Short-day onions produce bulbs where the amount of sunlight is approximately 10 hours, which is the amount of sunlight the southern US receives in the summer. The earlier short-day onions are planted, the larger the bulb. In the southern US, plant during the winter or early spring (110 day maturity) and in the northern US, plant in late spring (75 day maturity).
Intermediate-day onions (or day-neutral onions) produce bulbs where the total amount of sunlight is approximately 12 to13 hours each day. These type onions are know for their sweetness and mature in approximately 110 days.
Long-day onions produce bulbs where the total amount of sunlight is approximately 14 to 15 hours and are best suited for states in the northern US.