In the US, pistachio (Pistacia vera L.) is grown mainly in California, but it is a promising crop for some areas of the arid American Southwest, such as the southern parts of Arizona and New Mexico. The Dynamic Model used for calculating winter chill portions has yielded useful relationships in California between dormant-season temperatures and bud break timing for pistachio. Our objective was to characterize the relationship between bud break timing and winter chill accumulation in southern New Mexico, where, compared to California, there are much larger diurnal temperature fluctuations in winter. This experiment was designed to measure the effects of winter chill accumulation in immature pistachio over the course of one dormant season in Mesquite, New Mexico. The orchard used for the study was planted in 2021; the cultivar was Golden Hills budded onto seedling ‘UCB-1’ rootstocks. Dormant shoots were sampled every two weeks from 52 trees in the orchard from 3 January through 14 March 2025. On each sampling date one 25-30 cm shoot was collected from each tree. Samples were placed in containers with 15 cm of tap water in a growth chamber (Conviron CMP 3244) modified with LED lights (Heliospetra Elixia). The growth chamber light cycle was set to 14 hours on (“day”) and 10 hours off (“night”). The growth chamber air temperature was set to 24°C during the day and 19°C at night and. The samples were arranged in the chamber in a completely randomized block design with six blocks (containers). The average duration from dormant sampling until bud break decreased as sampling dates progressed from early January to mid-March. The time to bud break in the growth chamber for the six samples averaged 37 days in early January. It had dropped to 30 days by late January, and 17 days by the end of February.
