Rows of pistachio nut trees

Chill Hours Getting More Complicated

Taylor HillmanTree, nut & vine crops, Weather

Chill hours
Calculating chill hours for pistachio trees has become more challenging as “normal” seasonal weather is changing. The warmer, wet winter in California may be complicating the issue even more.

Many trees need a certain amount of chill hours during the winter months. The pistachio industry was hit hard by a lack of chill in 2014, especially in Kern County. Chill hours are calculated by the period of time the temperature is under 45 degrees. “If we look back 10 to 20 years, we used to have days where it was foggy all day long. The temperatures were 40 to 45 degrees. You didn’t see much change, and you could accumulate 24 hours of chill,” California Research Board Manager Bob Klein said at this year’s Statewide Pistachio Day.

Seasons and weather have changed in California, however, and full foggy days are few and far between. Klein said this is changing how chill is calculated. “Now what we see is what we call ‘sunny chill.’ That’s when we get these really cold night temperatures that are in the 30s. Then it warms up to the 60s during the day, and the bud temperatures get even higher than that because the sun is shining on them all day,” he said.

Cooperative Extension Advisor Craig Kallsen showed visuals of the impact the sun has on a tree with the lack of fog. Kallsen’s pictures showed trees that had leafed out on one side, yet not at all on the sun-facing side.

Klein said this winter is complicating the chill calculation even more. “Now we have had all of these warm rains where the temperature didn’t really get below 45, but it wasn’t sunny either, so maybe the buds didn’t get all that hot,” he said. These different situations have added new variables to estimating chill and create a new question for researchers: “How do we exactly, in our minds at least, compensate for these different wet-warm chill versus cold-sunny chill?” Klein asked.