When leafing through a garden catalog, an onion variety was described as a long-day type. What is meant by the term long-day?
Onion bulb formation begins when a certain day length is reached. Short-day onion varieties begin to form bulbs when they receive 10 to 12 hours of daylight, intermediate-day onions need 12 to 14 hours of daylight, and long-day varieties require 14 or more hours of daylight.
Bulb size is largely determined by the number and size of the leaves at bulb initiation. The larger the tops (foliage area) at bulb initiation, the larger the bulbs will be. (The size of onion bulbs also depends on weather, soil conditions, and other factors.)
Long-day onion varieties are the best choice for gardeners in Iowa and other states in the upper midwest. Short-day varieties generally produce small bulbs when grown in northern areas because of the small amount of foliage present at bulb initiation. Long-day varieties, however, are able to produce large tops before bulb initiation occurs. As a result, long-day onion varieties typically produce the biggest bulbs.