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Hot summer temperatures remain, but the season for measuring chill hour accumulation for the dormancy period has arrived.  Chill accumulation is measured as the amount of time trees are exposed to cool temperatures.  For our region, chill accumulation begins on October 1 and continues through February 15 of the following year.

Chill accumulation is a factor for growers of tree fruit because the amount of chill accumulated will, in large part, dictate the success of the upcoming harvest season.  Adequate chill accumulation will result in good fruit quality and a more concentrated harvest.  Inadequate chill will result in misshapen fruit, long harvest period, and reduced yield.

Chill accumulation can be measured in several ways.  The method used will depend on geographical location.  In our area, chill accumulation is typically measured according to the 45 model, which is the amount of time of exposure to temperatures at or below 45 °F.  Peach trees can require as few as 50 hours or as many as 1,300 of chilling exposure.  Another way to measure chill accumulation is by the dynamic model, which measures chill portions.  The model measures chill by a wider temperature range but states that optimal chill accumulation occurs at 43 °F.  Efficiency of chill accumulation is reduced as temperatures rise above or fall below 43 °F.  The dynamic model also accounts for warming trends during the fall and winter which result in loss of chill.  The 45 model has been used traditionally, but the Dynamic model provides a more accurate measure of chill for our area.

The Alabama Cooperative Extension System hosts two chill calculators.  One model measures chill hours and the other measures chill portions. Links to the 45 model and dynamic model chill calculators can be found here.   Learn more about winter chilling requirements here.

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