Onions grown from seeds or seedlings get larger and keep longer than onions grown from sets.
Onions must be one of the most confusing vegetables to grow for new gardeners. Does it make a difference whether you grow them from seeds or sets? Yes, it does and here’s why.
Most onions are biennial, which means that they grow vegetatively the first year and bloom the second year. Onion sets are marble-sized bulbs that grew from seed the previous year. They can be planted in cool soil early in the spring, grow quickly, and naturally want to bloom and set seed in the summer. This makes onion sets ideal for scallions and fresh harvest throughout the summer. They don’t keep very long in storage, however, and the bulbs stop enlarging as the flower stalks develop. The variety selection is usually limited, too.
If you want to choose your own varieties and store big, solid onions for the winter, grow them from seeds or buy started seedlings. In the North, look for long-day varieties that begin forming bulbs when the day length is more than 14 hours. Southern gardeners grow short-day varieties that form bulbs when days are 12 hours or longer.
Sow fresh seed about ¼”- ½” deep in sterile soil in seed trays about 8 to 12 weeks before your transplant date in mid spring. To prevent them from forming bulbs too soon, give the seedlings no more than 12 hours of light a day. Keep the tops trimmed to 3-4” high and don’t allow the soil to dry out.
Transplant your own or purchased seedlings to a garden spot that gets full sun. Plant them about 4-6” apart with the roots just under the soil and the top of the bulbs exposed. You’ll find more information on growing onions and other root crops in our how-to article called Preserving the Harvest. For a fun fact sheet on cooking with onions and storing them, click here.