What Are Chilling Hours and Why Are They Important?

Chill hours are the total number of hours below 45 degrees F that a plant is exposed to during their dormant period to produce properly developed fruit. Some deciduous fruit trees (fruit trees that loose their leaves seasonally) must go through a certain minimum number of chill hours to release their dormancy, go into bloom, and set their fruit.

Chilling Requirements

The chilling requirements are essentially the minimum period of cold weather after which a fruit-bearing tree will blossom.

This is from the Clemson Cooperatives Extension:

Chilling Requirements: Stone fruit trees such as peaches develop their vegetative and fruiting buds in the summer, and as winter approaches, the buds go dormant in response to both shorter days and cooler temperatures. These buds remain dormant until they have accumulated sufficient chilling hours (temperatures of 45 °F or less). Varieties with low chilling requirements are recommended for coastal areas. As long as there have been sufficient chilling hours, leaves and buds develop normally.

What Happens When the Minimum Chill Times Are Not Met

In the article, Chilling Accumulation: Its Importance and  Estimation by David H. Byrne and Terry Bacon, one or more of these things may happen to a fruit tree if the cultivar has a certain number of chill hours that was not met:

• Delayed foliation and heavy suckering – Delayed foliation is where a branch will sprout a grouping of leaves at the tips and be devoid of leaves for the next 1 to 2 feet from the tips. Suckering is a large amount of suckers that sprout from the base of the plant which will negatively affect the following year’s budding. The leaves and flowers are delayed for the entire plant.
• Reduced fruit set and buttoning – The blooms are delayed, and the fruit remains very small as it ripens. Also, there will be a reduced fruit set overall. Buttoning may occur, where the fruit is misshapen and very small.
• Reduced fruit quality – This is when the color is not as vibrant (more green), resulting from reduced firmness and not changing from a green immature color to a fully ripened color. The actual fruit quality will be reduced.

The number of hours needed differ depending on the actual cultivar, not the type of fruit. You can search for ‘low-chill’ varieties that would grow and produce fruit better in certain warmer garden zones – for example, peaches are deciduous fruit trees but certain peach cultivars are specifically grown to adapt to areas that don’t get their chilling requirements throughout the year.

Different Winter Chill Hour Models

There are three different models that calculate the winter chill hours: Chilling Hours Model; Utah Model; and Dynamic Model. The most commonly used one is the Chilling Hours Model, and the formula is below, courtesy of the US National Library of Medicine and the National Center for Biotechnology. Wikipedia has simplified the model: “one chilling unit for every full hour at temperatures below 7 °C (45 °F).”

Examples of Chill Hour Requirements for Fruit and Nut Trees

Here is a list of general chill requirements for different fruits and vegetables. The range varies greatly in some species, depending on the specific cultivar. Some fruit and nut trees will not grow in certain zones. Find your zone, and learn what the average number of chill hours are in your area before you bring home a fruiting plant. Information below comes from the University of Arizona Cooperative Extension.

• Almond – 200 to 700 chill hours
• Apple – 300 to 1200 chill hours
• Apricot – 400 to 1000 chill hours
• Avocado – None
• Cherry – 600 to 1200 chill hours
• Chestnut – 400 to 750 chill hours
• Citrus Trees – None
• Date Trees – None
• Figs – 100 to 500 (low number of chill hours)
• Filberts (Hazelnuts) – 800 to 1600 chill hours
• Grapes – 100 to 500 chill hours
• Kiwi – 400 to 800 chill hours
• Olives – None
• Peaches – 150 to 1200 chill hours
• Pears – 400 to 1500 chill hours
• Pecan – 200 to 1600 chill hours
• Persimmon – 100 to 500 chill hours
• Pistachio – 800 to 1000 chill hours
• Plum – 400 to 1000 chill hours
• Pomegranate – 100 to 200 (low number of chill hours)
• Quince – 100 to 500 chill hours
• Walnut – 400 to 1500 chill hours

More Information on Chill Requirements and Chill Hours

Resources for this article:

• “About Chilling Units & Hours.” Fruit & Nut Research and Information Center. UC Davic, n.d. Web. 29 May 2015.
• “AZ Master Gardener Manual: Introduction to Fruit Trees (Cont.).” Arizona Master Gardener Manual. University of Arizona College of Agriculture Cooperative Extension, n.d. Web. 30 May 2015.
• Byrne, David H., and Terry Bacon. “Chilling Accumulation: Its Importance and Estimation.” Chilling Accumulation: Its Importance and Estimation. Texas A&M University, Dept. Horticultural Sci., n.d. Web. 29 May 2015.
• “Chilling Requirement.” Wikipedia. Wikimedia Foundation, n.d. Web. 29 May 2015.                         
• “Derived Temperature Indices Chill Units: Chill Units.” Department of Agronomy: Applied Agricultural Meteorology. Iowa State University, n.d. Web.
• Luedeling, Eike, and Patrick H. Brown. “A Global Analysis of the Comparability of Winter Chill Models for Fruit and Nut Trees.” International Journal of Biometeorology. Springer-Verlag, n.d. Web. 29 May 2015.