Chill Hours for Fruit Trees

Ever wonder why some years your fruit trees produce a lot and other years not? Old trees that used to produce bounties no longer do? I’m talking about deciduous fruit tree varieties (they lose their leaves in winter) like apples, peaches, nectarines, pears, cherries and plums.

One factor could be the difference between a cold winter and a warm winter when the trees are dormant. Deciduous fruit trees need it to be cold for a minimum amount of time to end dormancy, this is called a chill hour. A chill hour refers to any hour when the temperature is lower than 45°F. In general, during a cold winter those living in coastal areas are fortunate to have 100 and 200 hours of chilling, while inland areas may get between 500 and 600. This is why most bounties of deciduous fruit is grown in higher elevations and northern climate zones.

“Different plants have different requirements for the number of chill hours needed to end dormancy, called its ‘chill accumulation'. If a fruit tree or berry doesn't have sufficient chill accumulation during the season, the end of dormancy can be delayed. A late dormancy means the plants will leaf out later in the season, with fewer flowers and less or poorer quality fruit.” [reference: UC Master Gardener Program of Contra Costa]

It helps to be aware of the topography surrounding your home. Cold air flows downhill, so canyon bottoms, low-lying areas and towns at the foot of mountains get more cold air than hillsides and mesas. Beach communities may get damp fog or cool air funneled out of a nearby canyon that can add to chill hours.

Mission Fig

Do your homework before you go to the nursery or purchase fruit trees online -  know what the average chill hours are at your home, and what varieties match your location. UC Davis offers a calculator -  search Los Angeles County and select the nearest town. For those of us in the San Gabriel foothills, the closest weather station is in Monrovia, it recorded just 226 chill hours for the 2024-25 grow year. North Hollywood had 520 chill hours.

Most nurseries will not have this information on the label. And just because they sell it doesn’t mean it will thrive in the local area or your yard (a tree may grow but not produce fruit).  Bay Laurel Nursery has a list of low chill varieties for reference (I am not endorsing them, nor have I ever purchased from them, but I like this list they have on their site).

Another resource is this list from Pacific Horticulture, assembled for the area around Santa Barbara but it applies to us here in the foothills too.

Keep in mind our shifting weather patterns, our winters are generally warmer and dryer than in the past. An established tree that was once bountiful may not be as productive as in the past.

Planning ahead…. the best time to plant fruit trees in Southern California is late Fall to early Winter, especially for bare root trees, to allow them time to get well established during the cool and hopefully rainy months.

It’s possible to have a variety of fruit in your yard, it's just important to be informed and make good choices before falling victim to your purchase impulses at the nursery… oh wait, that’s me!