When growing onions in your vegetable garden you can plant seeds, sets or seedlings. Sets are tiny onion bulbs, harvested from onions grown from seeds the previous season. Planting sets is a popular method for growing onions, while planting seedlings or transplants usually will give the best results. Onions do well in a soil with a pH range of 5.5 to 7.0.
Schedule your onion planting for when the frost has past.
Acclimate the seedlings over a two-week period, gradually getting them accustomed to the outdoor environment. This process is referred to as "hardening" and involves setting the seedlings outside in a protected area, out of the direct sun for several hours each day. Over the two-week period, gradually expose the seedlings to more sunshine, and each day leave the seeding outdoors for a longer period of time.
Plant the seedlings in a row, spacing them 4 inches apart. Do not plant deep, just enough so that they are in the soil.
Water the onions once a week, with about 1 inch of water. Keep the soil moist. Because onions have shallow roots, they require consistent irrigation.
Fertilize the onions when the plants are about 6 inches tall and again when they are 12 inches tall. Do this by scattering 1 ounce of fertilizer for each 10 plants, along the sides of the row.