Onions come in three types – Long, Intermediate and Short Day varieties.
Long Day require 14 or more hours of daylight to form bulbs. They are grown in Oregon, Idaho, Wyoming, Nebraska, Iowa, Illinois, Indiana, Ohio, Pennsylvania, New Jersey and state north of these.
Intermediate require 12 to 14 hours of daylight and are best grown in zones 5 and 6. They will grow well in zones farther north or south, however.
Short Day onions produce bulbs with 10 to 12 hours of daylight. They grow well in Southern California, Arizona, New Mexico, Oklahoma, Arkansas, Tennessee, North Carolina and States south of these.