The pistachio tree (Pistacia vera) a member of the Anacardiaceae family, (the cashew family, which also includes mango and sumac) is a small to medium tree originating in Persia. The tree produces seeds that are widely consumed as food. It thrives in the long hot summers and chilly winters of U.S. Department of Agriculture plant hardiness zones 8 through 10.
The pistachio tree is believed to be indigenous to Iran. It is widely cultivated from Afghanistan to the Mediterranean region and in California. The seed kernels can be eaten fresh or roasted and are commonly used in a variety of desserts, including baklava, halvah, and ice cream. They are also used for yellowish green colouring in confections. The seeds are high in protein, fat, dietary fibre, and vitamin B6.
Mature pistachio trees typically grow to a height of 20 to 30 feet (6 to 9 meters).The spread of the canopy is usually similar to the height. However, through pruning or training, the spread can be controlled or modified for easier harvest or space management in orchards. They have pinnate leaves, which are compound with 10-20 leaflets that are oblong to lanceolate, and they turn a beautiful yellow in autumn.
The trunk of a Pistachio tree is generally upright and becomes more robust as the tree matures. In young trees, the trunk might be somewhat slender, but it thickens over time. Pistachio trees have a tendency to form a central leader in their youth, but many cultivars are pruned to encourage a more open, spreading form for better light penetration and air circulation.
In the first few years, growth might be slower as roots establish. Once established, they can increase in height by about 1 to 2 feet per year until they reach their mature size. Young bark tends to be smooth and can have a grayish-green to brown hue. Over time, the bark becomes rough, with deep furrows and ridges. It can develop into a pattern of plates or scales.
Pistachio trees are dioecious, which means that male and female flowers are borne on separate trees. This necessitates having both male and female trees in an orchard for successful pollination and fruit set. Male flowers are borne on catkins, which are long, slender clusters. Each catkin contains numerous small, inconspicuous flowers that have stamens but no petals. They are a greenish-yellow or yellowish-green.
Female flowers are less showy; they are small and have a more spiky appearance due to the pistils. They have a reddish hue when they first emerge, which can darken to a burgundy color. Each female flower consists mainly of a single pistil with a small ovary at the base, which will develop into the fruit if pollinated. They do not have petals.
Pistachio trees flower in the spring in March or April. This is before the leaves fully emerge. Pollination occurs via wind. The male catkins release pollen that is carried by the wind to the female flowers. Typically, one male tree is planted for every eight to twelve female trees to ensure adequate pollen distribution. Some farmers use hand-pollination or mechanical blowers to distribute pollen if natural wind conditions are not favorable or if pollination rates need to be improved.
To break dormancy, pistachios require approximately 850 hours of winter temperatures below 45 degrees Fahrenheit. Although the trees require winter chilling, the rootstocks vary in their cold resistance.
Borne in clusters, the white drupe fruits are 1.5 to 2 cm (0.6 to 0.8 inch) long and tend to split at one side without discharging the seed. The “nut” is a greenish kernel enclosed in a thin, tightly adhering, reddish skin. The single, solid kernels have a pleasing mild resinous flavour.
Pistachio trees develop an extensive root system, which is particularly deep in well-drained soils. This deep root system helps them survive in dry conditions but also makes them less susceptible to being blown over by wind compared to trees with more shallow root systems.
Pistachio trees can live and produce nuts for 50 to 100 years or even more with good care. It takes about 5-8 years for a pistachio tree to begin producing nuts after planting. The trees usually reach their peak production capacity around 10-15 years of age. They can remain productive for many decades, although the yield might decrease as the tree ages.
Harvesting in the United States and in Greece is often accomplished using equipment to shake the drupes off the tree. After hulling and drying, pistachios are sorted according to open-mouth and closed-mouth shells, then roasted or processed by special machines to produce pistachio kernels.
In 2022, world production of pistachios was one million tonnes, with the United States, Iran, and Turkey combined accounting for 88% of the total.
A pistachio tree might be considered at the end of its life when it shows significant decline in health, reduced yield, or extensive structural damage that can’t be overcome with pruning or other interventions.
Factors Affecting Lifespan
- Climate: Suitable climate conditions (hot, dry summers, and cold winters) are vital for the health and longevity of pistachio trees.
- Soil and Water Management: Proper soil drainage and irrigation management are crucial. Over-watering or water stress can adversely affect the tree’s health.
- Pests and Diseases: The presence of pests like the navel orangeworm or diseases like verticillium wilt can shorten a tree’s life if not managed properly.
- Pruning: Regular and correct pruning can extend the life of the tree by removing dead or diseased wood and maintaining tree vigor.
- Cultural Practices: Good agricultural practices, including nutrient management, can keep trees healthy over a long period.
Rejuvenation
- Regrowth: Even older trees can sometimes be rejuvenated through severe pruning, which can stimulate new growth and extend productive life.
- Rootstock: The choice of rootstock can influence the tree’s vigor and potential lifespan. Some rootstocks are selected for their ability to confer longer life or better resistance to soil-borne diseases.
Pistachio Tree Cultivation
- Pistachios thrive in regions with hot, dry summers and cold winters. They require hot temperatures for fruit development and a sufficient period of cold to break dormancy. Trees need around 800-1000 chill hours (hours below 7°C or 45°F) annually.
- Prefer well-drained, deep loamy soils with a pH between 6.5 and 8.0. They are tolerant of alkaline conditions but sensitive to poor drainage which can lead to root diseases.
- Trees are planted with 18 to 24 feet between rows and 12 to 18 feet between trees within rows for optimal growth and to allow for machinery access.
- Late winter to early spring is ideal for planting, allowing the tree to establish before the hot summer.
- Young trees need regular watering for establishment. Once mature, trees are somewhat drought-tolerant but require irrigation during the nut filling stage for optimal yield and quality.
- Plant both male and female trees. The ratio is generally one male tree to every eight to twelve female trees.
- Since pistachios are wind-pollinated, ensure there’s no obstruction to wind movement. In some cases, mechanical pollination might be used.
- Prune to remove dead, diseased, or crossing branches to improve air circulation and light penetration.
- Harvest when the hulls split naturally, usually from late summer to early fall.
Post-Harvest
- Drying: Nuts are dried to reduce moisture content, which prevents spoilage.
- Processing: Removing the hulls (dehulling), sorting, and sometimes roasting or salting for market.
Pests and Disease Management
- Common Pests: Aphids, navel orangeworm, and various mites. Use integrated pest management (IPM) practices.
- Diseases: Verticillium wilt, botryosphaeria panicle and shoot blight, and Alternaria late blight. Crop rotation, resistant rootstocks, and good sanitation help manage diseases.