Onions can be grown from seed, transplants (also called slips) or bulbs (also known as sets).
Planting from seeds gives you the greatest choice, but needs the longest time as many onions need 100 to 130 days to mature when grown from seed.
Transplants speed things up as you are buying immature onions but they are more expensive than seeds. Transplants are grown from seed to a scallion or green onion stage, when they are harvested and sold.
Bulbs or sets are faster still, often giving a harvest up to a month earlier than starting from seed. Sets are grown until the bulbs are about 3/4 inch in diameter, then harvested and allowed to dry down. They store well and grow quickly when planted, producing scallions in a few weeks. If left in the ground for about 2 months they grow into full sized onions.
Yellow Stuttgarter, White Ebenezer and Red Wethersfield onions are all long day varieties. They need 14 to 16 hours of daylight to form a good sized bulb and are typically grown in colder winter areas Zone 6 and colder is a good rule.
Long day onions are planted in the fall for scallions or green onions and early spring for a full size onion.
Sweet Candy is an intermediate day or day-neutral onion. It is adapted to a much wider range of day-lengths from north Texas to Maine and above. 12 to 14 hours of daylight will produce a good bulb. These can be grown in Zones 5 to 9.
Day-neutral onions are planted in fall for warmer climates and in early spring for colder climates for a large, sweet onion.