Onions fit into three categories: short-day, intermediate-day and long-day varieties. Gardeners in plant hardiness Zone 7 and south will succeed best with short-day onion varieties. Zones 5 & 6 will probably be able to grow these varieties, too, if onions are planted in late winter instead of in the fall. Intermediate-day varieties will work for most gardeners in Zones 5 & 6 while long-day varieties will succeed in Zone 6 and colder zones. There is obviously some overlap with varieties that require shorter days being adapted to climates where much longer days occur. The key factor here is whether these varieties have sufficient time to make enough growth before lengthening days signal the bulbing response, and of course, the days have to get long enough to signal the response.
Source: The Cook’s Garden