Onions are a cool-weather crop. Because they require a relatively long growing season, they are usually started from sets, which are simply small onion bulbs. However, the selection of onion varieties available as sets is often limited, so if you want to try unusual varieties you'll have to start them indoors from seed. In general, the stronger-flavored onions keep longer, while mild, sweet varieties are more perishable.
Choosing a site to grow onions
Select a site with full sun and well-drained soil. Prepare the garden bed by using a garden fork or tiller to loosen the soil to a depth of 12 to 15 inches, then mix in a 2- to 4-inch layer of compost.
Choose onion sets that are 1/2 to 3/4 inch in diameter for best results. Plant your sets early in the spring -- onions do best if the temperature is cool when they start to grow, and warm as they mature. There's no need to make trenches or special holes for the sets. Just grasp them at the top (the pointed end) with the root end down and push them into well-prepared soil the full depth of the bulb. The soil should just barely cover the top of the onion sets. If you have some tiny sets, plant them at least an inch in the ground, so they get good contact with the soil. Space sets 4 to 6 inches apart.
If starting from seed, sow seeds indoors 2 to 3 months before your last frost date or sow seeds directly in the ground as soon as the soil can be worked in spring. You can also buy started plants or sets for spring planting. Set transplants 4 inches apart after all danger of heavy frost is past.
For mild winter areas of the South and West, grow short day varieties in fall and winter for a spring harvest.
Keep onion bed well weeded and water plants during the summer if rainfall is less than 1 inch per week. Contact your local County Extension office for controls of common onion pests such as root maggots.
How to harvest onions
Harvest bulbs when 1/4 of the tops have naturally fallen over and bulbs have a papery skin. Pull the bulbs and let them dry in a warm, airy, dark place for 1 to 2 weeks. Once dry, hang storage onions in mesh bags and place them in a cool, dark location.