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An avocado flower is perfect, carrying both stigma and stamens, but it does not use them at the same time. It opens once with a receptive stigma and stamens that are still closed, shuts for several hours, then opens a second time with the stigma no longer receptive and the stamens shedding pollen. Each flower is therefore female in one session and male in the other, and it cannot pollinate itself in either.
The whole tree keeps that rhythm in step, which is what makes the pattern visible from the ground. On any given morning a tree is either offering receptive flowers or shedding pollen, not both. This arrangement exists to force cross-pollination, and it is the single fact behind almost every question about avocado fruit set.
Type A and type B: the two timetables
Cultivated avocados fall into two flowering groups, conventionally called type A and type B, which run the same cycle offset by half a day. A type A tree opens female in the morning and male in the afternoon of the following day. A type B tree opens female in the afternoon and male the next morning. Set the two side by side and the female phase of one overlaps the male phase of the other, twice a day.
This is why nurseries label trees A or B, and why the label is worth writing down when you plant. It is not a quality grade and neither type is better; it only describes the clock. If you have space for one tree, choose whichever suits your climate, and if you have space for two, take one of each and site them within about thirty feet (9 m) of one another.
| Flowering type | Female phase (stigma receptive) | Male phase (pollen shed) |
|---|---|---|
| Type A | Morning of day one | Afternoon of day two |
| Type B | Afternoon of day one | Morning of day two |
| A and B together | Receptive flowers present most of the day | Pollen available most of the day |
Record which type each of your trees is at planting, on paper and not only in your head. Once the label has faded you can still deduce it by watching the flowers for two mornings, but it takes a fortnight of patience you will not want to spend.
A single tree still sets some fruit
A lone avocado is not sterile. The synchrony within a tree is never perfect, especially in cool or unsettled weather, so there are always some late female flowers still receptive when the first male flowers begin shedding. That overlap gives a solitary tree a modest crop, and in warm gardens with a long flowering season it can be a perfectly respectable one.
What a partner tree buys you is consistency and volume. Two compatible trees typically set several times more fruit than one on its own, and they carry through bad flowering weather far better. If you cannot plant a second tree, a branch of the opposite type grafted into the canopy achieves the same thing in a fraction of the space.
Temperature runs the whole system
The two-phase clock only keeps time within a fairly narrow temperature band. Below about 60F (16C) the sequence stalls and drifts: flowers open late, close early, or open in both phases at once, and pollen tubes grow too slowly to reach the ovule before the flower is finished. Above roughly 95F (35C) pollen viability and stigma receptivity both collapse within hours.
Night temperature matters as much as day. Cold nights around 50F (10C) during flowering delay the male phase enough that the two types fall out of step with each other, which is why a cool spring produces a poor set even in a well-matched pair of trees. There is little to be done about it beyond siting trees where they get morning sun and shelter from cold wind, and accepting that avocado crops vary a great deal from year to year.
Bees do the actual carrying, and they work the flowers only when it is warm enough to fly and the nectar is running. A flowering avocado should be audibly busy on a warm morning. If it is silent, look at what else is in flower nearby competing for the bees, and stop any insecticide use entirely while the tree is in bloom.
Hand pollination, and what to expect afterwards
Hand pollination is worth trying on a single tree or in a cool season, though the timing is demanding. Collect pollen in the tree's male phase by tapping open flowers over a small dish or dabbing them with a soft brush, keep it cool and dry, and apply it the following session to flowers that are in their female phase, when the stigma looks fresh and glistening rather than shriveled.
Do not be discouraged by what happens next. An avocado opens well over a million flowers on a mature tree and holds perhaps a couple of hundred fruit. A wave of tiny fruitlets yellowing and dropping a few weeks after flowering is normal self-thinning, not failure, and a second drop when the fruit are marble-sized is normal too. Judge the season by what is still hanging at the end of early summer, not by the litter under the tree in spring.
Keep the tree evenly watered through and just after flowering. Water stress during fruit set causes far more drop than any pollination problem, and it is the one factor in this whole chain that you fully control.