From intermittent to persistent chill insufficiency in California’s specialty crops

Abstract Sufficient winter chill is critical for breaking dormancy and enabling normal flowering and fruit set in temperature fruit and nut crops. Here, we use observational data to show that increasing interannual temperature variability has already reduced winter chill in California’s Central Valley, contrary to previous projections that chill insufficiency would occur only by the mid-to-late 21st century. Inadequate chill is affecting crops such as pistachio and plum, particularly in Southern California. We evaluate adaptation strategies using subseasonal temperature forecasts to assess chill sufficiency by early February and guide dormancy-breaking treatments in low-chill years. Using this approach, we demonstrate that a simple in-season forecasting strategy, driven by subseasonal temperature forecasts, can predict chill accumulation within 10 percent of observed winter chill in 89–94 percent of cases. We also propose revised chill thresholds to inform breeding programs for low-chill varieties and guide management interventions to reduce risks in low-chill years.