What are Chill Hours?
The first answer to this question is that they are approximate! Chilling is achieved at temperatures that are approximately forty-five degrees or below, roughly between November 1 and February 15, with the most benefit being derived from chilling hours occurring in December and January. These hours are cumulative and need not be continuous. Daytime temperatures above sixty degrees during this period may negatively affect the cumulative total.
Most fruit trees require some winter chill in order to set fruit. We have included the chilling requirements for most of our stock, but they should be used only as guidelines. If you are unable to determine how many chill hours you get in your area, consult your local Agricultural extension office or a local independent nursery.
If you live in an area that gets little winter chill, see our list of Low Chill Varieties. Bear in mind that many people in low chill areas have had success with varieties that are supposed to need somewhat higher hours, so experimentation can be fruitful, within limits. We do NOT, however, encourage ordering varieties that are clearly incompatible with your climatic conditions.
If you have had success with varieties that are not, strictly speaking, suitable for your area, we would like to hear your comments!
Dave Wilson Nursery: Methods Used To Define Chill Requirements For Fruit Trees
Wikipedia: Chilling Requirement
About.com: What are Chill Hours?
USDA Plant Hardiness Zone Map: Look Up Your Local Hardiness Zone
Chill Hours for Most California Counties