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When to plant red onion sets?

Updated July 19, 2017

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 Varieties

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 Varieties

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 Varieties

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.

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About the Author

Barbara Barker has been writing about gardening since 1999. She is a food gardening consultant and the owner of The Gourmet Gardener. The author of "Container Gardening for Health," Barker also writes for several blogs and on her website. Barker has a bachelor's degree in English and is a certified Master Gardener in Florida.