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Chilling Hours in Stone Fruit Explained

Written by Stephanie on May 29th, 2011

If you want to grow fruit trees, you need to understand the concept of chilling units.  If you plant a fruit tree that expects 800 chilling units in a place that only gets 500, you will never get fruit.  The reverse is true, too.  A tree that expects 500 hours might bloom in the middle of winter and freeze.

Stone fruit trees such as peaches actually develop their buds and leaf buds in the late summer.  As winter approaches, they go dormant.  While dormant, these buds can withstand very cold temperatures.  They do not leave dormancy during the brief warm spells that happen in the winter.

These buds stay dormant because they have an internal clock counting down the number of hours of cold.  This is called a chilling unit.  There are three ways to calculate the chilling unit.  One is simply the number of hours below 45 degrees F.  The second is the number of hours between 32 degrees F and 45 degrees F.  The Utah model is complex and beyond the scope of this article.

Using information researchers in Georgia and Florida independently came up with, Texas A&M University has developed a method of estimating chilling units by looking at the mean temperatures in December and January.  They have graphs which indicate the correct number of chilling hours based on mean temperature.  More information can be obtained at the Stone Fruit Breeding Center.
You can find the average number of chilling hours in your area by calling your county extension office.  When you shop for trees, make sure you plant varieties that match those chilling hours.  Planting trees that need more chilling hours than your area gets will result in stunted leaves, very little fruit, and sick trees.  Planting trees that need less chilling hours than your area gets results in the buds and leaves breaking dormancy too early and getting frozen out.  No fruit in that case, either.  If, however, you match the chilling hours you will have a good chance of having a nice crop of fruit to enjoy year after year.


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