Many deciduous trees (trees that lose their leaves in winter), such as apples, peaches, pears, plums, flowering cherries, and dogwoods, require a period of dormancy and the accumulation of chilling to produce flowers and fruit. A chill hour is the amount of chilling received by a plant at 45 degrees F. The chilling requirement is the total number of hours required during the winter for a particular cultivar to induce the tree to break dormancy and produce flowers.

Regions vary greatly in the amount of chilling they receive. For example, peach varieties adapted to the Piedmont (for example Atlanta-Charlotte-Birmingham-Dallas) require 1,000 hours of chilling before breaking dormancy, while those adapted to south Florida (Orlando –Tampa) only require 200 hours of chilling to be able to bear fruit. A tree that needs 1,000 hours will not bear fruit or grow in areas with lower chill hours.

A tree with low chill hours may grow farther north, but it may bloom too early in the season (because its chilling requirement has been met with just a few cold fronts) and later freezes may damage flowers or fruit. It is very important to know what chilling hours you receive to make sure you plant the correct varieties for your location.

Chilling hours also vary year to year, depending on the number of cold fronts each winter. A region that averages 500 chill hours per year might receive only 350 hours in a warmer winter, but 800 in a colder winter. Therefore, it is good to plant several different cultivars with a range of chilling requirements. Keep in mind that you don’t want to plant a variety with a requirement for 800 hours if your average is only 250.

The following link from Agroclimate shows both current and average chill accumulation totals for areas all over the South Eastern United States. You can use it throughout the winter to track how much chilling has occurred to your site, and watch how the different varieties react to the chilling.

AgroClimate Chill Hours Calculator

The following chart lists the best varieties for each zone and chill hour requirements: