For the pistachio tree male and female flowers are borne on separate plants. Inflorescences of several hundred tiny, brownish-green flowers are borne laterally on 1-yr wood. Flowers lack petals, and have up to 5 sepals. Males have 5 stamens, and females lack stamens, and have a single tricarpellate, superior ovary. The inflorescence is a panicle in both cases with 13 primary branches, each bearing 1 terminal and 5-19 lateral flowers. Most fruit set occurs from terminal flowers. Fruit set averages about 10%.
Pistachios are wind pollinated. Although bees may be attracted to male flowers for pollen, they are not attracted to females since they lack pollen, nectar, and petals. Pollinizers (males) are often planted in the center of a 3 x 3 square of females, yielding an 8:1 ratio, or sometimes ratios up to 24:1 are used. In Australia ratios of 15:1 are becoming common.