Waiting too long to plant onions from seeds does not mean you have to miss out on planting onions. Selecting onion sets and storing them until ready to plant will give you a head start. Some interchange onion sets with onion transplants, but the transplants refer to older onion plants between eight and 10 weeks for growing large onions while the sets have bulbs less than 1 inch wide for smaller mature onions. When looking for sets, opposites reign. Smaller bulbs at the bottom of sets grow best into larger onions while sets with large bulbs should be used for green onions as they may flower before producing a large bulb. To keep your sets healthy for planting you must properly store them until ready for growth, usually between March and April.
Remove the onion sets from their container and separate them into two piles: those with bulbs larger than a dime and those with smaller bulbs.
Set a window screen on top of four bricks, each placed under one of the corners of the screen.
Lay the two piles of onion sets on top of the screen in a single layer with at least one inch of space around each plant to allow for air circulation.
Keep the onion sets in a cool, dry and dark place such as a basement or cool pantry for up to three weeks, or until ready to plant.