Chill Hours & Dormancy

During winter, deciduous temperate climate plants are typically dormant; they do not grow even in warm winter weather. They need a specific number of cumulative hours of temperatures lower than 45 degrees F., in order to break dormancy and begin to grow in the spring when the weather becomes warm enough to support growth.

Different species and varieties have different chilling requirements. For example, apple trees need 500 - 1000 chill hours, depending on the variety.

It is important to know the chilling requirements of deciduous fruit and nut trees because inadequate chilling causes a poor crop. Thus, knowing your local chill hours is helpful for selecting the right tree.

Chill Hours From Stanford University Grounds Services Weather Station

Year (October - March) Hours Below 45 degrees F.
1998 - 1999 1139
1999 - 2000 729
2000 - 2001 1074
2001 - 2002 788
2002 - 2003 679
2003 - 2004 660
2004 - 2005 732

The following table provides information about chilling requirements for different fruit and nut tree species.

Type of Fruit Approximate Hours Equivalent time in days or weeks if continuously
exposed to 45 degree F. or below
Almond 200 - 300 8 - 13 days
Apple 1200 - 1500 7 - 9 weeks
Apricot 700 - 1000 4 - 6 weeks
Cherry, sour 1200 7 weeks
Cherry, sweet 1100 - 1300 6 - 8 weeks
Chestnut 300 - 400 2 - 3 weeks
Fig few hours ---
Filbert (Hazelnut) 1500 9 weeks
Kiwifruit 600 - 850 3.5 - 5 weeks
Olive 200 -300 8 - 13 days
Peach/Nectarine 650 -850 4 - 5 weeks
Pear 1200 - 1500 7 - 9 weeks
Pecan 400 - 500 3 - 4 weeks
Persimmon < 100 4 days
Pistachio 1000 6 weeks
Plum, American 3600 5 months
Plum, European 800 - 1100 5 - 6 weeks
Plum, Japanese 700 - 100 4 - 6 weeks
Pomegranate 200 - 300 8 - 13 days
Quince 300 - 400 2 - 3 weeks
Walnut, Persian 700 (Payne) - 4 weeks
Walnut, Persian 1500 (Franquette) 9 weeks

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