- Author: Janet M. Zalom
The simplest models developed to estimate chill accumulation use a simple count of the number of hours in which the temperature is below 45°F (Chill Hours). However, there is a concern with the chill hour models, particularly under mild winter conditions, such as last year. From the perspective of tree physiology, when cold winter nights (below 45°F) are immediately followed by warm winter days (above 75°F), some of the accumulated chill can be cancelled out, and the Chill Hour model does not account for this. The Dynamic Model was developed to respond to these conditions. It counts chill accumulation (Chill Portions) during hours below a temperature threshold, but then eliminates that accumulation if cold temperatures are followed by warm temperatures within a 24 hour time span.
Regardless of which model you are watching, the current season is chilly, so far. Both Chill Portions and Chill Hours are higher than 5-year averages (tables below). Does this mean we will meet the chilling requirement for your trees? It is too early to say for sure but we are off to a great start!
For excellent references for the chilling requirements of various crops, including variety-specific details, see: Fruit & Nut Crop Chill Portion Requirements by UCCE Orchard Advisor Katherine Pope and the table at the bottom of the webpage: Prune Chilling Prediction Model by UCCE Orchard Advisor Franz Niederholzer.