For Pistachios, it can be difficult to predict the timing of shell hardening, before harvest. For this reason, researchers have created the Pistachio Shell Hardening Growing Degree Day Model, which uses heat measurement and time to predict hardening of pistachio shells.

Setting the Biofix Date

For this model, the biofix date, or date from which to begin accumulating growing degree days, should be set when 75% of Pistachio flowers are in bloom.

Calculating Growing Degree Days

For this model, a lower temperature threshold of 50F is used to calculate growing degree day accumulation beginning at the time of planting. Growing degree days can be calculated from daily high and low temperatures, or via the Pest Prophet app, which uses hourly temperature data for a given field. This can be used to predict shell hardening based on growing degree day accumulation which will occur at approximately at 1197GDD F after the biofix date (75% bloom).

Source:

Purcell, M., and S. C. Welter. 1991. Effect of Calocoris norvegicus (Hemiptera: Miridae) on pistachio yields. J. Econ. Entomol. 84: 114-119.