Many onion varieties will be sold as either short-day onions, intermediate-day onions, or long-day onions.
Most onion varieties begin to form bulbs when the temperatures and the number of daylight hours reach a certain level. Varieties listed as short-day onions will start to make bulbs when the day length is approximately 12 to 14 hours. Long-day onions begin to form a bulb when the day length is between 14 and 16 hours. Intermediate-day onions fall somewhere in between.
In the North, daylight length varies greatly as you move farther away from the Equator. Winter days are short and summer days are long. Long-day onions have a chance to produce lots of top growth before the day length triggers bulbing. If short-day onions are planted in the North, the onions will bulb too early and will be small.
Southern gardeners should plant intermediate-day or short-day onions. In the South, there is much less variation in day length between seasons. If long-day onions are planted in the South, they may not experience enough day length to trigger the bulbing process.