The History of Pistachio Trees
The pistachio tree’s (Pistacia vera) history is a murky one, but archaeological digs in Turkey have unearthed evidence that its nuts were on humankind’s menu more than 9,000 years ago. Sometime in the first century A.D., Syrian traders brought it to Italy and from there it spread through the Mediterranean.
Pistachio Trees in the U.S.
The first pistachios arrived on American shores in 1854 but commercial production didn’t begin until the 1970s. They’re suitable grow in USDA plant hardiness zones 7b through 10. But their very specific climate requirements limit commercial U.S. plantings to:
- California’s San Joaquin Valley
- Around Tucson and in southeastern Arizona
- South central New Mexico
- West Texas, between Pecos and El Paso
So, while they’re one of a handful of fruit trees (yes, fruit trees!) to flourish in the desert’s soil and heat, pistachios only produce harvestable nuts in a tiny part of their growing zones.
Climate Requirements
Pistachios’ climate needs shift with the seasons. They require:
- Dry, frost-free springs windy enough to transport the male trees’ pollen to the female trees’ flowers.
- Bone-dry, low-humidity summers to minimize the threat of fungal disease.
- Summers with more than 600 hours at 86°F (30°C) or higher. Temperatures consistently in the 95° to 100°F (35° to 37.7°C) range are ideal.
- A fall and winter dormancy with 850 or more accumulated chilling hours at temperatures between 34° and 45°F (1° and 7.2°C).
Why Is Climate So Important?
When winter isn’t sufficiently cold, a pistachio may have fewer flowers and fruit than normal. Or its compound leaves could produce three leaflets instead of five. Less photosynthesis means a smaller crop. Where summer isn’t sufficiently hot, much of its nut crop will be empty shells. To grow pistachio nuts, you need very hot summers and cold but mild winters.
Where to Plant Pistachio Trees
Having the right pistachio-growing climate is one thing; taking full advantage of it with the right planting site is another. Soil, sun and spacing all make a difference in crop size.
Soil
Pistachio trees have adapted to desert life with taproots that take up water from deep within the soil. They also handle salty or alkaline soils well. That said, they perform best on a planting site with loose, well-draining sandy loam and a slightly alkaline pH between 7.1 and 7.8. The soil should be at least 7 feet deep before encountering root-restricting hardpan.
Expert gardener’s tip: For pistachios, water-retaining soils can be a death sentence.
Sun and Spacing
Give pistachios a site in full sun (no to minimal shade). Because you need female and male trees to produce nuts, your site should have with room for at least two trees. Allow 15 to 18 feet between a single pair and 20 to 22 feet between orchard rows.
Expert gardener’s tip: In less-than-perfect soil, plant pistachios closer together to speed their first harvest.
Watering
Pistachio trees are built to survive drought. But insufficient watering between late winter and early summer diminishes the quantity and quality of their nuts. It also makes a big difference in the following year’s performance. During those months:
- Their flowers open and pollination occurs.
- Their leaves open, their new shoots appear and they set buds that will bloom the following spring.
- Their nuts develop shells and kernels.
How and How Much to Water
The most economical and safest way to water pistachio tree is with drip irrigation. By avoiding saturated roots and reducing humidity around the trees, it thwarts fungal disease.
From late winter to early summer, a budding, flowering or fruiting pistachio needs about 40 gallons (150 liters) of water per day. That increases to 50 gallons (190 liters) daily from July to mid-August, during nut formation. The goal is to keep the top 4 feet of soil moist until watering stops before harvesting the crop in late August or September.
Organic Fertilizer
Between spring budding and harvest, pistachio trees burn through lots of nitrogen. Supply what they need organically with a late-winter or early-spring application of well-aged, grass-fed livestock manure. Spread 20 to 25 pounds of it evenly over the soil around each tree and water well.
Expert gardener’s tips:
- Avoid chicken manure; with twice the nitrogen of other varieties, it may cause root burn.
- If you source your manure locally, make sure it’s really organic with no herbicide traces that could harm your trees.
Supplemental Nutrients
For the first three years after planting, pistachio trees benefit from a foliar spray of copper, boron and zinc. Start treatment after they leaf out in spring when the leaves are still thin. Reapply every three weeks until the leaves drop in fall.
Supplement older trees with a foliar spray of boron and zinc when their buds break in early spring, when their flowers bloom and as they enter dormancy in fall.
Pistachios and Pollination
Even the most pampered pistachio trees can’t grow nuts without proper pollination. And that only occurs between compatible male and female pistachio cultivars. One male typically produces enough pollen for 10 to 15 females. The most commercially successful pairings include:
- ‘Kerman’ females with a ‘Famoso,’ ‘Randy,’ or ‘Peters’ male.
- ‘Lost Hills,’ ‘Golden Hills,’ or ‘Kalegouchi’ females with a ‘Randy’ or ‘Tejon’ male.
Expert gardener’s tip: Pistachios are wind pollinated. Centering a male tree among a group of females is the best way to ensure even pollen distribution. Some growers recommend one male for every seven females.
Alternate Bearing
Like many fruit trees, pistachios alternate heavy and light crop years. In light years, they put very little energy into producing nuts and lots of it into setting buds that will flower and fruit the following summer. Alternate bearing is a natural adaptation and one reason why pistachio trees can survive poor growing conditions.
Balancing Alternate Bearing
Early-spring pruning balances — but won’t completely break — pistachios’ on-off crop cycle. Thinning the fruit buds (they’re larger than the leaf buds) reduces the crop in a heavy year and increases it in the following light one.
Pistachio Pests and Diseases
One tiny pest and a pair of fungal diseases can wipe out a pistachio crop. Fortunately, both can be managed organically.
Pistachio trees’ major pest is the navel orangeworm (NOW). It burrows into a nut as soon as the shell splits, devouring the kernels and covering them with excrement. In the wake of this pest, toxic fungi often attack the damaged nut.
Organic NOW Prevention
Navel orangeworms hatch from moth eggs on unharvested nuts. Organic controls include:
- Removing the unharvested “mummy nuts” form the trees and surrounding soil. Do it soon after harvest and before rain.
- Spraying the trees the following season with organic bacterial pesticide Bacillus thuringiensis at hull split. The treatment kills new infestations before they cause serious damage.
Fruit Molds
Aspergillus and Alternaria fungi colonize and decay pistachio shells and kernels, especially those infested with orange navelworms. Eliminate the worms and keep the trees well watered while the nut shells develop in spring. Consistent watering prevents early shell split — and intact shells stop fungal infections.
