Most onions are biennial vegetables, which means they need two full years to produce a harvest. The first year onions grow their leaves and vegetation, then the second year they'll produce flowers or bulbs. Part of the allium family, onions are closely related to garlic, chives and leeks. According to Texas A&M, onions are grouped into "long-day" and "short-day" varieties based on when they begin forming their bulbs. Long-day onions will start forming bulbs once the days are long enough to produce 14 to 16 hours of sunlight, and short-day varieties will begin forming bulbs when there's at least 10 to 12 hours of sunlight available.
Onion sets are produced by onions that were grown from seed the year before. These can be purchased from catalogs or nurseries, and you may find them at local grocery stores as well.
Your best results will come from buying onion sets that are marble sized and not yet sprouting.
When outdoor temperatures are about 48 degrees, choose a planting space that will get a minimum of 6 hours sunlight each day. Plant the onion set 1 1/2 to 2 inches into the ground with the pointed side up, spaced about 4 inches apart and add enough water to moisten the soil completely without saturating.
Choose a sunny, well drained location in the garden and plant your onion seeds 1/2 inch deep.
Water the newly planted seeds thoroughly. Be sure to add at least 1 inch of water per week throughout the growing season.
Onions can be harvested at any time, but when the white part of the stalk is about as thick as a pencil is a good indicator they can be harvested.
Start your onion seeds indoors 8 to 12 weeks before the date of last frost for your region.
Harden off the plants once the danger of frost has passed, by putting the seedlings outside for short periods of time over the course of two to three weeks.
Plant the seedlings 4 feet apart with just enough soil to cover the roots loosely. Keep them well watered with at least 1 inch of water per week.