Preparing for harvest and harvest timing*.
Pistachio drip irrigation, photo by Arthur Mostead
A very important factor of harvest preparation is determining when to harvest. Maturation of pistachio hulls is signaled by color development and is closely linked to shell splitting. As maturity proceeds the hull turns from green to ivory and then to rose, and the shell turns from translucent to opaque. Lack of color development generally signals a blank or aborted nut within the shell. At full maturationthe inshell nut will eject easily from the hull when the nut is held between the thumb and forefinger and pressure is applied to the long axis. As these visible changes occur, increases in fat and sugar content cause the kernel moisture, respiration rate and total protein content to decrease and kernel dry weight to increase. The final indication of full maturity is formation of the abscission zone between the nut and its rachis. At maturity nuts will detach readily when the rachis is gently shaken. Optimum harvest time is the two to three weeks bracketing these events and is best indicated by nut removal force. As with all crops, maturation is uneven throughout the tree. Light crops also mature earlier than heavy crops. Thus, normal harvesting does not remove all potential crop. However, most nuts remaining in the tree after shaking are blank (empty), or have non-split shells Ethephon, a foliar compound that hastens and reduces variability in maturity of some crops, is ineffective on pistachios.
The final factor in harvest timing of pistachios is availability of services: harvesters, transporters and processors. The harvest season is usually during the month of March. Because pistachios are mechanically harvested and the equipment is seldom owned by the smaller growers, harvests are contracted and are rapid. Thus, orchard readiness, and harvest contractor, transportation, and processor availability, must all be coordinated.
Timeliness of harvest is very important in maintaining nut quality.
Late harvests increase the potential for hull deterioration which stains the nut shells, early rains that halt harvest and exacerbate hull deterioration, increases fungal infestation, kernel deterioration, and strong winds that remove nuts. Further, any hull deterioration present at harvest will be exacerbated during transport delays to the processor.