There is still hope, but as of this writing, it looks like 2018 could be a repeat of 2014-15 in terms of chilling. CIMIS stations in pistachio production areas statewide report low chill accumulation based on the Chill Portion model. As you know, we have migrated away from the traditional use of the below 450 F model, due to the occurrence of fogless, unusually warm days during past winters.
The solar radiation associated with warm days raises the bud temperatures well above 450 F, and thus they do not contribute to the rest requirement of the tree. We often state that elevated temperature “negates” hours of chill accumulation, but we really do not know what that means.
Some argue that you cannot take away what you have already made. Other researchers suggest that this negation may be more related to the consumption of limited soluble sugars within the bud and adjacent branches, whose concentration is critical to pushing the bud under favorable spring temperatures. I have written previously about the effect high January temperatures has on respiration; an increase of 100 F can double the respiration rate, and rapidly deplete the stored carbohydrates.
Regardless of the actual physiological mechanism, use of the Chill Portion Model considers these warm temperatures with the intent of providing a more accurate ESTIMATE of effective chilling. Also known as the Dynamic Model, it calculates chilling hours between 35-550 F in units known as “chill portions”.
Dr. Ammon Erez and a team of researchers developed this more sophisticated model in the 1990’s to account for temperature variability, since it was common in their native country of Israel. Erez et. al, theorized that fluctuating warm temperatures inhibit physiological processes associated with satisfaction of deciduous tree rest. Rather than cancelling chill portions already created,
Dr. Erez suggests that warm weather prevents the creation of additional chill portions. So, even if there WERE temperatures below 450 F for a given day, the existence of warm daytime temperatures negates their effect on rest development, and thus, a chill portion is not created.
Dr. Katherine Pope, Yolo County Farm Advisor, was funded by the pistachio industry several years ago to test the validity of the Chill Portion Model. Although the results were not as conclusive as she hoped, Dr. Pope’s data suggest that Kerman pistachios require about 58 chill portions to prevent yield reduction from inadequate rest. Peters blooms best at about 61 or greater chill portions.
I have prepared Table 1 (see below) to contrast the Chill Portion accumulation this season, as of January15, with that of previous years on the same date. The values in parentheses are the total number of Chill Portions accumulated from September 1 to February 15. As you can see, we are generally behind last year, and with another month remaining in our traditional chill accumulation period, we need to average one chill portion every day to have 50 by the end of January.
From January 1 to the 20th, we have thus far averaged only half a chill portion daily. However, if one compares the statewide average chill portions for 2014, a poor chill year, and 2015, a good year, one learns that we received 15.2 additional chill portions from January 15 to February 15 in 2014, and 16.2 in 2015.
So, the cold winter of 2015 had only ONE more chill portion accumulated during that period than the warm winter! Thus, the averages suggest that we could reach about 52 chill portions this year. If my napkin math proves accurate, we should expect delayed leaf out and some issues with bloom overlap between Kerman and Peters. We will just have to wait and see. Lots of great things can happen in between now and February 15.