Chilling Hours

Chilling hours are the accumulated periods of cold temperatures that allow many fruit and nut trees and shrubs to provide us with their bounty each year.


We are all familiar with the buds and leaves of spring, the prolific growth of summer, and the harvest of autumn, but fruit and nut trees and shrubs (and strawberries!) are working through winter, too. Colder winter temperatures are part of a deciduous tree’s natural lifecycle. In preparation to survive potentially freezing temperatures, many fruiting trees and shrubs produce a hormone that initiates a state of dormancy.


What are chill hours?

Just as many seeds require vernalization (a period of cold temperatures) to germinate, many fruit trees have the same need for hours between 32°F and 45°F each winter, called its chilling requirement. In this temperature range, the growth-inhibiting hormone responsible for dormancy begins to break down, allowing trees and shrubs to produce buds that ultimately become the leaves and flowers of spring. Fruiting trees and shrubs that do not receive enough chilling hours in a year will generate fruit and leaves erratically. Any fruit produced will be lower in both quantity and quality. Also, insufficient chilling hours can extend bloom time, making delicate buds and flowers vulnerable to diseases like fireblight and brown rot.


How are chilling hours calculated?

There are different models used to calculate chilling hours, but they all take the same basic information into account:

• Chilling does not occur below 32°F.
• The ideal range for chilling occurs between 32°F and 45°F.
• Temperatures above 60°F can reverse the number of accumulated chilling hours.
• Fall and winter chilling hours are more effective than spring hours.

The Utah model provides chilling hours, while the Dynamic model provides chilling portions. Whichever model you use will give you a better idea of the best varieties for your microclimate.


How many chill hours do my trees need?

Different species need varying amounts of accumulated chill hours. Within each species, each variety has its own needs, as well. For example, northern varieties of blueberries have chilling requirements of 800 to 1,000 chilling hours, while southern varieties may only need 150 to 200 chilling hours.


Your local chilling hour station

Universities work with the USDA to provide valuable information to farmers and orchardists. You can access this information to learn the cumulative chilling hours in your area. The accuracy of this information will depend on where you live and how far you are from the nearest recording station. Click on the county and town closest to you for historical averages or current season figures for hours below 45°F and those between 32°F and 45°F.


Almonds are a staple crop in California. Part of the reason is most almond varieties only need 200 to 300 chill hours. Northern California receives an average of 800 to 1,500 chill hours, while southern California only gets 100 to 400 chilling hours.


Ultimately, your fruit and nut crop will depend on pollination rates, weather, plant age, soil nutrition and structure, irrigation, and chilling hours. You can get the most out of your fruit and nut trees and shrubs by selecting varieties best suited to your microclimate and the average number of chilling hours each year.