The instructions for growing many fruit trees include a range of chill hours. What does this mean?
For proper flowering and fruiting the following spring, fruit trees require a certain amount of chill hours each winter to stay in dormancy. If the tree does not get enough chill hours, the leaves and fruit may not form properly.
There are several ways of calculating this, but one way is to count any hour where the temperature falls between 32F and 45F. Other formulas count every hour under 45F or weight the temperature against a scale and grant chill hours accordingly.
This number is important to know so that you can pick a variety that will do well in your area. Fruit trees growing in Southern California will likely have chill hours that are vastly different than those in Northern Minnesota.
The amount of chill hours needed will vary with each different variety. For example, some kinds of peaches may require 1000 chill hours before the buds will start to break open. Others are meant more for hotter areas and only require 150 chill hours.
One key to choosing a variety for your garden is checking with your local extension office. They have done research on the kinds of fruit trees that will do the best in your area and should have a list of suggested varieties. They could also help you calculate how many chill hours your area gets.