Onions are one of the oldest vegetables in continuous cultivation dating back to at least 4,000 BCE. The ancient Egyptians are known to have cultivated this crop along the Nile River. There are no known wild ancestors, however, the center of origin is believed to be Afghanistan and the surrounding region. Onions are among the most widely adapted vegetable crops. They can be grown from the tropics to subarctic regions. This adaptation is primarily due to differing responses to day length. Unlike most other species, day length influences bulbing in onions as opposed to flowering. Onions are grouped into three groups based on their response to hours of day length. The short-day varieties bulb with daylengths of 10-13 hours, intermediate varieties bulb with day lengths of 13-14 hours and are found in the mid-temperate regions of this country. Finally, long-day onions are adapted to the most northern climes of the United States as well as Canada and bulb with daylengths greater than 14 hours.