Onions are grown from either seed or sets. The sets are actually small, immature onion bulbs that are stored for later planting. Sets go to seed much more quickly than seed-grown onion plants, but they also produce usable-sized bulbs much sooner than seed-grown onions. Sets are often grown for green onion production instead of bulb production. While sets are readily available for purchase, have more control over what variety of onions are available for your garden by growing your own onion sets each summer for the following spring's garden season.
Spread 2 inches of mature compost over a well-drained, full-sun garden bed then work the compost into the top 6 inches of the soil. Prepare the bed for planting approximately three months before your area's first expected fall frost.
Sow the onion seeds on the soil surface, spacing them ¼ inch apart in all directions. Press the seeds lightly into the soil with your hands then cover them with a ¼ inch layer of soil.
Mist the soil surface with water to moisten. Water daily as needed to keep the soil moist but not soggy until the seeds germinate. Water once every five to seven days once they sprout, or often enough to keep the top 6 inches of soil moist.
Dig up the sets once the tops yellow and fall over, usually around the time of the first fall frost. Spread the bulbs out in a well-ventilated, shaded area for 24 hours to dry. Set onions are about 1 inch in diameter.
Cut off the remaining foliage and brush off the roots from the bottom. Place the sets in a mesh bag and store them in a cool, dry area until you are ready to plant them in spring.