The onion grows its top in cool weather and the bulbs in warm weather.
Onions are sensitive to the length of day light and are divided into short day and long day types.
Be sure to use the variety right for your area. Long day onions are grown in northern areas of the U.S. where they get 14-16 hours of light. Short day varieties are grown in southern areas where by early summer the receive 12 hours of light. Onions require very fertile soil. Work manure and fertilizer into the soil before planting.