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Larry A. Sagers Regional Horticulturist Utah State University Extension Thanksgiving Point Office
Utah produces some of the best onions in the world. Travel north into Davis, Weber and Box Elder counties and you will see onion growers planting onions as soon as the weather allows them on their fields. Following a few ideas from them will allow you to have some excellent onions that are large and tasty.
Unfortunately, many gardeners do not have success because they choose the wrong varieties. Onions are day length sensitive. That is, the length of the day determines the size of the bulb. In the Mountain West, we use long day varieties that start bulbing when day lengths reach 14 16 hours and temperatures rise above 65 degrees Fahrenheit. Cold temperatures after early planting encourages seed stalk development.
Prior to this triggering threshold, the onions develop leaves. The secret to large onions is to plant them early in the spring to allow growth as early as possible before producing bulbs. Short day onion varieties, though often sold in Utah, will not produce good bulbs. Utah Yellow Sweet Spanish, Sweet Sandwich or Walla Walla are good varieties. For green onions use Evergreen White Bunching or Welsh Onion. (Most sets available at garden centers.)
Onions have a very poor root system and need an even supply of water and nitrogen fertilizer to produce well. Control weeds and improve the sweetness and size of the bulbs by using a thick organic mulch to cool the soil and conserve moisture.
Plant your onions in a sunny area as early in spring as soon as soil is workable. Optimal temperatures are between 45 60ƒµF. during development, and 60 75ƒµF. during bulbing and curing.
Onions are heavy feeders so apply 2 lb. of 16 16 8 (or similar fertilizer) per 100 sq. ft. Add 2 cup of ammonium sulfate (21 0 0) per 10 ft. of row 4 to 5 weeks after planting. Repeat this application 4 or 5 weeks later. Water immediately after fertilizing.
Plant sets 1 to 2 in. apart and thin them by harvesting. Place transplants 5 to 6 in. apart giving enough room for proper bulb growth. Onions can be planted in rows or wide beds so long as spacing is maintained.
Onion seeds are small so cover them lightly (3 in. deep). When using sets, firm them into the soil, root down, so that the set is just covered with soil. Amend your soil before planting because onions grown in soil containing a high percentage of organic matter germinate and emerge better and produce larger bulbs.