Growers keeping track of what winter temperatures have been doing in their orchards have a couple of different options. UC Cooperative Extension Orchard Systems Advisor for Yolo, Solano, and Sacramento counties, Kat Jarvis-Shean said the most commonly referenced is the chill hours model. Any hour below 45°F is measured as one chill hour. The dynamic model for tracking winter temperatures takes more information into account.
“Chill hours counts any hour between freezing and 45° as the same. Chill portions gives different values to different temperatures,” Jarvis-Shean explained. “So instead of saying, ‘well but they were warm chill hours, we don’t know how that will affect the trees,’ chill portions actually has a way to account for that being a warmer – like a cool, but not cold – temperature experience.”
The chill hour model can also have some limitations in only indexing hours below 45°. Jarvis-Shean noted that because of California’s mild climate, trees can still get some value in temperatures above that threshold. The dynamic model can provide growers with a more specific indication of what the trees are experiencing during the cold temperatures. “The prime, best chill is around that range of 43° to 47°, but you still get some chill up to 54° in the chill portions model. So that mild cool, but not all the way cold, still counts for something,” said Jarvis-Shean.
As of December 13, the UC Davis Chill Calculator shows that the CIMIS station in Shafter has registered 24.4 portions under the dynamic model, with 452 hours. The station in Five Points has recorded 24.7 portions, with 453 hours. There have been 552 chill hours in Merced, with 27.1 cumulative portions. In Manteca, there have been 552 chill hours, equating to 25.9 cumulative portions. Finally, the CIMIS station in Durham has registered 28.2 cumulative portions, with 515 hours.
Listen to the latest chill report below.