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.
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