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.
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. All rights reserved.