Warmer winters have been occurring more frequently in California, impacting dormancy in orchard crops. UC Cooperative Extension Orchard Systems Advisor for Yolo, Solano, and Sacramento counties, Kat Jarvis-Shean said some years are still experiencing adequate chilling. However, the overall warming trend has still been increasing.
“Historically, if you go back in the records, we would have a low chill winter that would be disruptive to production about every 20 years. We’ve had at least two, arguably three, in the last decade,” Jarvis-Shean explained. “This year actually, winter chill is pretty good, so that’s not to say that every winter has been a low chill winter. But we’re getting more of them and they’re coming more often.”
Walnut varieties in the ground right now have the desired traits that growers want but have a certain amount of chill required for optimal production. Jarvis-Shean noted that researchers are working on developing varieties with a lower chilling requirement that still have the other traits valuable to growers. That process will take time and growers will still need to get strong production from the varieties currently available, even with warmer winters impacting trees.
“Until recently we haven’t had a lot of tools in the toolbox, which is why with the funding from the Walnut Board and now CDFA, we’ve been looking at different dormancy breaking treatments to help encourage trees along to wake up in the spring when they haven’t gotten enough winter chill,” said Jarvis-Shean. “So, we have these tools now to help with those varieties while breeders are looking at other varieties to start planting 10-15 years from now.”
Information from the UC Davis Chill Calculator shows that as of March 2, the Durham CIMIS station has logged 81.7 portions under the dynamic model, with 1,447 hours below 45°F. The station in Manteca has registered 79.5 portions, with 1,316 hours. There have been 1,499 hours in Merced, with 79.8 cumulative portions. In Five Points, there have been 1,384 chill hours, equating to 75.2 portions. Finally, the CIMIS station in Shafter has registered 75.6 portions, with 1,340 hours.
Listen to the latest chill report below.