If you’ve done some shopping (window or otherwise) for fruit or nut trees, you might have noticed the phrases “chill hours” or “chill zones” in the plant’s description. Though there’s some debate on the topic, it’s generally acknowledged that chill hours are the average number of hours plants are subjected to temperatures between 32 degrees and 45 degrees during the dormant period from fall until spring. While chill hour zones show you how long cold temperatures usually last in a given region, this should not be confused with the USDA’s plant hardiness zones map, which shows the average coldest temperatures per region.
Chill hours are important because deciduous plants (plants and trees that lose their leaves in fall) must enter a stage of dormancy in the winter in order to survive freezing temperatures. Dormancy ensures that the trees are able to regulate their growth and do not flower or produce fruit until spring, rather than continuing the growth cycle and trying to fruit in the fall and winter. In fact, with the exception of citrus trees, most fruit- or nut-bearing trees need to be exposed to a certain number of chill hours in order to flower and produce fruit in the spring. This is known as a chill hour or chilling requirement. The requirement is the number of hours needed for the plant to leave dormancy and resume growth when warmer temperatures return.
Chill hour requirements are like a timer; once a tree enters dormancy for the winter, it sets a sort of internal alarm to wake itself up after a certain number of chill hours have passed. If your tree wakes itself up too soon, it or the fruit it bears could be damaged by lingering frosts and cold temperatures. If warmer temperatures bring the tree out of dormancy before it’s been exposed to the necessary number of chill hours, the tree will most likely be unable to bear fruit, or bear fruit that is poor in quality.
Obviously, chill hour requirements are important to consider when selecting what kind of fruit or nut tree you want to plant. It’s best to match the needs of the plant to your region’s average number of chill hours. If you pick a plant or plant variety that requires a high number of chill hours, but you live in a low-chill area, the plant will not be exposed to enough chill hours and likely not flower or bear fruit at all. If you pick a low-chill variety but live in a colder region, it’s likely that your plants will flower too soon and be damaged by late frosts.
The most important thing to consider when selecting your fruit or nut tree is not the kind of fruit or nut, but the variety. For example, apples have some of the highest chilling requirements of any fruit; however, this doesn’t necessarily mean that you can’t plant an apple tree if you live in a warmer climate. Apple trees also have one of the widest spreads of chill requirements among their many varieties, meaning you can most likely find a variety that will flower in your region.
After a certain level of exposure to chillier temperatures, dormancy becomes irreversible until spring, and will not be affected by unpredictable short-term warm winter temperature peaks. However, you want to make sure the tree is still exposed to the minimum number of chill hours it needs. To account for fluctuating temperatures, make sure you select a variety with a chill requirement slightly below your region’s average number of chill hours. Still, there are other factors that can affect fruition; your best bet is to ask neighbors or your local extension office what varieties they’ve had success with in the past.