Pistachio growers at high elevation in Arizona and in the Cuyama Valley uphill and southwest of Bakersfield are at increased risk of damage to their crops from late spring frosts compared to San Joaquin Valley (SJV) growers.
Such damage was experienced by growers this spring in both of these high-elevation locations. However, occasionally, we see damage from late spring frosts on the floor of the SJV as well. A frost in early April this year appears to have damaged some pistachio leaves, and perhaps may reduce yields slightly in some low-elevation areas of Kern County. Applying oil in the mid-January to late February time-window to control scale insects or for the purposes of creating a more even bloom can increase the risk of being hit by a late frost by advancing bloom seven to 10 days.
One of the most common symptoms of late-spring frost damage on leaves is a distinctive ‘wavy’ pattern of light and dark green areas. The light-green or necrotic areas are the result of mesophyll cell collapse as a result of freeze damage. This damage frequently occurs when the leaves are still bound within or just existing the vegetative bud. Freeze damage may also destroy the germ cells of the reproductive (i.e. flowering) buds, which may result in reduced yield or no yield, depending upon the temperature.
In a frozen orchard, the result on the rachis will be tiny dark nutlets that don’t grow larger than BB size and eventually fall from the tree. These nuts will act similarly to nutlets that the tree sheds early in the season during the course of crop-load adjustment in more normal years. Generally, if the orchard exhibits the “frozen leaf” symptom, but nuts continue to expand and grow on the rachis, the growing nuts will fill and produce a kernel and marketable nut.
So how cold will it have to be to cause damage to the flower bud in the spring? The following text in italics is an abstract from a Turkish paper by S. Arpaci, H.S. Atli, H. Tekin, and M. Burak in an article entitled, Studies on spring frost resistance of some pistachio (Pistacia vera) cultivars:
“This study was carried out between 1999 and 2001, in order to determine spring frost resistance of Siirt, Ohadi, Uzun and Uygur pistachio (Pistacia vera) cultivars. In the flowering time, the fruit buds were exposed to artificial freezing tests for 1, 3 and 5 hours at -1ºC (30.2ºF.), -3ºC (26.6ºF.). The plant materials (cuttings) were put into refrigerator for 24 hours prior to exposure to freezing tests. The freezing tests were done in a temperature controlled freezer unit. After the exposures to different freezing temperatures, the plant samples were taken out from the unit and put in a refrigerator for 1-2 hours then they were put in the jars with full with water at ambient air temperature for 24 hours. Then, the buds were cut along the longitude by a sharp blade, and those that showed brown-black colors were determined as dead (injured) buds. In the flowering stage, -3ºC for 2 hours caused more than 85% injury in all cultivars, -1ºC for 2 hours caused more than 60% injury, and in the same regime 1 hour application resulted in less than 40% mortality rate. In small fruit stage, -2ºC for 2 hours caused a mortality rate more than 66%, whereas 1 hour application resulted in 35% injuries in Uzun cultivar, 25% in Siirt, 41% in Ohadi and 45% in Uygur cultivars, respectively.”
Even though this study did not include ‘Kerman’ the cultivars they used were all P. vera and it is not likely that Kerman’s performance would be much different. The important thing to note is that the temperature does not have to be very cold during bloom time to do some significant damage to the developing crop.