Stone and pome fruit trees rely on enough chilling for flowers and leaf buds to develop normally. If the buds do not receive sufficient chilling temperatures during winter to completely release dormancy, trees may develop physiological symptoms such as delayed and extended bloom, delayed foliation, reduced fruit set and reduced fruit quality. Growers and industry traditionally keep track of chilling hours beginning in November to get a sense of the orchard management practices needed and comparison of past year's weather and crop load. The approximate number of hours/units/portions needed for normal development varies depending on variety and species. For more information contact local nurseries or farm advisors.
Chilling Hours and Units models: The publication Chilling Accumulation: Its Importance and Estimation (David H. Byrne and Terry Bacon, Dept. Of Horticultural Sciences, Texas A&M University) explains the different methods of accumulating chilling units. There are three common models used to calculate chilling, each one defining what a chilling unit is: 1) Hours below 45°F model; 2) Hours between 32°F and 45°F model; and 3) Utah model.
| The Bryne and Bacon Explanation of calculations used by the Utah model: | The program calculations for the Utah model accessed on this website: |
| 1 hour below 34F = 0.0 chill units | 1 hour below 34F = 0.0 chill units |
| 1 hour between 35F ~ 36F = 0.5 chill units | 1 hour between 34.01F ~ 36F = 0.5 chill units |
| 1 hour between 37F ~ 48F = 1.0 chill units | 1 hour between 36.01F ~ 48F = 1.0 chill units |
| 1 hour between 49F ~ 54F = 0.5 chill units | 1 hour between 48.01F ~ 54F = 0.5 chill units |
| 1 hour between 55F ~ 60F = 0.0 chill units | 1 hour between 54.01F ~ 60F = 0.0 chill units |
| 1 hour between 61F ~ 65F = -0.5 chill units | 1 hour between 60.01F ~ 65F = -0.5 chill units |
| 1 hour above 65F = -1.0 chill units | 1 hour above 65.01F = -1.0 chill units |
Dynamic Model or Cumulative Chilling Portion: "The ability to calculate chill portions using the dynamic model's calculation method became available in September 2006. The model is set to start each year on Sept 1, based on decisions made due to early chill some years, ongoing research interests, realities of programming, and figuring that inclusion is better than exclusion when it comes to data. The best use of the model for research purposes and for industry applications is a matter of change over time as we learn more about the subject and its applications. While this information is made available as a public service, I hope that all users will be aware that it is more a research tool at this point than policy written in stone with respect to recommended usage."
- Kitren Glozer, September 2006
Additional Resources:
Analysing variations in flowering time based on the dynamics of chill and heat accumulation during the fulfilment of cultivar-specific chill requirements in apricot
Alvaro Delgado, David Ruiz, Ana M. Muñoz-Morales, José Antonio Campoy, Jose A. Egea, European Journal of Agronomy, Volume 164, 2025, 127509, ISSN 1161-0301, https://doi.org/10.1016/j.eja.2025.127509. (https://www.sciencedirect.com/science/article/pii/S116103012500005X)
Evaluation of chill models from historical rest-breaking spray experiments on ‘Bing’ Sweet Cherry
Glozer, Kitren & Grant, Joseph & Coates, William. (2005). Evaluation of Chill Models in `Bing' Sweet Cherry Rest-breaking Trials in California from 1994 to the Present: Validation of the Dynamic Model. HortScience. 40. 1141D-1141. 10.21273/HORTSCI.40.4.1141D.
Rest-Breaking Alternatives for Sweet Cherry and an Update on Chill Accumulation
GLOZER, K., 2006. Rest breaking alternatives for sweet cherry and an update of chill accumulation, Tree Topics: University of California Cooperative Extension Vol. 31.