Read about tricks for bulbing onions HERE
Onions start to bulb up when they receive specific amounts of daylight each day. Since days are longer in the north and shorter down south, they need different types of onions. This map shows the zones for short, intermediate, or long-day onions. The areas overlap--grow long-day types in the blue and green zones; short-day in the peach zone, and intermediate-day onions in green, yellow, and orange areas. Illustration courtesy of Organic Gardening.