When you are shopping for onion varieties to plant in your vegetable garden, you will often see them listed as either short-day onions or long-day onions. Which ones to grow depend upon where you live.
Most onion varieties begin to form a bulb when the temperature and the number of daylight hours reach certain levels. Varieties listed as short-day onions bulb up when the day length is between 12 and 14 hours. Long-day onions, on the other hand, begin to form a bulb when the day length is between 14 and 16 hours.
Northern gardeners should plant long-day onions. In the North, daylight length varies greatly as you get farther and father away from the equator. Winter days are very short, but summer days are long. Long-day onions will have a chance to produce lots of top growth (hence produce bigger bulbs) before the day length triggers bulbing. If short-day onions were grown in the North, the onions would bulb up too early and they would be small by comparison.
Southern gardeners should plant short-day onions. In the South, there is much less variation in day length between seasons than up North. If long-day onions were planted in the South, they may not experience enough day length to trigger the bulbing process.