Temperate fruit trees must pass through some cold in order to know that winter is over and it is time to bloom. The Mid Klamath region has plenty of chill hours, so this is generally not an issue or our area, but some areas of Southern and Coastal California are "low chill", meaning they lack the amount of chill required to stimulate bloom. Chill hours are listed below for many fruit and nut varieties and a more detailed explanation.
Chill is measured by the amount of hours below 45°F from November to mid February. Temperate fruits require anywhere from 100 to 1400 chilling hours. Gauging cumulative chill and matching varieties for your area is more of an educated guess than an exact science, as low temperatures vary considerably within a climate zone and from year to year. Chilling requirement is a concern for USDA zones 9B and 10, predominantly southern and coastal regions where chilling hours average 100-600 chilling hours per year. If you are within this area, take note of the chilling requirements listed for fruits and choose accordingly. Persimmons, almonds, olives, berries, pomegranates and chestnuts all have low chilling requirements. Filberts need lots of chill (1200-1500 hours) and should be avoided in low chill areas. Use the map to see if you are in a low chill area and estimate your amount of winter chill. The “At a Glance” tables will tell you the chilling requirements of each fruit.