Measuring of chill accumulation in the Southeastern United States typically begins on October 1 and concludes on February 15 of the following year. During this time, temperate fruit crops, such as peach, will accumulate the necessary amount of chilling (exposure to cool temperatures) in order to break dormancy in the spring.
In the Southeastern region of the U.S., chilling is measured primarily by the 45°F (Weinberger) model, which calculates the number of hours of exposure to temperatures at or below 45°F. These hours of exposure are called chill hours. When peaches do not receive an adequate number of chill hours, the result will be a protracted harvest season, reduction in fruit marketability, reduction in yield, and substantial financial loss.
The amount of chill accumulation reached by the end of the year is an established benchmark, which provides an indication of the potential chill accumulation reached by February 15. Generally, if approximately half of the chill hours necessary to satisfy chill requirements for most varieties have been reached by December 31, then chances are good that chilling will be sufficient. As of December 18, 2020, 241 chill hours have been accumulated in Central Alabama. According to the current extended forecast, there is a strong possibility that Central Alabama, which is the leading peach producing region of the state, will be short of the benchmark chill accumulation of 400 – 450 chill hours. If this is the case, growers will likely begin adjusting their cultural practices in order to mitigate potential damage to the fruit crop as a result of insufficient chilling.
Other Chill Models May Indicate More Chill Accumulation
The Weinberger model only measures chill in hours of exposure to temperatures at 45°F and below. Another model, the Dynamic model, measures chill accumulation in chill portions. With the Dynamic model, chill accumulation occurs from 29°F to 55°F. On occasions when the temperature is above 45°F, the Dynamic model accounts for chill being accumulated in this range whereas the Weinberger model does not. Therefore, according to the Dynamic model, chill can be accumulated even when the temperature is warmer.
The Weinberger model assigns the same value to each hour of exposure at 45 °F and below. On the other hand, the Dynamic model assigns weighted values to each temperature within the 29°F to 55°F range. Optimal chill accumulation occurs at 43°F according to the Dynamic model; therefore, at this temperature, one full chill portion is accumulated after a certain duration. As the temperature rises above or falls below 43°F, more exposure is required to accumulate one full chill portion. Additionally, chill portions can be lost at 60°F and above, which is another feature missing from the Weinberger model. During seasons when chill accumulation is expected to be low or if the region has experienced warming trends, it is often useful to refer to both the Weinberger and Dynamic models to determine the degree of chilling insufficiency. Both models can be found on the ACES Peach IPM website at https://ssl.acesag.auburn.edu/department/peaches/peachipm. Currently, conversion from chill hours to chill portions or vice versa cannot be made, but the Peach IPM website provides a table of chill hour and chill portion comparisons.
Management Options for Insufficient Chilling
- Delay pruning. When chill accumulation is low, the number of viable buds will be reduced. It is recommended to delay pruning or bloom thinning until fruit set to have a better indication of what the fruit load will be. This may not always be practical since the availability of labor will determine when pruning can occur. Depending on the fruit set, a grower might decide to delay pruning until after the harvest of each variety is complete.
- Use or rest breaking compounds. Potassium nitrate is a fertilizer usually applied to the soil, but applied at a 5 percent rate to the canopy of dormant trees, it has been shown to stimulate bud break. However, 5 percent potassium nitrate solution has been most effective when chill accumulation has been at 80 to 90 percent of required chill accumulation. Therefore, if a peach variety requires 1,000 chill hours, potassium nitrate will be most effective when applied at 800 or 900 accumulated chill hours. More research is needed to determine the effectiveness of 5 percent potassium nitrate on trees that have accumulated less than 80 percent chill accumulation.
December chill accumulation has traditionally been an indicator of total chill accumulation, but several weeks of potential chill accumulation remain. Weather events that are more favorable for greater chill accumulation can occur in January and February to erase the deficit of chill accumulation the region currently has.
For more information about chill accumulation, consult the content piece Fruit Culture in Alabama: Winter Chilling Requirements on the Alabama Extension website. You may also contact your local Extension office.