Heat model for pistachio bloom and harvest

• Systematically tested base, optimum and critical temperature for pistachio trees.
• Determined growing degree hour from chill completion to bloom for ‘Sirora’ pistachio trees.
• Determined growing degree hour from bloom to harvest for ‘Sirora’ pistachio trees.
• Discussed issue for trees without enough growing degree hours.

To determine the heat accumulation required for bloom and harvest of ‘Sirora’ pistachio (Pistacia vera) trees, 6510 and 25,609 combinations of base temperature (Tb), optimum temperature (Tu) and critical temperature (Tc) were tested. Using eight seasons’ data, we attempted to find the smallest variation for heat accumulation for bloom and harvest. The combination of Tb = −1, Tu = 26 and Tc = 26 for bloom and Tb = 14 and Tc = 32 for harvest resulted in the lowest value for the coefficient of variation. Validation was performed using actual harvest data from 2 further years from the orchard used for the initial modelling and 10 years from other orchards in different regions. Reasonable results were obtained for the major Australian pistachio production areas. The data from the cooler Nhill area did not fit this model. The reason for this has been explored.