Small immature onion bulbs, called onion sets, are planted in the spring to produce large onions in the fall. How you intend to use the onions determines the proper planting technique. Large sets planted closely produce tender green onions, while small sets planted deeply produce the best onions for fall harvest. Both require the same soil preparation and basic care.
Select a growing site that receives at least six to eight hours of direct sunlight per day. All day sun is preferred.
Till the soil to a depth of 8 inches and remove any rocks or other debris, as onions require loose, rich soil.
Test the soil to determine its pH level. Amend the soil with the ingredients recommended by the testing kit, if necessary, to reach a pH level of 6.2 to 6.8.
Sow onions in late spring as soon as the soil can be worked.
Plant onion sets to a depth of 2 inches and cover with soil. Space them 3 to 4 inches apart and firm the soil down with your hands. Plant the base of the onion set in the soil with the pointed end above the surface of the soil, spaced 1 inch apart, if you are planting fresh green onions.
Water thoroughly to moisten the soil, and allow it to dry between waterings. Watering to moisten the soil to a depth of 4 inches once a week provides sufficient water for onions.
Sidedress with 5-10-10 fertilizer when the shoots are 6 inches high and again when bulbs begin to increase in size. Follow the recommended application rate on the package. Sprinkle fertilizer along the row 4 inches from the base of the plants and work into the soil with a garden hoe.