What Are Chill Hours For Fruit Plants & Trees?

Not all fruit plants and trees require a dormant period where they will receive a certain amount of what are called "chill hours." But many, such as blueberry bushes and apple trees do. If you want these fruit plants and trees to produce a large crop of fruit they will require a rest during winter, and a certain amount of cold temperatures.

A chill hour is equal to one hour that a fruit plant or tree spends in cooler temperatures ranging from 45 to 32 degrees Fahrenheit. Various types of fruit plants and trees require different amounts of chill hours. Some fruit trees, such as figs, only require 100 chill hours during the cool season. Others can require up to 1,000 chill hours.  Tropical fruit plants and trees require no chill hours.

Note:  As a general rule, fruit plants and trees that require higher chill hours are better suited for cooler northern climates while lower necessary chill hours are suited for warmer climates in the Southern region.

Some fruit plants and trees form their buds for next year’s crop during the late summer. Then, in cooler climates, temperatures start to drop in fall and the plant or tree begins to enter the cool season dormant stage. During this stage the plant or tree goes dormant (into hibernation) so as to protect themselves. This protects all parts of the plant, including the fruit buds that will become next years fruit.

Then, when spring arrives and soil and air temperatures start to warm up the plants and trees begin to emerge from dormancy. Once they do, and a plant has received its necessary chill hours, the fruit buds start to wake up and open at the proper time during spring.

If a plant doesn't get enough chill hours it might not bloom on time, or at all, therefore producing little or no fruit. Sometimes, this can lead to a later and/or longer bloom time, which results in disease on the flowers and reduced fruit set and poor fruit quality.

But what happens if a fruit plant or tree gets too many chill hours? Usually there's not a problem. But, when a low chill hour tree, which is typically more suited for warmer or less cold climates, there is a possibility of the plant emerging from dormancy during an early warm spell, before winter is really over. When this happens, new growth or flowers that emerge can be damaged, thereby diminishing fruit production.  

Note: Chill hours aren’t exact. For example, if you have a fruit tree that requires 500 chill hours this doesn't mean it has to get exactly 500 hours...just somewhere around that. A dormant season with 50 or even 100 hours less or more won't be devastating to fruit production.

Below is a map you can use to determine how many chill hours typically occur in the area where you live and garden.