Winter fog compensates for the lack of elevation in the pistachio producing areas of California, helping trees meet their chilling hour requirement.
Chilling hour accumulation has been a problem for pistachio orchards the last couple of years and the damage showed in yield totals this season. Cooperative Extension Farm Advisor Craig Kallsen says other producing areas of the world have higher elevations to help with that requirement and fog is a key substitute for that in California.
More About Chill Requirements
Pistachios (Pistacia vera L.) are widely grown in the arid rainfed areas of Tunisia. However, pistachio production is still low with important variation between years. This study investigated the responses of pistachio trees to variable winter chilling and annual precipitation. It was carried out in an experimental orchard of Mateur, the most important Tunisian cultivar in central Tunisia (34 degrees 94’11 ”, 10 degrees 60’82 ”) for a period of twelve years from 1997 to 2008. Yield correlated poorly with precipitation (varying on the range of 80-300 mm/year during the 12 years) and showed a moderate alternate bearing index of 0.63. However, flowering and nut yield of pistachio trees was a function of chill accumulation computed as chilling hours (CH), chill units (CU) or chilling portions (CP) depending on using Crossa-Raynaud, Utah or Dynamic models, respectively.
For Mateur cultivar, the threshold chilling accumulation appeared to be 206 CH, 539 CU or 36 CP depending on the model used. Under this level, pistachio production was clearly affected. Warm winters with low chilling caused erratic floral bud break, delayed flowering, sparse foliage and decreased yield. With inadequate winter chilling, chemical treatments with hydrogen cyanamide (Dormex) at 2 and 4% increased floral bud break, advanced flowering period and improved vegetative growth of pistachio trees in comparison to the untreated control. As a conclusion, better prediction of chill accumulation must be achieved in the warm production region of Tunisia. To that effect, the Dynamic model showed the lowest coefficient of variation compared to the chilling hours and Utah models and seemed to better reflect what is happening on the field under our warm winter regions. Read more from ScienceDirect.com