Onion bulb formation begins when a certain day length is reached. Short-day onion varieties (cultivars) begin to form bulbs when they receive 10 to 12 hours of daylight, intermediate-day onions need 12 to 14 hours of daylight, and long-day cultivars require 14 or more hours of daylight.
Bulb size is largely determined by the number and size of the leaves at bulb initiation. The larger the tops (foliage area) at bulb initiation, the larger the size of the mature bulbs. (The size of onion bulbs also depends on weather, soil conditions and other factors.)
Long-day onion cultivars are the best choice for gardeners in Iowa and other states in the upper midwest. Short-day cultivars generally produce small bulbs when grown in northern areas because of the small amount of foliage present at bulb initiation. Long-day cultivars, however, are able to produce large tops before bulb initiation occurs. As a result, long-day onion cultivars typically produce the biggest bulbs.
Suggested onion varieties (cultivars) for home gardens in Iowa include:
Using a well-drained growing medium, sow onion seeds in flats approximately eight to 10 weeks before you intend to plant them outdoors. As soon as the seeds germinate, place the onion seedlings under artificial lighting. An ordinary fluorescent shop fixture, containing one 40 watt cool white and one 40 watt warm white tube, works fine. The light fixture should be placed no more than 4 to 6 inches above the seedlings. Growing temperatures should be 60 to 65 degrees Fahrenheit. Insufficient light and temperatures above 65 F promote spindly growth.
Thin the onion seedlings (within two to three weeks of germination) if the plants are crowded. Harden or acclimate the onion seedlings outdoors for seven to 10 days before planting into the garden. Initially, place the plants in a shady, protected location. Then gradually expose the plants to longer periods of sunlight. Bring the seedlings back indoors if nighttime temperatures are forecast to drop to 32 F or below.