Growing onions from seeds gives you a larger variety of onion types to choose from as opposed to planting small onion sets. In most areas, start onion seeds indoors, then later move them out to the garden in order to give them enough time to reach full harvesting size. The variety of onions you plant depends on where you live and the length of your average day. In areas with 12-hour days in summer, plant short-day-length seeds; in areas with more than 12 hours of summer sunlight, plant long-day-length onion seeds.
Fill seed-starting pots or cell-packs with a quality potting medium. Pre-moisten the medium prior to planting by watering until the excess moisture drips from the bottom holes on the pots.
Sow three seeds per pot or cell, sowing them to a 1/4-inch depth. Cover with a layer of plastic wrap, and place in a room with a temperature of 65 to 75 degrees Fahrenheit to germinate.
Remove the plastic once seeds germinate, approximately seven to 10 days after planting. Place onion plants in a sunny window, and water as needed to keep the soil moist.
Trim off the onion tops once they reach 4 inches tall. Continue to trim and maintain this length until transplanting outside; this encourages bulb production instead of flowering.
Transplant onion seedlings outside once all danger of frost has passed in your area. Set them outside during the daytime for one week prior to planting so they can become accustomed to outdoor conditions.
Plant seedlings 4 inches apart in a well-drained, full-sun garden bed. Sow each seedling so the bottom roots lie just beneath the soil surface and the developing bulb sits even with or slightly above the soil.
Water plants once weekly, providing approximately an inch of water per plant per week. Harvest the onions once the foliage begins to yellow and fall over.