Pistachios are the seeds of a tree, specifically the Pistacia vera species. Although commonly called a nut, botanically, the pistachio is categorized as a drupe, which is a fleshy fruit with a single seed enclosed in a hard shell, similar to an olive or a peach. The shell we crack open is the stony layer of the fruit.
Identifying the Pistachio Tree
The pistachio tree is a deciduous plant that typically grows up to 33 feet and is known for its long lifespan, often living for up to 300 years. Cultivation requires specific environmental conditions, favoring areas with long, hot, and dry summers, but also needing moderately cold winters for chilling requirements. They are highly tolerant of drought and saline soil, which is why major production occurs in arid regions like California, Iran, and Turkey.
Pistachio trees are dioecious, meaning individual trees are either male or female. For a successful harvest, both sexes must be present in an orchard, as the female trees bear the fruit and rely on the wind to carry pollen from the male trees. Growers usually plant one male tree for every eight to twelve female trees to ensure adequate wind pollination. A young tree takes around seven to ten years to begin producing a significant crop, with peak yields not reached until about twenty years of age.
The Journey from Flower to Fruit
The pistachio’s journey begins with wind pollination in the spring, which leads to the formation of the fruit. This fruit starts as a small cluster that resembles a miniature grape bunch. It is a drupe composed of three layers, including the outermost fleshy hull, known as the epicarp and mesocarp, which must be removed after harvest.
Beneath this outer hull is the hard, cream-colored shell, which is the endocarp we commonly crack open. Inside this stony shell is the edible seed, the kernel, which develops throughout the summer. As the fruit ripens, the fleshy hull changes color, shifting from green to a reddish-yellow hue, signaling that the nutmeat inside is fully mature. The harvest generally occurs in late summer or early fall when the hull easily separates from the hard inner shell.
Why Pistachios Split
The most distinctive characteristic of a marketable pistachio is the split shell, a process called dehiscence. This natural opening occurs while the fruit is still on the tree, typically starting around late July and continuing through mid-September. The split is caused by the rapid expansion of the inner seed, which creates pressure that forces the shell to pop open along its suture line with an audible sound.
The splitting is a highly desirable trait, as it makes the nut easy to consume and indicates a fully developed, high-quality kernel. Pistachios that fail to split, often called “blanks” or “closed-mouth,” usually have an undeveloped or absent seed. To harvest, large commercial operations use specialized equipment to shake the nuts from the trees onto catching frames. The outer hull is then removed quickly, usually within 24 hours, because the moisture in the hull can stain the shell, which is why commercial pistachios are often a clean, pale color.