California Pistachios: Dormancy Studies – Fuzzy Findings

Tests continue using winter-applied kaolin clay or calcium carbonate-based materials to either reflect solar radiation or diffuse as a way to improve dormancy.

  • Results from David Doll, UCCE Farm Advisor, Merced County, and Valley Orchard Management, showed a 200 to 250 pound increase in APC yield over untreated trees when Surround (kaolin-clay) was applied prior to the 2015 season.
  • In 2016, trees treated with dormant oil yielded more than either the untreated or Surround treated trees.
  • In 2017, the untreated, Surround, and Surround plus oil treatments yielded more than the oil alone treatment, but not the Surround plus oil treatment.

Thus, the Surround plus oil treatment has yielded the most over two years. However, we have insufficient data to recommend any of the treatments. David believes the test results are partly associated with differences in rest satisfaction, since Surround application provided approximately 10 percent greater chill portion accumulation from lower flower bud temperatures.

The 2014 winter was well below the chill portions required for adequate pistachio rest; hence the kaolin treatment was more valuable. High chilling during the 2015 winter rendered the kaolin treatment less valuable in 2016, and the reduced yield in 2017 from the winter oil treatment could simply be an alternate bearing effect. David hopes to continue this experiment to determine if one treatment clearly separates itself from the others.

Our lack of understanding the physiological effects of oil, reflective, and diffusion materials makes this research very difficult.

Calcium carbonate-based diffusion materials work differently than kaolin-based clay materials. Kaolin clays reflect light to reduce the absorption of solar radiation by plant tissue such as flower buds. It is also marketed as a finely ground powder, which growers report to be more difficult to apply than liquids.

In contrast, calcium carbonate crystals modify the incoming light through a process called double refraction. This essentially divides the light rays as they intercept the crystals, and thus reduces their energy. Incoming light can also hit the crystals whose size matches the incoming wavelength, resulting in a so- called “sparkler effect” in which light is dispersed in multiple directions.

Both light division and the sparkler effect reduce energy absorption by the plant, resulting in lower temperature. My intent in describing the methodology of calcium carbonate is NOT to suggest it is better than kaolin-clay. It is simply to inform the reader that kaolin-clay and calcium carbonate are distinctively different in their mode of action.

We cannot tell you if kaolin-clay or calcium carbonate provides statistical improvement in chill accumulation and subsequent yield benefit, because this research is slow to progress, due to the complexity of its performance. The weather also cannot be controlled to secure the needed temperature differences.

Field reports indicate some growers have begun treatment of these products in early December as a precaution. The use rates of the various kaolin-clay products vary from 25 to 40 pounds per acre.

The liquid calcium carbonate is typically applied at four gallons per acre. Re-application is recommended after significant rainfall. Applications are not presently advised in February, unless one desires to delay bud break and bloom due to the risk of spring frost in your growing area. The cost per application is estimated at $80-90 per acre.

Happy New Year, Farming, and see you at Pistachio Day, Wednesday, January 17, 2018, at the Visalia Convention Center.



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