Onion is a coolseason vegetable that can be grown successfully throughout most of temperate North America. Onions may be grown from sets, transplants or seeds.Onions start bulb formation when the day length is of the proper duration and different varieties of onions require different day lengths to initiate bulbing. In general, most common varieties fall into one of two classes, longday for northern latitudes and shortday for southern latitudes. For this reason, onion varieties that are grown in the South are not adaptable to the North and vice versa. Late plantings of the suggested varieties also result in small bulbs or lack of bulbing altogether in any location.

Website: http://www.urbanext.uiuc.edu/veggies/onion1.html