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Index : Vegetable Gardening
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- 15. Plant Onions Now for Spring Harvest - Top
February is the month for onion planting here in Tucson. We are lucky that our Winters are mild and conducive to growing the sweet onions most people refer to Vidalia onions.
Vidalia and other sweet onion types, such as Texas Grano, Granex, and Sweet Spanish are short-day types. When grown from seed they are planted in the fall. But, it's much easier to purchase and grow small onion plants, from mail order garden plant catalogs. These "transplants" can be set out in the garden in February and harvested in May. Don't worry about freezing temperatures. Onions are hardy down to 20 degrees F.
Onions prefer to be planted on a bed mounded or raised up a 3 or 4 inches high and about 20 inches wide. Two rows of onions, spaced 10 inches apart, can be planted down the bed. As with other vegetables, the bed should be in a location receiving full sun. The furrow should be about four inches deep. In the bottom of the furrow, scatter in a fertilizer rich in phosphorous, such as ammonium phosphate (16-20-0). Evenly spread one-half cup of fertilizer for every 10 feet of row. Phosphorous will greatly increase the growth and size of the onions.
Do not, however, add sulfur to the soil! Although other vegetables benefit from the mixing of sulfur into the soil, it will cause sweet onions to turn hot!
Before planting the onion plants in the furrows, cover the fertilizer up with a two inch layer of soil. When onion plants arrive, plant them immediately. Set plants in the ground approximately one inch deep with a four inch spacing between plants. Firm the soil around the base of each plant, then water. Continue watering to keep the soil moist. The closer to harvest, the more water will be required.
Onions also require regular fertilization with nitrogen. Use a nitrogen source such as ammonium nitrate, ammonium phosphate or urea. Do not use ammonium sulfate, because it contains sulfur that can cause the onions to be hot. Unfortunately, organic sources of nitrogen and phosphorous are not effective in the cooler months of February and March.
Apply one-half cup of a nitrogen-containing fertilizer for every 10 feet of row. Scatter the fertilizer and lightly scratch it into the top of the soil. After fertilizing, apply water to soak the fertilizer into the soil. The first application of nitrogen should be made about three weeks after planting. Continuing fertilizing every 2 to 3 weeks. Discontinue fertilizing by mid-April, or when the neck of the onion (just above the bulb) starts feeling soft. Each fertilization should be followed with a thorough watering.
As the onion bulb develops, loosen the soil around the expanding bulb to allow it room to grow. Be careful not to cut too far into the soil or too close to the bulb. It's important not to damage the bulb during cultivation. As it grows, the bulb will expand up and out. Do not cover the bulb with soil. It should be exposed above ground. At the time of harvesting, nearly half of the bulb may be above the soil.
As onions mature, the leaves will begin to fall over at the base. As they do, reduce watering. The tops (leaves) will start turning brown. The onions are now ready to harvest. Because of their high moisture content, sweet onions will not store as long as other types of bulb onions. They will however, keep for a month or two if properly stored. Onions should remain dry and not touch one another. The best way to accomplish this is by placing them in a nylon stocking. Tie a knot between each onion to keep them apart. Hang the stocking in a cool, dry location. When your ready for one, simply cut off the bottom onion from the one above. Another way to store onions is to wrap them separately in foil and place them in the refrigerator.
Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
- Updated: February 2, 2003
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