Onions are one of the most popular vegetables in the home garden. They can be grown for green onions and dry bulbs.
Onions are easy to grow. They perform best in well-drained, slightly acidic, fertile soils in full sun. Heavy soils can be improved by incorporating organic matter, such as compost, into the soil. Onions require higher fertility levels than most other vegetables. Apply 1 to 2 pounds of an all-purpose garden fertilizer, such as 10-10-10, per 100 square feet and till into the soil prior to planting. Four to five weeks after planting, sidedress with additional fertilizer. Sprinkle 1 pound of an all-purpose garden fertilizer per 100 feet of row. Place the fertilizer in a narrow band about 2 to 3 inches from the base of the onion plants.
An important aspect of onion development is photoperiod or day length. The photoperiod, along with temperatures, control bulb formation. The cool temperatures and short days of early spring promote leaf and root growth. Bulb formation begins when a certain day length is reached. Short-day onion varieties begin to form bulbs when they receive 11 or 12 hours of daylight, intermediate-day onions need 12 to 14 hours of daylight, and long-day varieties require 14 or more hours of daylight. Long-day varieties are the best choice for gardeners in Iowa and the upper Midwest. Short-day varieties in Iowa begin to bulb when the plants are small and produce small bulbs. Small bulbs can also be expected if long-day varieties are planted in late spring in Iowa. Onions may be grown from seeds, sets, and plants. The planting method selected is based on cost, use, availability, and planting ease.
Seeds. Growing onions from seeds may be the most difficult planting method. However, it is the least expensive. Germination may be sporadic, plant growth is slow, and weeds may be a problem. Plant onion seeds as soon as the ground can be worked in spring (late March or early April in southern Iowa, early to mid-April in central Iowa, and mid to late April in northern portions of the state). Plant seeds in rows 12 to 15 inches apart. Cover the seeds with one-half to three-quarters inch of soil. When the seedlings are 2 to 4 inches tall, thin the planting. For large, dry onions, plants should be spaced 2 to 3 inches apart after thinning. A full season of growth is needed for mature onions.
Sets. Sets are small onion bulbs that were grown the previous year, harvested, stored through winter, then distributed to garden centers in early spring. Specific onion varieties are usually not available. They are sold simply as red, white, or yellow onion sets. Since the variety is unknown, the flavor, use, and keeping quality of onions grown from sets vary considerably. Before planting sets, separate the bulbs into two size groups -- those smaller than a nickel in diameter and those larger than a nickel. The larger sets often bolt (produce a flower stalk) and don't produce good-sized bulbs. Use the larger sets for green onions. The smaller sets can be allowed to develop into mature onions.
Plant sets from early April to early May. Sets should be planted at a depth of 1 to 1.5 inches in rows 12 to 15 inches apart. For dry onions, plant the sets 2 to 3 inches apart. Sets grown for green onions can be planted closer together.
Plants. Plants are onion transplants grown in southern areas of the United States in winter, bundled into bunches of 50 to 100 plants, then shipped to garden centers in early spring. Onion varieties are available when purchasing plants.
Plant onion plants from early April to early May. Place plants 1 to 1.5 inches deep in rows 12 to 15 inches apart. To produce large, dry onions, space plants 2 to 3 inches apart.
Suggested onion varieties for home gardens in Iowa include 'Copra' (main season, yellow-brown skin, excellent storage), 'Candy' (yellow-brown skin, globe-shaped, short term storage), 'Red Burgermaster' (bright red, globe-shaped, good storage), 'Red Zeppelin' (deep red, globe-shaped, excellent storage), 'Stuttgarter' (flattened globes, light brown skin, excellent storage, from sets), 'Sweet Sandwich' (late season, yellow-brown skin, excellent storage), and 'Walla Walla Sweet' (late season, yellow-brown skin, short-term storage).