Chilling hours are the accumulated hours 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, as well. 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 properly, many fruit trees have the same need for a certain range of 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. This allows trees and shrubs to begin producing buds which will 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, and the fruit that is 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, such as 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:
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. UC Davis provides a list of chill portions needed for several different fruit trees. You can see a more extensive list here.
How many chill hours do my trees need?
Different species need varying amounts of accumulate chill hours. Within each species, each variety has its own needs, as well. This is why learning about plants before you buy them is so important. 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. For specific fruit and nut trees, you can use the UC Davis Backyard Orchard page, click on a specific tree, and then click on the tree selection option on the righthand side of the page for the best information.
Your local chilling hour station
Universities work in conjunction with the USDA to provide valuable information to farmers and orchardists. You can access this information to find out more about the cumulative chilling hours in your area. Depending on where you live, and how far you are from the nearest recording station, the information will be more or less accurate. Simply click on the county and town closest to you for either historical averages or current season figures for the number of hours below 45°F and the the number of hours between 32°F and 45°F.
Another way you can calculate your chilling hours is to use the Wundermap website to find the recording station closest to you (just click on the bubble and copy and paste the station number), and then enter that station number on the Get Chill website (this one can be really slow sometimes, but the information is very good).
Almonds are a staple crop in California and part of the reason is that 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 chill hours.
Ultimately, your fruit and nut crop will depend on pollination rates, weather, plant age, soil nutrition and structure, and irrigation, along with 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.
You can grow a surprising amount of food in your own yard. Ask me how!