Dormancy and Chill Units

This winter, temperatures have been fluctuating greatly with unseasonable mild days, thanks to El Niño. These temperatures have brought out questions on what the effects are on the apple buds and how do chill units affect dormancy. Chill units usually aren't much of concern for Ontario because our winter temperatures usually don't rise much more than a few degrees above zero which will keep buds dormant until spring. In more southern temperate areas, like California, chill units are a concern because seasonal temperatures are higher and they need to get enough chill hours to overcome dormancy. This winter in Ontario the number of chill units that have accumulated is a concern because warm temperatures may signal trees to start coming out of dormancy. To better understand the significance of chill units, it is important to know that there are two types of dormancy that buds go through in the winter:


Buds are dormant because of internal physiological blocks that prevent growth, even under ideal external conditions for growth. A specific amount of chilling units are required to break through this dormancy. If buds are still in the endodormancy stage, warm temperatures will have no influence on bud growth (Westwood, 1993).


Buds are dormant as a result of external conditions unfavorable to growth. Buds will go into the ecodormancy stage once endodormancy is complete. Buds will remain dormant until temperatures are favourable for bud growth (Westwood, 1993).

Apples require 1000 to 1200 chilling units. There are several models that exist for calculating chill units:

  • The Chilling Hours Model gives 1 chill unit for every hour that temperatures are between 0 and 7.2°C
  • The Utah Model assigns partial, whole and negative chill units depending on the temperature:
    • 1 hour below 1.4°C = 0 chill units
    • 1 hour 1.4-2.4°C = 0.5 chill units
    • 1 hour 2.4- 9.1°C = 1 chill unit
    • 1 hour 9.1- 12.4°C = 0.5 chill units
    • 1 hour 12.4-15.9°C = 0 chill units
    • 1 hour 15.9- 18.0°C = - 0.5 chill units
    • 1 hour above 18.0°C = -1 chill units
  • Cornell Extension uses a model developed by Hauagge and Cummins, 1991 which follows this formula: Chilling Unit= 0.277 + 0.1940t -0.0124t2 , t= Celsius temperature. It works similarly to the Utah Model, where maximum chilling units are achieved around 7°C and partial units are assigned for temperatures between -1 and 16°C with a larger fraction of a chilling unit given to temperatures that are closer to 7°C.

In Table 1, I used the Chilling Hours Model and the model used by Cornell Cooperative Extension to calculate the chilling hours accumulated for various locations in the province and the date when 1200 chilling units were accumulated. Chill units were calculated from October 1st to February 26th. There is quite a large a difference between the two models. The dates when 1200 chilling units were accumulated, under the model used by Cornell Extension, match very closely with what was calculated for New York State.

Table 1. Chilling units accumulated in various locations in Ontario using two different models using temperature data from October 1st 2015, February 26th, 2016.
  Chilling Hours Model (0-7.2°C) Model used by Cornell Cooperative Extension
Weather Station Location Chilling Units
Accumulated as of Feb. 26
Date that 1200 Chilling Units
Chilling Units
Accumulated as of Feb. 26
Date that 1200 Chilling Units
Cedar Springs
Feb. 19
Dec. 16
Feb. 21
Dec. 16
Feb. 19
Dec 14
Prince Edward County
Feb. 7
Dec. 6
Feb. 1
Dec. 12

Data for all locations except Collingwood was taken from the Vine and Tree Fruit Innovations website. Collingwood data was taken from the Environment Canada website.

For more information:
Toll Free: 1-877-424-1300