Adapted from
"The Remarkable Avocado Flower"
By Dr. B. O. Bergh

Introduction
The avocado (Persea americana Mill.) flower behavior is noteworthy in that there is nothing quite like it known in any other plant. The flower has both female and male organs, but they don't function at the same time. Each flower is female when it first opens. That is, its stigma will receive pollen from other avocado flowers, but its stamens (male organs) do not shed pollen at this first opening. The petals and sepals, which look alike in the avocado, protect the delicate sex organs while they develop. The nectaries and staminodes secrete nectar that attracts bees. To obtain an avocado fruit, a bee (or other flying insect) must transfer pollen from a different flower to the stigma of this first-opening flower. Sperm from the pollen grows down under the slender style to fertilize the female egg inside the ovary, which then grows into the mature fruit.

The unique avocado flower
The first or female stage remains open for only 2 or 3 hours. The flower then closes and remains closed the rest of the day and that night. The following day it opens again. But now the stigma will no longer receive pollen. Instead, the flower is now shedding pollen. That is, each flower is female at its first opening, male at its second. After being open several hours the second day, the flower closes again, this time for good. If it had been successfully pollinated at the first opening, and if other conditions are right, it will develop into delicious fruit.

"A" and "B" type flowers
Nature has provided for avocado cross-pollination by creating varieties of two kinds. The "A" type is female in the morning of the first day and male in the afternoon of the second day (when the weather is warm). The "B" type is just the reverse: its flowers are female in the afternoon and male the following morning. For one flower of each variety, the timing will be as follows:

But with hundreds of flowers opening in both stages day after day, the daily situation appears like this:

Flower opening and temperature
The two flowering types behave with clock-like exactness only when the average temperature (night minimum and day maximum) is above 70 degrees F (21.1 C). As temperatures fall, the daily openings become delayed and become irregular, so that a single tree may have flowers in both female and male stages at the same time. This helps to explain how large blocks of just one variety sometimes set heavy crops. But, as the average temperature falls below about 70 F, the flower parts function less well. Below about 60 F (15.5 C), there may be zero set.

Video: Avocado Flowering Cycle
Video capture of a film showing an avocado flower as it cycles from female to male. (Video has no sound).

Current research
Efforts are currently underway to help determine if interplanting "A" and "B" flower type trees will increase pollination and yield. For some early results of this study see "Avocado Flowering Basics".