Guide to Calculating Chilling Hours

Chilling hours are determined by the accumulation in time that peach trees are exposed to a specific range of temperatures (for our discussion, between 32˚-45˚ F) during the winter. In figure 1, you can see that this winter (and most likely all winters), multiple areas will experience significant differences in chilling hours over a small geographical area. The Florida Automated Weather Network (FAWN) weather stations on the ridge have consistently lower number of chilling hours compared to other weather stations off the ridge.

Dover Chill Hours _45Figure 2: Accumulated chilling hours below 45˚ F for Dover FAWN site. Shaded green area projected chilling hour accumulation.

Dover Chill Hours 32-45
Figure 3. Accumulated chilling hours between 32˚ and 45˚ F for Dover FAWN site. Shaded green area projected chilling hour accumulation.

This variation at first glance might resemble variation in summer rainfall patterns in Central Florida. It is more likely that these geographical areas have consistent topographical features (ridge versus depressional areas) that cause this variation. Everybody knows about cold pockets on freeze nights and these (colder areas) would be consistently cooler even on nights when freezes don’t occur, hence more chilling hours.

Chilling hours, depending on the crop or chosen chilling model (see figures 2 & 3) can be calculated differently. At the AgroClimate website (http://agroclimate.org/tools/Chill-Hours-Calculator/), you can see a map that displays the number of chilling hours acquired to-date from weather stations located in the southeastern United States. There is also additional and more detailed information on calculating and displaying chilling hours for the identified weather stations. Once you get the idea, it is fairly simple to do your own calculations from your weather station data.

CREDIT

by CHRIS OSWALT, UF/IFAS Extension Agent, Polk County