soils near Vidalia, Georgia. The combination of low sulfur and favorable climate is responsible for the sweet taste. Mississippi onions can be of the same sweetness if the right variety is grown with no sulfur and no stress.
A 2007 trial of yellow Granex onion at Delta Branch Experiment Station yielded 20,000 pounds per acre of onions that were 2 or more inches in diameter.
Production Costs: Rough estimate per acre based on yield of 400 50-pound bags.
The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended of other products that may also be suitable.
By Dr. David Nagel, Extension Professor, Plant and Soil Sciences
Discrimination based upon race, color, religion, sex, national origin, age, disability, or veteran’s status is a violation of federal and state law and MSU policy and will not be tolerated. Discrimination based upon sexual orientation or group affiliation is a violation of MSU policy and will not be tolerated.
Information Sheet 1506
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. MELISSA J. MIXON, Interim Director