What Are Chill Hours?

Chill hours refer to the time needed under cold temperature conditions for a plant to break dormancy. Specific chill hour requirements are different for each crop and variety within that crop. If not enough chill hours are accumulated during the winter season, some plants may have physiological problems, such as: delayed foliation, reduced fruit set and “buttoning” (flowers that are not fully formed, and reduced fruit quality. In some cases, “over-chilling” plants may result in physiological changes as well, such as higher plant vigor, or altered bloom timing.

Knowing chill hours accumulated for a given growing season is an important part of managing an orchard or crop, and can also be useful in planning for crop production timing and amount.

How to Calculate Chill Hours

Chill hour accumulation usually begins on Nov. 1st and last through the winter until April. There are several different methods to calculating chill hours. The first, and simplest, method is to count the number of hours under 45F (7.2C). The second way is to use the temperature of freezing as a lower threshold and only count hours between 45F(7.2C) and 32F (0C). The Utah Model for chill accumulation applies different chill “units” to different temperatures to account for different temperatures within the chill “range” using the table below:

1 hour below 34°F = 0.0 chill unit

1 hour 34.01 – 36°F = 0.5 chill unit

1 hour 36.01 – 48°F = 1.0 chill unit

1 hour 48.01 – 54°F = 0.5 chill unit

1 hour 54.01 – 60°F = 0.0 chill unit

1 hour 60.01 – 65°F = -0.5 chill unit

1 hour >65.01°F = -1.0 chill unit

All three of these models can be calculated easily on the Pest Prophet app using specific weather data for your field, without needing data loggers or weather stations.