In keeping track of chill accumulation during the winter, growers may be best served by using the dynamic model. UC Cooperative Extension Orchard Systems Advisor for Yolo, Solano, and Sacramento counties, Kat Jarvis-Shean said that “portions” measured under the dynamic model provide more context for chill accumulation.
“In some recent warm winters, you were a little bit surprised because it looked like we got enough chill hours for the trees to behave normally but it was a trick because that model doesn’t really count as the way the trees count,” said Jarvis-Shean. “So, if you’re looking at kind of predicting how your trees will come out of dormancy, and especially if you’re looking at trying to apply a dormancy breaker based on the physiological feeling within the tree of how much chill it has experienced, it’s a much safer bet counting in terms of chill portions.”
Cumulative chill portions will assign different values to different temperatures. Jarvis-Shean explained that chill portions can better account for temperature variability. “That cold at night, it doesn’t really count if it’s then counteracted within the same 24-hour period by warmth. Chill hours would still count it anyway, but the tree doesn’t really feel it that way. So chill portions has a way to subtract when that heat follows an experience of cold,” said Jarvis-Shean.
As of December 28, the UC Davis Chill Calculator indicates that the Shafter CIMIS station has registered 35.2 portions under the dynamic model, with 751 hours below 45°F. The station in Five Points has recorded 36 portions, with 755 hours. There have been 839 hours in Merced, with 38.6 cumulative portions. In Manteca, there have been 724 chill hours, equating to 37.3 portions. Finally, the CIMIS station in Durham has registered 38.6 portions, with 765 hours.
Listen to the latest chill report below.