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Farmers keep watch on amount of orchard ‘chill’

Issue Date: December 24, 2014
By Christine Souza

As winter begins this year, farm advisors and orchard farmers focus their attention to the chilling temperatures needed to help buds, flowers and fruit develop—and they're expressing a bit of concern about the lack of chill during the autumn.

"I am concerned about this year. Regarding chill hours, we're way behind from what I understand," said Sunny Dale, who grows prunes, peaches and walnuts in Sutter County, and almonds and grapes in Kings County.

Researchers and growers start tallying chill hours on Nov. 1 and continue to track these units until early spring, depending on the crop variety.

Dale said he wants the chill hours to accumulate to around 800 by mid-February.

"If you are close to that number, you don't need to be worried, but some years if you get to about half of that number, it can get serious," Dale said. "From where we are at currently, we're drastically behind in Sutter County."

University of California Cooperative Extension specialist Ted DeJong said deciduous fruit and nut trees do better with moderate to high chill, between 32 and 45 degrees, followed during bloom by daytime temperatures between 50 and 70. On average, he said, California orchards experience from 800 to more than 1,000 chill hours annually.

"Rarely do we see a total crop failure due to lack of chilling in California, because we get enough (chill units) so that there's actual bloom," DeJong said.

What keeps farmers up at night, he said, is the potential for an uneven bloom, a poor fruit set and problems with fruit quality.

"Looking at the current (chill) figures, we've got a long way to go," DeJong said. "It could be a bad year, but it is still early and if we have cool weather setting in and if we don't have negating temperatures like we did in January, we might recoup."

To gain a more sophisticated picture of orchard crops' chilling needs, farm advisors have begun using "chill portions," units of measurement they say account more fully for warm daytime temperatures that may cancel out at least a portion of the benefits brought by chilly overnight weather.

DeJong said the chill portions calculation "accumulates it (chill) for a period of time and if there are no intervening periods of high temperatures, it dumps it into a permanent bucket. If you have warm temperatures, it (this build-up of chill) gets negated."

That's what happened during the past winter, "when almost every day, it went from the 30s to the 70s," he said.

Researchers now recommend that growers start keeping track of chill portions rather than traditional chill hours.

"If we count in chill hours, we are going to keep getting surprised," UCCE farm advisor Katherine Pope said. "Now is the time to start counting chill in a new way. Get comfortable with it and figure out what an average winter counts up to in chill portions."

The Dynamic Model used to record chill portions is more complicated to use but, Pope said, most agricultural researchers are realizing it does a better job at measuring cold or moderately cold temperatures and shows whether they count toward an orchard's chill requirement.

"This winter, so far, shows a reverse of chill units that growers saw last year, with traditional chill hours showing that things are behind, and showing average when looking at chill portions," Pope said. "Looking back at chill accumulation, at this point last year we were also average, but there's still plenty of winter left for things to quit being good."

Based on historical figures, Pope said, low chill years only happen once every 10 to 20 years, "so there is very little reason to think that this would be a year like last year in terms of chill—but it is not a guarantee."

Almonds have among the lowest chilling requirements—between 400 and 600 hours on the traditional scale—with low-chill stonefruit varieties requiring about 300 hours and higher chill varieties needing about 1,100 hours.

UCCE pomology farm advisor Rachel Elkins in Lake and Mendocino counties, who tracks chilling units for pear and apple varieties, said, "The bad thing that happened last winter was that it got really warm in January, which offset what (chill) had been gained last November and December, and that is what caused the problems."

Richard Buchner, UCCE orchard advisor and county director for Tehama County, reported similar problems in prunes in his growing region. He noted that last season, Northern California growers saw fewer chilling hours and as a result, "the bloom in prunes was erratic and the trees did not set a crop well, which means growers had a more difficult time supplying markets with product."

UCCE farm advisor Kevin Day in Tulare County reports that in Tulare and Fresno counties last season, stonefruit yields were not affected and "most commodities had record years." He said he emphasizes to growers the importance of winter irrigation during dry years.

Also important to the success of chilling, he said, is the presence of rain or fog, or a reduction in the amount of sunlight.

"In the last week or two, we had some rains, it was overcast and we're starting to get some fog, and that is very nice to see," Day said.

In general, the amount of fog is decreasing in the Central Valley, said Pope, who advises farmers in Sacramento, Solano and Yolo counties.

"About three out of 10 winter days were fog-covered days and in the last 20 years, it's dropped down to two of 10 winter days," she said.

Weather records show that temperatures are getting warmer in the Central Valley, Pope said, revealing that annual minimum temperatures have gone up more than 4 degrees since 1970.

During the winter of 2013-14, she said, "average chill was down 25 percent in the Central Valley, falling behind in January and never catching up."

Looking at the figures, the traditional chill hour model showed that there were enough chill hours, yet the chill portion model indicated that hours were quite low.

To calculate chill portions using the Dynamic Model, go to

(Christine Souza is an assistant editor of Ag Alert. She may be contacted at

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.

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