Insufficient chilling last winter significantly reduced some California deciduous fruit tree crops, Northern California cherry growers and Southern California home growers being among those affected. This winter, as of January 10th, due to unusually warm temperatures until about Christmas, accumulated fruit tree chilling for most California locations was generally even lower than a year ago.

 

As measured by chill hours (hours of temperatures below 45°F with no adjustments), temperature data from 36 of 62 selected weather stations indicate chilling to January 10th* was less than 60% of last year's amount. Only 9 of the 62 locations had chill hours equalling 90% or more of last year's.

 

However, accumulated chilling "portions" (units used by the Dynamic Model, a more complex method of estimating fruit tree chilling) compared to last year were off not nearly as much. Only 8 of the 62 stations recorded temperatures that correspond to chill portions of less than 60% vs. a year ago, and 27 stations had chill portions of at least 90% of last year's amount. For example, as of January 10th the Gilroy station had only 59% of last year's chill hours, but 97% of last year's chill portions; Modesto had 48% and 87%, respectively.

 

Depending on specific variety, climate and year, fruit tree chilling is accumulated through January or into February and March.

 

Aside from the chilling models, DWN regional sales manager Tom Spellman reports Southern California fruit tree defoliation, dormancy and hardening appear to be much closer to normal this year compared to last, likely attributable to the cooler early winter afternoons. This promises a better bloom and fruit set.

 

The linked data page, Chilling Hours & Portions, January 10, 2015, compares Hours and Portions for this year and last, for the 62 California locations.

 

*From the UC Davis Chill Calculator pages.

 

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