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Dormant peach trees.

Chill Accumulation

The standard chill hour logs chill hours at and below 45°F. The modified chill hour models log chill hours between 32 °F – 45°F. Both models count each chill hour the same regardless of the temperature in these range.

The Dynamic model measures chill accumulation as chill portions but chill is accumulated differently at each temperature over a broader range. In the Dynamic model, optimal chill is accumulated at 43°F. Chill accumulation is reduced as temperatures rise above or fall below 43°F. The Dynamic model also takes into consideration warming trends when calculating chill portions, which indicates that some chilling can be lost when temperatures become unseasonably warm. The table provides an approximation of chill portions in relation to standard chill hours.

It is important to remember that currently no conversion between chill hours and chill portions has been developed, but temperatures have been warm enough that the same trends in chill accumulation have been observed. The comparisons in Table 2 were made from observing fruit shape at harvest over a period of a few years. Values in this table will likely change as more comparisons are made over subsequent seasons.

Table 1. Chill Accumulation in Various Locations in Alabama as of January 14, 2020

LocationStandard Chill Hours (45 degrees and below)Modified Chill Hours (32 degrees - 45 Degrees)Chill Portions ( Dynamic Model)
Auburn University Turf Unit33816125
Black Belt REC (Marion Junction)53227331
Brewton Ag Research Unit (Brewton)46723326
Chilton County REC (Thorsby)50924534
Durbin Farms (Clanton)47524734
EV Smith Research Center (Shorter)56030431
Gulf Coast Station REC (Fairhope)28014624
Sand Mountain REC (Crossville)71738544
Tennessee Valley REC (Belle Mina)70435544
Upper Coastal Plain (Winfield)74938941
Winfred Thomas Ag Research Unit (Hazel Green)76240245
Wiregrass REC (Headland)31718729

*Chill accumulation is measured from October 1 until February 15 of the following year.

Table 2. Comparison of Chill Hours and Chill Portions

Chill Hours NeededCorresponding Chill Portions
650 Hours30-35
75035-40
80040-45
85045-50
95050-55
105055-60

*Chill hour/chill portion comparisons do not include modified chill hours

Information for Chill Requirements

Chill hour and chill portion calculators may be accessed online. For more information about chill hours, chill portions, or Old 45 and Dynamic Chill models, read Fruit Culture in Alabama: Winter Chilling Requirements.

As a reminder from the last chill accumulation update, the Peach IPM webpage, which houses the chill accumulation calculators, has undergone a series of changes with more scheduled changes in the future. You will notice that several other locations have been included in the standard chill hour and modified chill hour calculators similar to the chill portion calculator. This will help you to select the location closest to your operation for the most accurate measure of chill accumulation.

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