Sun Reflecting Products for Increased Winter Chill?

Written by Bob Beede (UCCE Kings County, Emeritus) and David Doll (UCCE Merced)

Tests of winter applied kaolin clay or calcium carbonate-based materials intended to either reflect solar radiation or diffuse it continue.  Results from David Doll, UCCE Farm Advisor, Merced County, and Valley Orchard Management, showed an increase in chill portion accumulation and a 200 to 250 pound increase in CPC yield over untreated trees when kaolin-clay was applied prior to the 2015 season. More can be found here.  The data thus far suggests that spraying these materials to mitigate the negative effects of warm winter temperatures does not assure you of a normal crop, but it might prevent NO crop!This past winter, Carl Fanucchi and Bob Beede collaborated with ORCAL, the company which manufactures ultra-fine, dry ground calcium carbonate, which is marketed in liquid form as Mask^® and Diffusion^®. They performed UNREPLICATED screening trials in two locations; one in Buttonwillow, and the second east of Highway 99 on Pond Road.  The screening trials included single and double applications applied January 12 and February 12.  A December treatment was planned, but the field could not be accessed.  Flower bud temperatures were monitored in treated and untreated areas using tiny thermocouples inserted into the buds without causing their death.  The resulting data showed bud temperatures were reduced by as much as 10⁰F, and the rate of heating during the morning hours was also slower.  Calculations indicate that the January treatment increased chill portion accumulation by about 13%, due to the lower bud temperatures.  Weekly rating of the treatments for bud break and bloom were also performed.  The treated trees emerged more evenly, and the second treatment of Diffusion applied in February delayed development by four to five days.  The single January treatment developed at about the same rate as the untreated.  These observations are from a site in which the rest requirement for pistachio was satisfied during the 2015 winter.  The results may differ when the chill portions are insufficient.  Carl successfully tracked yield from the treated and untreated trees at both sites with the outstanding cooperation of the growers and found a yield response. Results are not being reported due to being an unreplicated trial.Never-the-less, this trial documents that there may be several materials that reflect or diffuse heat. Based on our understanding, there are several products with varying "modes of action." Below is a list of a few products, alond with some thoughts on rates and mechanisms. The intent in describing the methodology of these products is NOT to suggest one is better than the other, but rather to inform the reader that there are distinctive differences in their mode of action. We are positive that there are many other products available that are not listed, and we will work to identify these in the future. Any product that is not listed should be washable in order to reduce the permanent effects of reflecting heat (if the trees wanted to be white, they would be white naturally!).

1. Kaolin clay products (e.g. Surround). 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. Exact rates to optimize heat reflection are unknown, but trial work was around 30 lbs/acre in a 100 gallon ride. Follow up applications will be needed after heavy rain events. Kaolin clay was used in 2015 and 2016 trials with Valley Orchard Management and a chill portion increase was documented.
2. Calcium Carbonate (e.g. Mask, Diffusion, Purshade). 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. This product was tested in 2016 in unreplicated screening trials near Buttonwillow and Highway 99 on Pond Road. Both trials showed an increase in chill portion accumulation as measured through placed thermo-couplers.  Four gallons/acre in a 100 gallon ride were applied.
3. Titanium oxide (e.g. Crop Shield). Applications of titanium dioxide create multi-facetted nanoparticles that refracts and disperses light and heat. It is also thought to create an insulating barrier which reduces heat exchange. Although this product has not been utilized in pistachio trial work, thermal aerial imaging has shown a reduction in heat on vegetable crops, and use within almonds has delayed bloom (as intended). Applications can be made by air or ground and are made at 1 gallon per acre.

Applications should occur at the rate required for the specified product. Full label rate may not be required for some products due to the reduced leaf area of the tree. 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