Chilling hours are the number of cold hours that are needed for a deciduous crop to break winter dormancy. Lack of adequate chilling can result in poor bud break, extended flowering season, poor bloom overlap with pollenizers, flower bud death, and weakened trees. Extended flowering season may sound nice for the homeowner, but for commercial growers it means greater expense with repeated harvests. Commercial blueberry and peach growers often must resort to applying growth regulators to fool the plants into thinking they have had enough chilling so they can get compact harvest dates.
How do we measure chilling? There are various models used. The traditional model has been Cumulative Chilling hours, adding the number of hours between 32 0 and 450F from November 1 through the end of February. AgroClimate provides this information for various sites in Florida, and these are the “chill hour” requirements that are provided for the various fruit cultivars when you look at descriptions. However, with warm Florida winters with variations from warm to cold, we find this model does not work well for all plants, especially the low-chill southern highbush blueberries. The Dynamic Model is better for predicting where southern highbush blueberries will produce best and to time growth regulator applications.
In the Dynamic Model “chill portions” accumulate with cool temperatures and are lost with warmer temperatures from 33-550F. To make it even more complicated, some cool temperatures are more effective than others. The Dynamic Model, using “chill portions”, is a much more stable measurement – it varies less from year to year in any one place than “chill hours” do. The University of California produced a program you can use with hourly data from a datalogger to calculate chill portions. The University of Florida calculates chill portions for you at the Florida Automated Weather Network (FAWN) site. When you look at the current observations for any selected site, on the left side it offers the Chill Portions (Dynamic Model).
How Much is Enough?
How do you use chill portions though? They are different from chill hours and how do you know when you have enough? Since it is a new way to measure, we will need to determine for each cultivar what they need. California peaches require anything from 8- 75 chill portions, and Flordaprince is reported to require 8 chill portions. We will need to work on determining chill portions to report for cultivars rather than chill hours if we want to have a better way to determine adaptation of fruit crops to the various regions of our state.