Do “Rest Enhancement” Cover Sprays Pay? Simple answer: I do not know.
No long-term, a replicated trial has been done using kaolin or calcium carbonate-based products to the point that we could declare their economic value. Field reports indicate some growers have begun treatment of these products in mid-November 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 a specific growing area. The cost per application is estimated at $80 to $90 per acre.
What’s The Difference, Materially Speaking?
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, liquid 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, only to say that kaolin-clay and calcium carbonate are distinctively different in their mode of action.
Limited Research And Results
Application of these products is intended to reduce the flower bud temperature. Carl Fanucchi and I worked with Tim Baker, co-owner of the Diffusion product, three years ago to measure what effect calcium carbonate crystals had.
Tim inserted tiny thermocouples into the flower buds without injuring them; the data showed a 100F decrease in bud temperature. Any reduction in winter bud temperature theoretically benefits the tree by decreasing plant respiration rate, and thus, stored carbohydrate consumption.
However, a 100F reduction in temperature would theoretically only improve rest satisfaction if lowered bud temperatures to 450F or below. That is the recognized threshold for chill accumulation. That is possibly why Dr. Gurreet Brar found that whitewashing pistachio trees in 2015 lowered bud temperatures, but did not affect chill accumulation significantly.
It is unfortunate that another UCCE farm advisor did not continue the “rest enhancement” research started by David Doll, former Farm Advisor, Merced County. Teamed with Valley Orchard Management, Doll began a kaolin-clay (Surround) trial during the 2014-15 winter.
Kaolin-clay showed a 200 to 250 pound increase in APC yield over untreated trees when 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 oil alone.
The Surround plus oil treatment has yielded the most over two years. However, this is insufficient data to recommend any of the treatments.
David believed the test results were 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.
If You Decide To Treat…
Our lack of understanding the physiological effects of oil, reflective and diffusion materials makes this research very difficult. It is going to take a more organized, multi-disciplined effort to better define the causes and effects.
If you do decide to apply kaolin clay or calcium carbonate crystals, leave some untreated areas for comparison. Also, re-treatment after an estimated half inch of rain is necessary to maintain the temperature-depressing effect. That could easily mean three treatments over the winter.