Sets are a common way to grow onions in other parts of the country, but most varieties that are available as sets do not perform well on the Gulf Coast. Sets are immature onions that have already begin to bulb. They have a tendency to bolt rather than continue bulbing. Virtually all the sets you can find will be for long day varieties. I have not personally seen sets available locally for short day varieties, but I have read that they are available in some places.
Locally, onion seedlings are often called “sets”. When you see a nursery or feed store advertise that “sets are now available”, they may be referring to seedlings. Make sure before you make the trip.
Sets that are less than 1/2″ in diameter sometimes don’t develop well, and those that are 1″ or more will sometimes bolt before they bulb, especially if they are exposed to deep cold after planting. Try to select sets that are as close to 3/4″ in diameter as possible. Make sure the varieties you select are short day onions.
Sets are very simple to plant. Just press the little bulb into the soil 1″ deep, pointed tip up, basal plate down. It is easy to determine the basal root plate of an onion set. Onions should be spaced 4″ – 6″ apart for full sized bulbs. You can plant them as close as 2″ apart if every other one is harvested as a spring onion, leaving the remainder to bulb fully.
Water well after planting. Keep the soil evenly moist until the onions are making new growth.
For more information see the Growing Guide for Bulbing Onions.