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, such as compost or peat, into the soil. Onions require higher fertility levels than most other vegetables. Apply 1 to 1 1/2 pounds of an all-purpose garden fertilizer, such as 10-10-10, per 100 square feet prior to tilling. 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. Photoperiod, along with temperature, 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 will begin to bulb when the plants are small and not produce large bulbs. Small bulbs can also be expected if long-day varieties are planted in late spring in Iowa.
Onions may be grown from seed, sets, and plants. The planting method selected is based on cost, use, availability, and planting ease.
Growing onions from seed 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 seed as soon as the ground can be worked in the spring (late March or early April in central Iowa). Plant seed in rows 12 to 15 inches apart. Cover the seed with 1/2 to 3/4 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 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 varies 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 seedstalk) and don't produce good dry bulbs. Use the larger sets for green onions. The smaller sets can be allowed to develop into mature onions.
Plant sets in early spring as soon as the ground can be worked. Sets should be planted at a depth of 1 to 1 1/2 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 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.
Select healthy green transplants and plant them 1 to 1 1/2 inches deep in rows 12 to 15 inches apart. To produce large, dry onions, place the plants 2 to 3 inches apart. Plant them as soon as the ground can be worked in the spring.
Suggested onion varieties for home gardens in Iowa include Norstar (early season, short term storage), 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), Southport Red Globe (purple-red skin, globe-shaped, medium storage), and Sweet Sandwich (late season, yellow-brown skin, excellent storage).
Onions, like most vegetables in the garden, require one inch of water per week. During dry weather, water onions weekly. Control weeds in the garden to reduce competition for water, nutrients, and sunlight. Shallow hoeing, hand pulling, and mulches are the most practical control measures for home gardeners. Mulches also help conserve soil moisture.
This article originally appeared in the April 7, 2000 issue, pp. 28-29.