California Pistachios: Enough Chill, So Far, For 2018 Yields?

High temperatures prevailed until the first week in December, when we finally got frost on December 5. The first frost normally occurs about the second week in November.

We had Thanksgiving outside at our neighbor’s house in shirt sleeves. I cannot remember the last time I did that. Maybe never.

It was 770F November 23, and 820F on Black Friday. I am therefore surprised that the Chill Portion accumulations for selected CIMIS stations are not too far behind last year. Can this be true?

Gee, I know a way farmers could determine that. It’s called YOUR OWN WEATHER STATION IN THE ORCHARD, something I have been whining about for years. I think you guys don’t respond to my pleas, just to watch me froth at the mouth, like an English bulldog.

Table 1 provides the chill portions for various sites throughout the Valley between September 1 and December 10 for the past five winters, as well as 2010 in which over 70 chill portions were accumulated by February 15. This exceeds the 58-60 chill portions estimated to satisfy the rest requirement of the Kerman cultivar.

The Peters male may have a chill portion requirement as great as 65.

The values in parentheses are the total chill portions accumulated by station and year. As you can see, 2013 and 2014 were significantly warmer than 2010, in which dormancy was well satisfied throughout all areas of the state.

The 2015 winter data shows good chill accumulation throughout the Central Valley in mid-December, and continued cold temperatures through January contributed to the record 2016 crop.

In contrast, 2014 was already showing deficient chill accumulation at several locations by mid-December. The Arvin/Edison and Coalinga stations might be considered the “canary in the coal mine” for early assessment of future low chill winters. HOWEVER, now that all of you have your OWN weather stations, you can obtain much more valuable data than a poorly maintained CIMIS station being used as a scratching pole by a white-faced steer.

The importance of orchard- accurate chill data is realized by the 2017-18 CIMIS data I have summarized for your consideration. As you can plainly see, chill portion accumulation is marginal thus far, and we need a shift towards colder days to meet the pistachio chilling requirement.

Table 1. Chill portion accumulation for various CIMIS stations statewide from 9/1-12/10 for selected years. Numbers in parentheses are the total chill portions accumulated at each station by year from 9/1- 2/15.

Year 2017-18 2016-17 2015-16 2014-15 2013-14 2010-11
Durham 21 23(64) 25 (66) 22 (55) 20 (54) 28 (70)
Patterson 19 16(54) 20 (59) 23 (63) 22 (63) 26 (73)
Madera II 20 22(68) 25 (66) 25 (52) 15 (57) 23 (NA)
Parlier 16 14(56) 26 (67) 27 (64) 22 (53) 27 (74)
Five Points 15 15(56) 24 (65) 15 (52) 20 (55) 24 (69)
Coalinga 14 16(60) 25 (62) 13 (48) 20 (53) 28 (70)
Shafter 17 12(49) 24 (59) 25 (61) 24 (63) 23 (70)
Delano Missing 15(56) 25 (65) 16 (58) 22 (56) 24 (73)
Blackwell’s 16 18(60) 24 (67) 15 (52) 21 (50) 27 (75)
Arvin/Edison 13 15(54) 23 (61) 10 (44) 21 (55) 22 (66)
Porterville 16 14(49) 30 (76) 20 (63) 22 (59) 27 (63)


I have gone back and reviewed the chilling observations of Dr. Julian Crane, as well as the research Dr. Louise Ferguson and I performed individually and collectively. It all clearly states that Kerman and Peters do not grow normally when winter rest is inadequate.

Our research efforts suggest Kerman requires 750 hours below 450 F, and Peters 850 hours in order to leaf out and bloom promptly in the spring. One experiment suggested that Peters continued to benefit from cold temperatures up to 1200 hours below 450F.

It was also reported in these studies that a minimum of 500 hours below 450 F was needed to initiate much bud break from Peters. University of California Circular 179, “Deciduous Orchards in California Winters”, by W.H. Chandler and

D.S. Brown (1936), states that December and January are the two most critical months in California to satisfy the rest requirement. During the 2013 and 2014 winters, the unusually warm temperatures in January did not provide its complement of chill hours.

The effect of high winter temperatures is thought to be two-fold:

  1. They negate the effect of chill hours already accumulated by altering the complex physiological processes occurring during dormancy.
  2. They elevate the bud respiration rate which consumes the limited amount of carbohydrates critical for spring growth.

UC Davis Plant Sciences Associate Professor Maciej Zwieniecki (Dr. Z) has joined our pistachio industry research team to study this important aspect of tree biology. Dr. Z suggests there may be a critical amount of carbohydrates and other growth substances needed to produce normal growth in the spring.

This may explain why oiled trees performed so poorly in 2015. Oil is thought to enhance rest breaking by causing a slight stress to the tree,which is not phytotoxic. In the process of metabolizing the oil, the tree may increase its respiration rate, which renders it more responsive to favorable spring temperatures for growth.

Thus, high January temperatures and oil treatment possibly have a compound effect on carbohydrate depletion from elevated respiration. When the time comes for bud break, the deficiencies in both chilling and available sugars create the perfect storm for poor leaf out and fruit set.

There could also be detrimental effects to male and female flower development and receptivity. Because of the current uncertainty of this winter’s weather pattern, oil application is NOT being suggested at this time due to the negative impact it had on the 2015 season.

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