Chill Hours

Apple blossoms

Many fruit varieties require a certain amount of winter cold, measured in chill hours, in order to grow well, set flowers, and produce a good crop. Since the Bay Area no longer gets as much winter chill as it has in the past, older fruit trees may not be as productive as they have been in the past. Bing cherries, for instance, require 700 chill hours which may not be met in warmer winters. This UC Home Orchard page describes how temperatures affect fruit trees growth and why they need chill hours.

UC has climate stations that have recorded historical chill hours for many locations in California.

Gilroy is the only location located in Santa Clara County. Chill hours have ranged from 600–900 chill hours in recent years.
Union City may be more relevant for warmers parts of Santa Clara County. Chill hours have ranged from 400–800 in recent years.

When selecting new fruit trees, be sure to consider their chill hour requirements. UC Davis has fruit variety lists that include chill hours for apricot, cherry, nectarine, peach, and plum varieties. This PDF from UC Santa Cruz includes a list of low-chill varieties for a wide range of fruits. Sites for commercial fruit tree growers often list chill hours too.

Chill hours are generally calculated by measuring the number of hours between 32°F and 45°F. Temperatures below 32°F do not count as chill hours. Warm daytime temperatures (above 65–70°F) can count against accumulated chill hours. This USDA article describes different models for calculating chill hours and how climate change is affecting regional chill hours.

Microclimates in your landscape, such as cold air pockets at the bottoms of hills or winter shade can increase the chill hours. This PDF from the California Rare Fruit Growers includes a useful discussion of chill hours with illustrations.