Kaolin Clay May be Useful in Increasing Chill Accumulation in Pistachios

David Doll, University of California Cooperative Extension, Merced, CA; Clay Beck and Joe Coelho, Valley Orchard Management, Three Rocks, CA.

A preliminary study conducted by the University of California Cooperative Extension and Valley Orchard Management in which Surround® was sprayed onto dormant pistachio trees has shown an increase in cluster count and yield. This technique may be useful in increasing chill unit accumulation by 5-7 portions in winters with minimal fog and high amounts of sunshine.

Dormant pistachio tree covered with kaolin clay to help reflect sunlight.

Dormant pistachio tree covered with kaolin clay to help reflect sunlight.

Over the past two years, pistachio orchards have shown classic low chill symptoms, which include weak, delayed, and protracted bloom, poor overlap with males, and weak leaf-out. This can lead to a severe reduction in yield due and high blanking due to poor pollination.

Chill is weather related and a function of temperature and time. In cool, cloudy weather, chill portions accumulate quickly as temperatures stay within the ideal range of 33° and 54°F. When temperatures are below or above this range, chill units do not accumulate and could be subtracted if the temperatures are too high. This means that during warm, sunny weather during tree dormancy, chill portions do not accumulate, and if too warm of weather, negation of earlier accumulated chill occurs (For more information please see: http://thealmonddoctor.com/2015/01/13/chill_portions_model/).

Chill units are calculated using ambient air temperatures. This may be inaccurate, however, due to the temperature of the wood. In sunny weather, wood is usually warmer than the ambient air temperature due to radiant heat, while in cloudy weather, the wood temperature matches ambient air temperature. With this consideration, sunny weather may increase wood temperatures above the ideal range, negatively impacting the accumulation of chill.

In attempts to increase chill unit accumulation in the winter of 2014-2015, several applications of Kaolin clay (Surround®) were made to a ‘Kerman’ with ‘Peters’ pistachio orchard located near Coalinga, CA. Seven rows were randomly selected and sprayed, and the remainder of the orchard was left untreated. The first application was made on January 10th, 2015 with 40 lbs/acre of Surround® applied by ground rig. Follow up applications were made after rains on January 23rd and February 10th, 2015. Data loggers were installed in four trees, two data-loggers per treatment, with three sensors per tree on February 6th, 2015. Sensors in untreated and treated trees were 100 feet apart. Temperature sensors were wrapped around the wood in order to measure the temperature of the wood with one sensor in the northeast and southwest quadrant of the tree, while the third measured ambient air temperature. Upon data logger removal, chill portions were calculated by taking hourly temperature data and using the spreadsheet available at http://ucanr.edu/sites/fruittree/How-to_Guides/Dynamic_Model_-_Chill_Accumulation/. At harvest (September 15th, 2015), clusters were counter per tree and individual tree harvest were taken. Ten randomly selected trees in each treatment were measured for the paired study.

The kaolin treated trees were found to have a higher cluster count and yield (Table 1). Kaolin trees had an average of 50.4 clusters and 2.004 lbs per tree, while untreated trees averaged 21.0 clusters and 0.015 lbs per tree. Blanking was considerably higher in the untreated trees. Temperature information collected from the data loggers found an increase in chill portions between the period of February 6th and 20th, 2015 (Table 2). Over this two week period, kaolin coated trees accumulated one more chill portion than the untreated trees and ambient air temperature. Furthermore, kaolin coated trees leafed out and bloomed 14-20 days after untreated trees during March/April, 2015.

This preliminary study suggests potential benefits of spraying kaolin for chill portion accumulation during sunny, warm winter. Further study is needed to determine the correct rates and timings. Based on the reduced chill portion accumulation, this material should be applied earlier to increase chill accumulation. In order to reduce the delay in leaf-out, February applications should not occur to increase heat unit accumulation during endo-dormancy.

With this in mind, the following should be considered if kaolin is planned to be applied to a pistachio orchard. Applications should begin in late November with re-applications after rain events through late January. The first application should be high enough to cover the trees (~20-30 lbs/acre) while successive applications may be reduced based upon the amount of rain received (~15 lbs/acre). Kaolin should not be applied after the first week of February unless there is a desire to delay bloom.  Applications may not provide a benefit and thus not be needed if the weather is cloudy, foggy, and cool. Please keep in mind that the rates and timings of start and completion of spraying are speculative and further work is needed.


Table 1: Tree cluster counts and yield per tree for kaolin treated1 and unsprayed ‘Kerman’ pistachio trees.

Variable Measured Kaolin Sprayed Untreated
Cluster Count 50.4* 21.0
Yield/Tree (lbs CPC) 2.004* 0.015

1: Kaolin sprays were made using Surround® on January 10th, 23rd, and February 10th, 2015 at 30 lbs/acre.

*: Indicates statistical difference at p<0.05 (pair-wise T-Test).


Table 2: Cumulative chill portions1 calculated from hourly temperature data from two data loggers placed within ‘Kerman’ pistachio trees between the dates of February 6th and 20th, 2015.

Location of Sensor Kaolin #1(North End) Kaolin #2(South End) Untreated #1(North End) Untreated #2(South End)
Northeast quadrant of tree 1.523 1.992 0.000 1.007
Southwest quadrant of tree 0.723 0.101 0.331 0.000
Ambient air 0.000 0.000 0.000 0.000

1: chill portions were calculated by taking hourly temperature data and using the spreadsheet available at http://ucanr.edu/sites/fruittree/How-to_Guides/Dynamic_Model_-_Chill_Accumulation/.

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11 thoughts on “Kaolin Clay May be Useful in Increasing Chill Accumulation in Pistachios

  1. It is common to add powdered milk or similar “adhesive” to whitewash to resist wash- off from rain and weather. One would think that any number of white products would work other than Surround which is really not all that white compared with other materials. The addition of a “sticker” would likely cut back on the number of needed applications.

    1. Dale,
      I am unsure about the powdered milk and sticker helping keep the material on the trees longer. Really depends on rain. Also unsure if it can be legally and feasibly used.

      I agree that any inert product that reflects light will help reduce radiant heat would probably work.

      Thanks for the comment,

      1. David,
        There is a lactose based sticker that would work great in this situation, it is called SKH organic adhesive. We use it all the time with Crop White and surround applications. Developed right here in CA over 40 years ago, Distributed By Brandt. Let me know if you need some.
        Great trial, looks promising!

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  4. Hi David,

    A comment from Australia, we have a warm and wet winter forcast here, so chill accumulation maybe a problem. Calcium carbonate resists wash off better then kaolin products, have you any experience with these products like MicroCal? Comments on these products would be greatly appreciated.

    Best Regards Russell

    1. Hey Russell,
      Yes, they can be used. The idea is to reflect heat. My only concern is that I want the material to wash off. If the material stays on through the rains, it will decrease heat unit accumulation and cause a delay in bud break.

      Also, keep in mind, the material only works by reflecting radiant heat from sunlight. If the skies are cloudy and warm, the “cooling” effect will be much less.


  5. Thanks David,

    Applying MicroCal calcium carbonate at leaf fall would give a good start for the chill accumulation. This could be more effective on higher chill crops like walnuts or cherry. Our leaf fall occurs in May, we will run trials this year on these crops to measure the difference in chill accumulation but more importantly time to blossom and blossom period and fruit set.

  6. Hello David
    A question from Chile….
    Do you know if applying Surround will increase the chance of frost damage? If you get a 0 or -1Celcius it would be more dangerous having the caolin applied?
    Thanks for the advice
    Best regards

    1. We do have nights that drop below freezing. My guess is that the damage wouldn’t occur due to the plants being in dormancy. Then again, if the days and nights are cold, the application probably wont be needed.


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