pistachio flowers

Pistachio growth habits – flowers : are dioeciously with male and female flowers on separate trees. Male and female trees must be present for fruit to set, or a branch from a male tree may be grafted on a female tree. The small, brownish green flowers are without petals and borne on maxillary racemes or panicles in early summer. Wind carries the pollen from the male to the female flowers.

pistachio fruit

Pistachio growth habits – fruit : The reddish, wrinkled fruits are borne in heavy clusters somewhat like grapes. Although known as a nut, the fruit of the pistachio is botanically a drupe, the edible portion of which is the seed. The oblong kernel is about 1 inch in length and 1/2 inch in diameter and protected by a thin, ivory-colored, bony shell Normally the shells split longitudinally along their sutures when mature. Under unfavorable conditions during nut growth, the shells may not split open. 
The color of the kernel varies from yellowish through shades of green, which extends throughout the kernel. In general the deeper the shade of green, the more the nuts are esteemed. Pistachio nuts are rich in oil, with an average content of about 55%.

The trees begin bearing in 5 to 8 years, but full bearing is not attained until the 15th or 20th year. Pistachios tend toward biennial bearing, producing heavy crop one year followed by little or none the next. 
Production of nuts is also influenced by drought, excessive rain, heat or cold and high winds.