Planting dates for red onion sets are determined by temperature, photoperiod (day length), and onion variety. Red onions develop rapidly in cool weather, 10 degrees C to 15.6 degrees C, prior to the bulb development period. They require warmer temperatures, 21.1 to 26.6 degrees C, when bulbs are developing.
Short-day red onion sets, such as Early Red, are planted when days are 12 hours or fewer in length. Short-day cultivars are planted primarily in Southern states in early October.
Intermediate-day red onion sets, such as Southport Red Globe, are planted when days are 13 to 14 hours long. Intermediate-day varieties will not develop bulbs in Southern climates. They develop best in middle and northern states when planted in early spring.
Long-day red onion sets, such as Red Wethersfield, require 14 to 16 hours of light each day. Long-day cultivars are planted in northern states in early spring. Long-day varieties will not develop bulbs in southern climates.