California Chill Report: Storm Impacts on How Trees Experience Chill

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Most of California has been experiencing a substantial amount of rain since the end of 2022 and that can have an impact on how trees are experiencing chill. UC Cooperative Extension Orchard Systems Advisor for Yolo, Solano, and Sacramento counties, Kat Jarvis-Shean said that the weather can be measured as “anywhere from neutral to good” as it relates to chill accumulation. The impact on winter chill has less to do with overall temperatures and more to do with sun exposure.

“When we have these overcast days or these stormy, cloudy days during the daytime, the suns not getting through to the buds. So, they’re sort of shrouded from the sunshine and the energy and the heat that that sunshine gives,” Jarvis-Shean explained. “When we have low chill winters, it often coincides with dry, drought winters, because we have clear sunny days, and those flower buds and vegetative buds are just cooking out there in the sunshine.”

Temperatures have not been dramatically different for this part of the season due to the storm systems that have come through. But the lack of direct sunlight is impacting orchards similar to what foggy conditions can provide for chill accumulation. “Even if the air temperature is in a decent range, the tree feels it as warmer because it’s out there getting all that sun. So, these overcast days, stormy days are good for winter chill accumulation,” said Jarvis-Shean.

The most recently available information from the UC Davis Chill Calculator indicates that the Shafter CIMIS station has logged 42.9 portions under the dynamic model, with 810 hours below 45°F as of January 11. The station in Five Points has registered 44 portions, with 820 hours. There have been 882 hours in Merced, with 47.8 cumulative portions. In Manteca, there have been 769 chill hours, equating to 46.8 portions. Finally, the CIMIS station in Durham has registered 48.8 portions, with 845 hours.

Listen to the latest chill report below.