California Chill Report: The Crucial Role of Winter Chill in Fruit and Nut Trees

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Winter Chill

Farmers are closely monitoring a key factor that plays a crucial role in the success of their orchards – winter chill accumulation. Winter chill is the cumulative exposure of deciduous trees, such as tree nuts and stone fruit, to cold temperatures during the dormant season. UC Cooperative Extension Orchard Systems Advisor, Kat Jarvis-Shean, said that inadequate chill can create a host of issues for growers.

“When trees don’t get enough winter chill accumulation, we see a straggled bloom in the springtime,” Jarvis-Shean said. “So, a longer window of time between when the first flowers open and the last flowers open, which can then cause problems down the road with straggled harvests, different sizes in your lot, that sort of thing.”

In moderate cases, insufficient winter chill may result in a prolonged flowering period, causing a ripple effect on harvests with varying sizes and timing. “When you really, really don’t have enough winter chill, you have decreased bud break overall. So, fewer buds opening, fewer flowers, fewer things that you can harvest at the end of the year,” said Jarvis-Shean.

The impact of winter chill extends beyond the flowering stage, especially in nut crops. Jarvis-Shean notes, “When you have that straggled bloom, different nuts starting the race at different times, you end up with differences in size.” This is particularly critical in nut crops, where varying sizes can impact the overall yield and quality of the harvest.

Information from the UC Davis Chill Calculator shows that as of December 19, the Durham CIMIS station has logged 26.9 portions under the dynamic model, with 470 hours below 45°F. The station in Manteca has registered 24 portions, with 416 hours. There have been 557 hours in Merced, with 21.6 cumulative portions. In Five Points, there have been 431 chill hours, equating to 20.7 portions. Finally, the CIMIS station in Shafter has registered 19.9 portions, with 422 hours.

Listen to the latest chill report below.