Deciduous plants lose their leaves in fall and enter a state of dormancy in order to survive the freezing temperatures of winter. A dormant fruit tree will not resume normal growth, including flowering and fruit set, until it has detected an amount of cold equal to its minimum “chilling requirement” followed by a certain amount of heat.
Fruit tree chilling requirements can vary widely from one variety to another. If a variety’s chilling requirement is considerably lower than typically received where it is planted, a warm spell might cause it to end dormancy and bloom too early, subjecting the tree and bloom to freeze damage. If too much higher, bloom will be delayed and erratic, perhaps with little or no fruit set.
For dependable crops in most climates a variety’s chilling requirement should approximately match the amount of chilling typically received. Note that many apple varieties, however, will set crops with far less chilling than generally recommended (though fruit quality and color may depend on cool nights in summer and fall).
A simple and widely used method for quantifying fruit tree chilling – the one used by Dave Wilson Nursery - is the Hours Below 45°F model, which equates chilling to the number of hours at temperatures below 45°F occurring in the dormant period: autumn leaf fall to spring bud break. These hours are termed “chill hours”. As chilling is complex and difficult to measure precisely, published fruit tree chilling requirements are necessarily approximate or estimated.
Varieties adapted to colder climates usually, but not always, have chilling requirements of 800-1,000 or more hours. “Low-chill” varieties adapted to warm-winter climates are defined at Dave Wilson Nursery as requiring 500 or fewer chill hours. For coastal southern California, low-chill varieties are considered to be those requiring less than 300 hours.
Much of California is blessed with a wonderful deciduous fruit growing climate: plenty of chilling, no particular spring frost problem and a long, hot, dry growing season. In these areas it is possible to grow almost any kind of deciduous fruit including varieties with chilling requirements anywhere from 100 to 800 or more hours.
When choosing fruit varieties, always refer to area retail nurseries, agricultural universities, county extension offices and Master Gardeners for local fruit growing information and ideas.
For a more complete discussion see Fruit Tree Chilling Requirement in the Product Information section.
