While any member of the botanical genus Pistacia is ambiguously called a pistachio or pistache tree, the plant that yields the tasty nut is scientifically known as Pistacia vera. The pistachio tree is susceptible to verticillium wilt and oak root fungus, particularly when soils are too wet. The fruits are shaken from the tree in fall and the husks are removed. The nuts are wetted with water to hasten the shell splitting and then the seed is dried, roasted and sometimes dyed.
The pistachio tree is native to the eastern reaches of the Mediterranean and into central Asia. According to the authors of "Economic Botany: Plants in Our World," pistachio trees have been cultivated for more than 3,000 years. Archaeological evidence suggests that the trees were used as a food source in Turkey as early as 7,000 B.C.
Pistachio trees develop into broad, bushy trees with a singular or multiple trunks. Mature size ranges from 25 to 30 feet tall and 25 to 25 feet wide. The branches often droop and, according to the California Rare Fruit Growers Association, a healthy pistachio tree will live for centuries and reliably producing edible nuts. The leaves are gray-green and comprise three to five leaflets. The leaves turn shades of orange or red in fall before dropping from the tree. Trees are either male or female and produce greenish flowers that lack petals. Only female flowers develop into the drupe fruits that crack open to reveal the shell that protects the seed (what we eat as the "nut"). Crops of nuts tend to occur more heavily in alternating years.
Pistachio trees grow their healthiest in deep, well-draining soils that are fertile. They need a winter dormancy that is cool to cold in temperature to produce flowers, and the summers are best long and hot with little ambient humidity. Often, pistachio trees are grafted onto the more drought-tolerant and heat-resistant rootstock of the Mount Atlas pistache (Pistachio atlantica) according to the "Sunset Western Gardening Book." Generally, pistachio trees grow best in USDA hardiness zones 8 through 10 or Sunset climate zones 7 though 12, 14, 15, and 18 through 21.