Onions can be grown from seed, bulb transplants or bulb sets, with seeds being the most cost-effective way to raise onions in the garden. In the northern areas of the United States, onion seeds are started indoors in the late winter and transplanted to the garden in the early spring. In the Deep South, onion seeds can be sown directly in the garden in the late fall.
Place 1 to 2 inches of potting soil in each seed planting flat tray that you plan to use for onions.
Water the soil until it is moist to the touch.
Sprinkle onion seed, which is very tiny, evenly over the surface of the moist soil.
Press the seed into the soil using your fingers. There is no need to cover the seed, as you only want to give the seed good contact with the soil.
Place a single layer of plastic wrap over the seed flat tray and set the flat in a warm area.
Remove the plastic wrap from the tray when the majority of the seeds have germinated.
Water the tray as needed to keep the soil moist. Thin the seedlings to a single plant every 1/2 inch in the tray as the seedlings grow.
Plant the seedlings in the garden as soon as the soil can be worked in the early spring. Space the seedlings 3 to 4 inches apart in rows 12 inches apart. For scallions, plant the seedlings 1 1/2 to 2 inches deep; for large onions, plant approximately 1/2 inch deep.