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Commercial Horticulture > Comm Hort Blog > Posts > Chill Accumulation in Chilton County: Current Status and A Historical Perspective

Peach and other temperate fruit trees require a certain exposure time to winter temperatures in order to break dormancy after exposure to warmer temperatures during the following spring.  Peach trees that were not sufficiently exposed to winter temperatures will have reduced ability to break both flower and leaf buds during the spring.  The amount of exposure to winter temperatures required to break dormancy is dependent on cultivars.  Over the last two years many peach cultivars struggled to break flower and leaf buds in the spring due to lack of sufficient chill for many cultivation.

Chill accumulation for the 2018 peach season is following a different course compared to some previous years, particularly last year when the region experienced historically low chill accumulation. As of December 17, 2017, 423 chill hours or 25 chill portions have been accumulated in Chilton County, which is the major peach producing area in the state of Alabama.  This represents 2/3 of total chill hours accumulated for the area last year.

The following charts provide a historical prospective of the chill accumulated in Chilton County at two benchmark periods over ten years (Figures 1 and 2).  These benchmark periods are October 1 to December 31 (orange columns) and October 1 to February 15 of the following year (gray columns) or the total chill accumulated.  Figure 1 follows the accumulation of chill hours using the 45 °F model, which measures chill hours accumulated at 45 °F and below.  Figure 2 follows accumulation of chill portions using the Dynamic model.  Compared to the 45 °F model, the Dynamic model is complex assigning quality to chill accumulated.  Quality of chill is temperature dependent.  The Dynamic model also takes into consideration warming trends that may result in subtraction of chill portions accumulated.  Subtraction of chill will depend on both duration and timing of the warming trend.   The Dynamic model is considered the best fit for measuring chill accumulation in peach crop. 

For the following observations, chill data were obtained from the website.  In addition to the 45 °F and Dynamic models, also provides chill accumulation data based on Utah, Positive Utah, and Old 32-45 °F chill models.  This site uses weather data from weather stations throughout the country.  For these observations, chill accumulation data was obtained from a weather station in Jemison, AL in the Union Grove community of Chilton County.  When using, go to the US map provided (WunderMap) and select the station that is closest to your location or has similar elevation.  Using to assess chill accumulation from multiple sites near your farm may provide a more accurate assessment of chill accumulation. 

Over the 10-year period, chill hour accumulation in the 45 °F model (Figure 1) is more variable than chill portion accumulation in the Dynamic model (Figure 2).  Historically, if chill hour accumulation were above 400 by the first benchmark date of December 31, the total chill hour accumulation would be sufficient for most peach cultivars by the second benchmark date of February 15.  It is often stated that if half of the chill requirement for peach varieties is not met by December 31, accumulation of sufficient chill by February 15 becomes more uncertain. 


Figure 1.  Historical Chill Hours Accumulated during Three Periods in Chilton County using the Old 45 Chill Hour Model.  Weather data acquired from and accessed on November 28, 2017.


Figure 2.  Historical Chill Portions Accumulated during Three Periods in Chilton County using the Dynamic Model.  Weather data acquired from and accessed on November 28, 2017

If the current weather conditions are maintained, peach trees in the region should reach sufficient chill overall. Amount of chill accumulated by December 31 will provide a clearer picture.

Edgar Vinson, Ext. Specialist

Elina Coneva, Ext. Specialist

James Pitts, Director of Research Extension Center



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