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Following a bad chill hour winter (2014-15) and an “okay” winter in 2015-16, California pistachio growers are looking for a larger accumulation of chill hours by the end of February to bring in a top quality crop.

“We are in better shape at this time than we were last year,” said Setton Pistachio grower services rep. Mike Smith estimated in mid January. “The rain and fog are adding more hours, but we still have a long ways to go this season.”

Chill hours, a summation of hours below 45 degrees F, are counted from November 1-February 28.

Pistachio trees in the northern growing areas of the state are ahead in chill hours, said Bob Klein with the Pistachio Research Board. In the south, where 95 percent of the state’s pistachio trees are growing, the outlook is that chill will be sufficient, but not great, Klein said. With the memory of the warm winter of 2014-15 and subsequent poor yields, growers have experienced what very low chill hours can bring.

“When it is marginal, it’s hard to say,” Klein said, noting there are still several weeks to rack up more chill to give trees a better dormant season. Insufficient chill hours can result in delayed or irregular bloom, late vegetative development, altered leaf morphology, poor pollen production, death of stigma, reduced fruit set, increased proportion of blanks and unsplit nuts, late maturation and general reduction of crop yield.

Using 900 hours of temperatures below 45 degrees F as the benchmark for desired chill hours, UCCE farm advisor Elizabeth Ficthner notes that some pistachio production areas in the state did attain enough chill hours in 2015-16 but other areas fell short.

The amount of time California pistachio trees experience temperatures below 45 degrees F may not tell the entire chilling story. There is some heat and chill interdependence according to UC researcher Louise Ferguson. Excess chill hours means fewer heat hours are necessary in the spring for good bloom and fruit set. Spring heat compensates slightly for winter chill deficiency.  Very warm spring temperatures can adversely affect flower quality.

Ambient air temperatures may not reflect what trees are experiencing, Ferguson said. Other environmental conditions such as fog or bright sunlight may influence tree response. For example, less fog or wetness decreases evaporative cooling. More sunlight hours may warm even the shaded parts of a tree. Another method for measuring chill hour accumulations, called chill portions, places weight on intermittent warm temperatures during the November 1-February 28 time period. The chill portion formula subtracts from the total hours accumulated when temperatures reach 55 degrees. There is no correlation between the two chill measuring methods.

UCCE farm advisor emeritus Bob Beede, in his 2017 task list for pistachios, noted that in mid December, chill portion accumulation was about 40 percent less than last year at that time. Chill portions- the temperature measuring method that is weighted, were counted from September 1-December 13. Shafter, Delano and Blackwell’s Corner areas in Kern County had accumulated only 12, 15 and 18 chill portions respectively compared to 24, 25 and 24 last year. Madera and Durham areas were closer to the 2015 mark with 21 and 22 chill portions. The only cold weather during that time occurred early in December and the state experienced unseasonably warm temperatures later in the month.

Growers can check on their local chill accumulation at the UC Fruits and Nuts Center web site, clicking on the “Weather—related Models and Services and selecting chilling accumulations models, then cumulative chilling portions. The site shows chill portion accumulation for every CIMIS station in the state and also provides historical data. Beede cautioned that the stations were designed for irrigation scheduling, not for chill portion measurement, and the absence of fog at the stations can cause temperature differences up to 20 degrees F between ambient air and the buds.

Beede noted that research he and Ferguson did found that Kerman and Peters varieties do not grow normally when they do not receive adequate winter rest. Beede said the research suggests that Kerman requires 750 hours below 45 degrees F and Peters 850 hours to leaf out and bloom adequately in the spring.

In addition to shortening the rest period for the trees, Beede said warm temperatures also elevate the bud respiration rate which consumes carbohydrates critical for spring growth. This finding by UC Plant Sciences professor Maciej Zwieniecki, may explain why oil applied to pistachio trees to enhance rest breaking cause poor production in 2015, Beede said. In response to the applications, the trees increased respiration rates to metabolize the oil and depleted their carbohydrate stores. At bud break, the combined deficiencies in available sugars and low chill caused poor leaf out and fruit set. Beede said due to the uncertainty of this winter’s weather pattern, oil application is not being suggested.

Some growers, Beede reported, took the precaution of applying kaolin clay or calcium carbonate in early December to reflect or diffuse solar radiation. The use rates for the clay vary from 30-50 pounds per acre, he said, while the liquid calcium is typically applied at four gallons per acre. After significant rainfall, the products must be re-applied. Costs per application is in the $80-$90 per acre range.

The effects of winter applied kaolin clay or calcium carbonate to pistachio trees

continue to be studied. Each has a different mode of action. The clay products, which are finely ground powder, work by reflecting light to reduce the absorption of solar heat. Calcium carbonate crystals modify the incoming light, reducing its energy and dispersing it in multiple directions. These actions reduce energy absorption by the plant, resulting in lower temperatures.

Merced County farm advisor David Doll’s research showed a 200-250 pound increase in CPC yield over untreated trees when kaolin clay was applied prior to the 2015 season. Beede said the data suggests that the treatment to mitigate the negative effects of warm winter temperatures does not ensure a normal crop, but may prevent extremely poor yields.

Beede noted unreplicated field trials with calcium carbonate in Kern County last year were able to increase chill portion accumulation by about 13 percent due to lower bud temperatures. The trials included single and double applications January 12 and February 12. Flower bud temperatures were monitored in treated and untreated trees. The data showed bud temperatures were reduced by as much as 10 percent and the rate of heating during the morning hours was slower. Beede said the January treatment increased chill portions accumulation by about 13 percent. The treated trees also leafed out more evenly and the second treatment in February delayed development by about four to five days compared to untreated trees. Beede that the trial was done where the chill requirement for pistachio was satisfied during the previous winter and the outcome for trees that did not have sufficient rest could be different.

Yield improvements were found in the trial using the calcium carbonate product Diffusion and the return on the cost of application was positive. Without replication, Beede stressed, they were not reporting the actual yield increase.


Cecilia Parsons