Onions are one of the most popular vegetables in the home garden. They are also easy to grow. Onions are grown for green onions or dry (storage) bulbs.
Onions perform best in well-drained, slightly acidic soils. Heavy clay soils can be improved by incorporating organic matter, such as compost, into the soil. Onions also require six or more hours of direct sun each day.
Onions may be grown from seeds, sets, and transplants. The planting method selected is based on cost, use, availability, and ease of planting.
Prior to planting, apply 1 to 1½ pounds of an all-purpose garden fertilizer, such as 10-10-10, per 100 square feet. Incorporate the fertilizer into the top several inches of soil.
Growing onions from seeds is the least expensive planting method, but can be rather difficult. Germination may be sporadic, plant growth is slow, and weeds may be problematic. 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). Cover the seeds with ½ to ¾ inch of soil.
Another option is to sow seeds indoors and then transplant the onion seedlings outdoors in spring. Sow onion seeds indoors 8 to 10 weeks before the intended outdoor planting date.
Sets are small, dormant bulbs that are planted directly into the garden. Sets are typically sold as red, white, or yellow types with no distinction as to cultivar. Since the cultivar is unknown, the flavor and keeping quality of set-grown onions can 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 don't produce good-sized bulbs as they often bolt (produce flower stalks). 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 1½ inches.
Onion plants or transplants can be purchased from garden centers and mail-order retailers. Specific cultivars are available. Plant onion transplants from early April to early May. When planting, place the roots and lower white portions of the plants below ground level.
Onions grown from seeds should be thinned when the seedlings are 2 to 4 inches tall. For large, storage onions, seedlings should be spaced 2 to 3 inches apart after thinning.
Onion sets and transplants should be spaced 1 inch apart when grown for green onions and 2 to 3 inches apart when grown for mature, storage onions.
In the home garden, onions are commonly planted in rows spaced 12 to 15 inches apart.
Suggested onion cultivars for home gardens in Iowa include 'Candy' (yellow-brown skin, globe-shaped, short-term storage), 'Copra' (yellow-brown skin, round, excellent long-term storage), 'Ebenezer' (yellowish brown skin, flattened globe, good long-term storage), 'Red Burgermaster' (bright red skin, globe-shaped, good long-term storage), 'Redwing' (dark red skin, globe-shaped, excellent long-term storage), 'Red Zeppelin' (deep red skin, globe-shaped, excellent long-term storage), 'Stuttgarter' (light yellow-brown skin, flattened globe, excellent long-term storage, grown from sets), 'Sweet Spanish' (white or yellow skin, globe-shaped, short-term storage), and 'Walla Walla' (yellow-brown skin, flattened globe, short-term storage).