Horticultural Subjects

Chill Hours and Dormancy

During winter, deciduous temperate climate plants are typically dormant; they do not grow even in warm winter weather. They need a specific number of cumulative hours of temperatures lower than 45 degrees F., in order to break dormancy and begin to grow in the spring when the weather becomes warm enough to support growth.

Different species and varieties have different chilling requirements. For example, apple trees need 500 - 1000 chill hours, depending on the variety.

It is important to know the chilling requirements of deciduous fruit and nut trees because inadequate chilling causes a poor crop. Thus, knowing your local chill hours is helpful for selecting the right tree.

Chill Hours From Stanford University Grounds Services Weather Station

The following table from the University of California Resource for Fruit and Nut Crops provides information about chilling requirements for different fruit and nut tree species. This web site also provides additional helpful information.