White onions are a common choice for backyard vegetable gardens due to their ease of growth and the fact that they thrive throughout most of North America. They can be grown from seed or from transplants, but growing onions from onion sets, which are just small, immature onions, is the simplest method of obtaining an onion crop. If you do not want to purchase sets, you can start your own by seeding onions the previous year and harvesting them when they are about the size of a marble. They must then be dried and stored for early spring planting.
Prepare the soil for your onions. Send a soil sample to your local extension office to find out if it is lacking any nutrients and if the pH is correct. Onion sets prefer soil with a pH of 5 to 6.5. Add in compost or fertilizer, and add lime as directed by the manufacturer to adjust pH appropriately, if needed.
Rake the soil to loosen it and break apart any clods or clumps. Heavy soils like clay may benefit from added loam or garden mix to lighten them up and help them hold moisture.
Walk over your onion patch prior to planting to tamp the soil back down. Onion sets prefer a firmed soil so their small roots can take hold more easily.
Sort your sets into two groups: those with larger than a dime diameter and those that are less than a dime in diameter. Use the larger group for green onions, since they will put up a larger bolt very quickly, making them unsuitable for lengthy storage. Smaller sets grow larger onions that dry well and store for longer periods since they mature more slowly. If desired, you may also sort according to shape. Round sets grow flat onions, elongated sets grow round onions.
Sink each onion set up to 2 inches into the soil, spaced 3 inches from one another. Plant with the onion's point facing upward.
Firm the soil over and around the set.
Weed and cultivate your onions often; they do not compete well with other plants. Weeds will quickly choke out your onion patch, so pull or hoe frequently to prevent them from foiling your hard work.
Water you onions just enough to keep them moist as they grow.