Across California’s Central Valley, warming winter temperatures over the past several decades are reflected in declining trends in “winter chill,” a measure of cold temperatures required for certain fruit and nut trees to produce flowers and fruits.

Many fruit and nut trees require a period of cold temperatures over the winter to become and remain dormant before blooming in the spring. The amount of cold experienced by these trees during this dormant period is referred to as “winter chill.” Meeting winter chill requirements is critical for many of the state’s high-value fruits and nuts, such as cherries, apricots, and almonds. For more details, download the Winter Chill chapter.

The graph shows annual chill hours based on weather station data for selected locations. These and all other locations studied are shown on the map below. Chill hours are the total number of winter hours when temperatures are between 32 degrees Fahrenheit (°F) and 45°F.

The 20 locations studied are shown in the map below. Outlines correspond to county boundaries.

• Chill hours are declining at more than half of the Central Valley locations studied (12 out of 20); at the locations shown in the graph, declines ranged from about 40 to 105 chill hours per decade.

• California accounts for over half of the nation’s fruit and nut production. Serious economic loss could result when fruit and nut trees can no longer experience sufficient winter chill.

• Tracking trends in winter chill will allow growers to anticipate the need for adaptation measures, such as using chill-compensating products or growing low-chill varieties.

• When trees do not meet their chill requirements, they bloom later and over a long period, resulting in a more prolonged, costly harvest and increased risk of pests damaging crops.

• The indicator is derived from temperature data and as such, is influenced by the same factors that influence air temperatures.

Fruit and nut trees have different winter chill requirements. For example, almonds (pictured) require 250 to 350 chill hours; apples, 1200 to 1500 chill hours; and Bing cherries, 1000 to 1300.

Credit: Florence Low, California Department of Water Resources

• UC Agriculture and Natural Resources, Fruit and Nut Research and Information Center, About Chilling Hours, Units and Portions