Published in the Contra Costa Times on January 7, 2006

Q. How can I tell how many chilling hours my area received?

A. Chill hours refer to the hours of time a deciduous fruit or nut tree  spends with temperatures between 32 and 45F. Chill hours are necessary  to break dormancy, which is the state of rest that these trees enter during the winter months in response to growth inhibitors. Growth will  not occur even if the temperatures rise, which prevents the trees from  beginning to grow during atypical warm weather when they could be  damaged by normal cold temperatures later in the winter or early  spring. When sufficient cold temperature breaks down the growth  inhibitors in the trees, growth resumes. This is called vernalization, “chilling’, or “winter chill”. Different varieties of trees need  different numbers of hours to break dormancy.  Most of northern  California gets between 800 and 1500 hours each year, but some of the warmer coastal and Bay side areas may receive fewer.  Check before  purchasing fruit and nut trees to make sure that your area gets enough  winter chill for the trees to bloom and set fruit. Some varieties such  as Apricots are unlikely to bear if they do not receive enough chill  hours.  IF you are in a low chill area, look for trees that are  tolerant to your climate.  The Fruit and Nut site at UC Davis;  http://fruitsandnuts.ucdavis.edu/ has a lot of useful information.   For a  list of  the chill hours required by different types of trees ,   PDF link to low chill varieties and a PDF link to 'Temperate Tree  Fruit and Nut Varieties for Planting in the Home and Garden Landscape' .  For recommended varieties as well as much more information follow  these steps:  Open the http://fruitsandnuts.ucdavis.edusite; click on 'Fruits and Nuts; Click on 'Backyard Orchard';  click on Fruits and Nuts; Click on Tree Selection.