Unlike other vegetables, onions aren't always planted from seed. Instead, they are grown from small onion bulbs called sets. Selecting the proper sets for both your climate and your desired product is vital. Some sets are better suited to producing green onions; the edible onion stalk is used as a garnish and to add flavor in many recipes. Other sets produce mature onion bulbs. The day-length requirement is also vital because it indicates whether the onion variety will grow well in your area.
Choose long-day varieties of onions if you live in the north and short-day varieties if you live in the south. The north has longer summer days than the south, so varieties that require 14 to 16 hours of sun a day thrive there, while the 12-hour short-day types thrive in the shorter days of the south.
Purchase ¾- to 1-inch diameter sets for growing green onions. Choose ½- to ¾ -inch diameter sets for growing full onion bulbs.
Inspect the sets for signs of rot or disease. Look for soft spots, mold and cuts in the side of the sets. These sets may rot in the ground before they have a chance to grow.
Buy sets from onion varieties that are labeled as slow to bolt. Onion sets are prone to bolting, or flowering, which inhibits the growth of the onion bulb.
Plant only sets that are firm. Dispose of any that have already begun to sprout, because these may rot in the ground, especially in wetter regions.