Plant sets 2 to 4 weeks before the average last-frost date (your county’s cooperative extension service can tell you when this is). In mild-winter climates, plant onion sets in fall or winter. Place the sets in a shallow furrow and cover with just enough soil to leave their pointed tips at the soil surface. The spacing between onions should eventually be 4 to 6 inches, depending on the mature size of the variety, but you can place the sets closer together initially and harvest thinnings for use as green onions.
A disadvantage of relying on sets is the limited choice of varieties. Most garden centers label their bins of sets by color (white, yellow, or red) instead of by cultivar. You might be tempted to pick out the largest sets from the bin, but these can go to seed quickly instead of forming a large bulb. Sets that are 1⁄2 inch in diameter—about the size of a dime—are the best buy.
Transplants. Bundles of bareroot onion transplants are available from mail-order retailers in winter and early spring. With a greenhouse or indoor light setup, you can also produce your own. In the north, sow seeds in a flat of seed-starting medium 8 to 10 weeks before the last frost, to be planted outdoors about 6 weeks later. In the south, time the seedlings to be set out in fall or winter. Thin the emerging seedlings in the flat to stand at least 1⁄2 inch apart in rows about 2 inches apart so they will be easier to separate at transplant time.
Harden off the seedlings by setting the flat in a sheltered spot outdoors for a few hours a day, gradually acclimating them to a full day of direct sunlight. On the day they are to be planted in the garden, lift the seedlings carefully from the flat and shake the soil from their roots. If the tops have grown tall and wispy, trim them back to about 6 inches.
Dig a trench for the seedlings and place them slightly deeper than they were in the flat. As with sets, seedlings can be planted closer than their ultimate spacing of 4 to 6 inches, with the extras harvested as green onions.
Seeds. To grow the biggest bulbs, onions benefit from the head start they get from sets or transplants. But bunching onions or scallions are quicker to mature, and they can be seeded directly into the garden. Sow seeds outdoors beginning about a month before the frost-free date and then again every few weeks through fall for continual harvests; in the south, the season is fall through spring. Start with fresh seeds, or seeds that are no more than a year old, because onion seeds lose viability quickly in storage.
For most onions, the lengthening days of late spring are the trigger that tells them it’s time to transition from growing leaves and roots to the business of forming bulbs. The types of onions that succeed in northern states, with their extra daylight hours, are different from the onions grown in the south. Seed packets and catalog descriptions should reveal which varieties are intended for short-day regions (those that begin to form bulbs when day length is only 10 to 12 hours), intermediate-day regions (12 to 14 hours), or long-day regions (14 to 16 hours). A few onions are considered day-neutral and can be grown anywhere. Dixondale Farms, which grows and sells onion transplants, provides this map, right, to its customers to aid in variety selection. Note the area of overlap between long-day and intermediate-day regions, where varieties of either type can be grown, and a similar overlapping area for intermediate-day and short-day varieties. Canadian gardeners should select long-day varieties.
Photos: Patrick Montero