Chill hours are defined as the number of hours below 45F (7C) but above 32F (0C) that your tree will experience in a given calendar year. Most apples trees need at least 1,000 chill hours each year. That’s around 42 days or 11% of the available days or hours in a given year. However, you need half the year for growing so that means you need around 22% of the available winter hours to be cold but not too cold. You can get trees that need less chill hours, Anna is an apple commonly marketed in many warmer climates like Florida or Central America. It is known to only need about 300 chill hours per year. Many of the low chill hour varieties originated in Israel, including Anna, Michal, and Shlomit but you can also find some from Florida, Bahamas, Brazil, and other locations(1). While related, chill hours are not the same as cold hardiness. In the US, cold hardiness is defined by the US Department of Agriculture Cold Hardiness Zones. This defines the coldest temperature the plants can survive. Colder zones will be more likely to have lower chills hours but not necessarily. For example, Quebec, Canada, and Southern England, have similar latitudes. However, Southern England has almost infinite chills hours because the winter temperatures tend to hover in the chill hour range versus Quebec, which has months that never get above freezing.
Why are chill hours important? Apple trees need rest and dormancy to store up energy and be able to produce an abundant crop during the next season. Think of trees that can be biennial, meaning they produce heavy and then light in offsetting years. This is often because they exhaust too much energy in the heavy years and need the off-year to recover. Chills hours are similar. If you don’t give trees time to go dormant, it reduces their ability to create fruit. They will put energy into growing but not fruiting. Chill hours can also have the reverse impact. If you try to plant trees that require low chill hours in a cold climate, you might find they begin blooming too early making them subject to late frosts. If you are planting apple trees, understanding the temperature capabilities of the variety and the rootstock are important, especially if you live in either very warm or very cold climates. Cold hardiness and short growing season are key for extreme cold while chill hours and heat tolerant are key for warm locations.
(1) P. Anderson, Low-Chill Apple Cultivars for North Florida and North Central Florida, University of Florida, Horticultural Sciences Department, UF/IFAS Extension, HS764, 2000.
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