Winter is the time that fruit trees have the chance to chill out.  In fact, for most fruit tree varieties, it is essential for hardy fruit production.  This process is know as chill, chill hours, or vemalization.  Like most plants, fruit trees do the majority of their growing in the spring time or early summer and go into a state of dormancy in the fall and winter.  In order to break dormancy and properly flower in the spring, fruit trees experience a certain number of hours that they are exposed to cold temperatures below 45 degrees.

These periods of cold temperatures are known as chill hours and the needed hours will vary on the species.  Once the tree has reached its recommended chill hours, then the tree is ready to break dormancy (as soon as the temperatures become warm enough for this to take place).  Ideally, the cold temperatures should be between 32 and 45 degrees Fahrenheit.  Temperatures that are too cold can prevent growth in the spring.  If the tree does not experience enough chill hours, this can cause delayed and limited growth.

There are several theories on how to calculate chill hours.  These calculations can get very complex, so you might want to ask a local extension agency or arborist if the conditions in your area are able to support the type of fruit tree that you want.  Taking into consideration the recommended chill hours that are needed will help ensure that you get the optimal production of your fruit trees.  For more information on chill hours, you might want to check out this website from the University of California.

http://homeorchard.ucdavis.edu/general-tree.html#chill