Biology
The tree grows up to 10 meters (30 ft) tall. It has deciduous
pinnate leaves 10–20 centimeters (4-8 inches) long.
Pistachio is a desert plant, and is highly tolerant of saline soil.
It has been reported to grow well when irrigated with water having
3,000-4,000 ppm of soluble salts.[1] Pistachio trees are fairly
hardy in the right conditions, and can survive temperature ranges
between −10°C (14°F) in winter to 40°C (104°F) in summer. They need
a sunny position and well-drained soil. Pistachio trees do poorly in
conditions of high humidity, and are susceptible to root rot in
winter if they get too much water and the soil is not sufficiently
free draining. Long hot summers are required for proper ripening of
the fruit.
The plants are dioecious, with separate male and female trees. The
flowers are apetalous and unisexual, and borne in panicles.
The fruit is a drupe, containing an elongated seed, which is the
edible portion. The seed, commonly thought of as a nut, is a
culinary nut, not a botanical nut. The fruit has a hard, whitish
exterior shell. The seed has a mauvish skin and light green flesh,
with a distinctive flavor. When the fruit ripens, the shell changes
from green to an autumnal yellow/red and abruptly splits part way
open (see photo). This is known as dehiscence, and happens with an
audible pop. The splitting open is a trait that has been selected by
humans. Commercial cultivars vary in how consistently they split
open.
Each pistachio tree averages around 50 kg of seeds, or around
50,000, every two years.
Cultivation
Commercially prepared pistachios in shells
The trees are planted in orchards, and take approximately seven to
ten years to reach significant production. Production is alternate
bearing or biennial bearing, meaning the harvest is heavier in
alternate years. Peak production is reached at approximately 20
years. Trees are usually pruned to size to make the harvest easier.
One male tree produces enough pollen for eight to twelve nut-bearing
females. Harvesting in the United States is often accomplished by
using shaking equipment to shake the nuts off the tree.
Pistachio nuts in and out of the shell
Pistachio trees are vulnerable to a wide variety of diseases (see
List of pistachio diseases). Among these is infection by the fungus
Botryosphaeria. This fungus causes panicle and shoot blight (i.e.,
kills flowers and young shoots), and can damage entire pistachio
orchards.
In California almost all female pistachio trees are the cultivar
"Kerman". A sprig from a mature female Kerman is grafted onto a
one-year-old rootstock. Male pistachios may be a different variety.
Bulk container shipments of pistachio nuts are prone to self-heating
and spontaneous combustion because of their high fat and low water
content.
