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Chill Hours

It might sound like the time that you spend with friends, listening to music and enjoying a glass of homemade apple cider. Actually, "chill hours" refers to the amount of time an apple tree needs to spend below 45°F in winter in order to be productive the following season.

There’s some tricky science behind all this, but the upshot is that fruit-bearing trees need to stay cold for a certain amount of time in order for the hormones responsible for dormancy to break down. Only when these hormones break down will the tree bloom, and a tree must bloom in order to produce fruit. The “chill hours requirement” is the number of hours a specific variety requires to complete its dormancy cycle. As you may have guessed, different species and varieties have different requirements, but the vast majority of apples have a requirement that is in the area of 600-800 hours.

Fortunately, the majority of Americans do not need to worry about chill hours because we live in climates where the winter is long enough and cold enough that pome fruits can safely complete their dormancy cycles. However, those of us who live in growing zones 8 and above will need to be attentive to chill hours. I’m talking to you, Alabama, Louisiana, and Nevada. In warm-winter climates, growers will need to shop for trees that have a low chill hours requirement.

Fortunately, some varieties have either evolved or been bred to have very low chill hours requirements, and these varieties are productive in warm-winter climates. At Cummins Nursery some of the apples that we sell with low chill hours requirements are Anna (bred in Israel), Sundowner (bred in Australia), and Kandil Sinap (first known from Turkey).

For more information on warm-winter apples, see Low-Chill Apples (PDF) from the University of California Cooperative Extension.

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