I miss Bob Bergh. If you want to learn about avocado flowering and pollination, read everything that this longtime University of California researcher wrote. His studies were creative and useful, and his reporting was honest and skeptical. In short, you can apply everything that Bergh learned, and you can believe everything that he shared.

In 1967, Bob Bergh wrote a piece for that year’s California Avocado Society Yearbook called, “Reasons for Low Yields of Avocados.” It contains a thousand gems of insight, but one slapped me in the face when I reread it a few days ago.

This was the story of the “unusually heavy avocado set” throughout Southern California during the spring of 1965. “Avocado set” refers to the amount of new fruit that begins to grow on trees soon after flowering.

New avocados set on my Reed tree in July 2019.

What preceded this great new crop of 1965 were two things that will sound very familiar. First, the winter of 1964-1965 “was marked by much more late rain than is usual.” Writes Bergh: “University of California at Riverside records show that March, 1965 had 1.23 inches of rain — well above average. Then April that year had 4.63 inches — which is much more than the combined rainfall totals for the preceding six Aprils on which I checked the records.”

While I don’t have the numbers for UC Riverside, I know that my yard’s March 2020 precipitation total was 6.29 inches, and my April total (up until today) is 6.27 inches — which is also much more than the combined rainfall totals for the preceding six Aprils in my yard, according to my records. Much, much more late rain than is usual, to be sure.

(Most of Southern California was about 200% above normal for precipitation during those months; see this video from the National Weather Service.)

Secondly, in 1965, this cool, wet weather delayed the bloom of the avocado trees until later in spring. Later in spring the weather is usually warmer than early in spring. So when the avocados bloomed in the late spring of 1965, “temperatures above normal for avocado bloom were in fact experienced.”

What is your weather like today? At my place, last two days were 80 or above, and the next six days are forecast to be between 80 and 90 too. Above normal, indeed.

Effects of much late rain and warm weather on avocado fruitset

But what do these events mean for our avocado trees? How might they result in an “unusually heavy avocado set”?

One effect of our March and April rains is that rains leach salts out of a tree’s root zone and improve its condition. “Trees with leaf tip-burn [caused by salts] are manifestly not in the best condition to set and mature fruit,” explains Bergh.

(See my post, “Avocado leaves turning brown? Here’s why and what to do.”)

Much more tip burn than this will cause a tree to put energy into growing new leaves at the expense of flowering and fruiting.
This is more like what we want to see.

As for the warm weather, there is a high correlation between warmer temperatures during bloom and heavier fruitset in avocados. Or as Bergh puts it, “Ordinarily, the higher the temperature mean, the more favorable the set prognosis.” 

The temperature mean is the average of the daily maximum and minimum temperatures. And in another article Bergh says that means above 65 degrees give the best fruit set. Yesterday, my yard’s high was 88 and the low was 51, for a mean temperature of 69.5. That’s auspicious. (See details about this temperature-fruitset correlation here.)

(Important note: B-type avocados have been found to need higher mean temperatures than A-type avocados for good fruitset. One study done in Australia estimated that the B-type variety called Shepard needs five degrees Fahrenheit more compared to the A-type Hass. And during the heavy set of 1965 in California, Fuerte — a B type — was the main variety grown.)

Why do high temperature means help avocado fruitset? Among the reasons are that bees and other pollinating insects are more active, avocado flowers open in their female stage more regularly, and once pollinated, the pollen tube in a female flower grows faster in warm weather.

What could go wrong?

So I’m feeling positive. I like how this spring of 2020 is shaping up; it’s shaping up like the spring of 1965. Unfortunately, there are numerous other factors that affect the fruitset of an individual avocado tree in a yard.

A few of the factors that could make a tree produce little to nothing despite the conducive climatic conditions include:

-Tree age (young trees never produce as much as older trees)

-Preceding crop and therefore amount of bloom (if the fruitset of last spring was heavy then the tree is unlikely to repeat)

-Health of tree going into last winter (an unhealthy tree sometimes grows mostly leaves instead of flowers)

-Cold damage (branch tips that were injured cannot flower)

-Variety (certain varieties always produce more than others)

-Rootstock (some have shown higher fruitfulness than others)

-Timing of bloom (an early blooming variety might have already finished its bloom)

-Pollinators (if few bees are present, then who will get the pollen to the female flowers?).

Female avocado flowers waiting for bees to deliver the pollen.

And on and on. There’s soil, and cross-pollination opportunities, and humidity levels. “. . . and so the complicating interactions pile up,” writes Bergh.

It’s also possible that the weather gets too warm. “It is felt that temperatures above 95 degrees can influence young fruit drop,” wrote Len Francis. I feel the same. I’m now in the habit of watering down an avocado tree’s foliage midday to cool it if it gets over 95 during bloom or within a month after, and this appears to help fruitset and prevent fruit drop.

(See my post, “Protecting avocado trees from heat.”)

But at least we had those good rains of March and early April, eh? And now we’ve got a stretch of warm days that will open up all of the avocado flowers on time and invite the bees to do their work from dawn to dusk. Like they did back in ‘65?

Nice crop on Carmen avocado tree last year in Saticoy.

More about avocado flowering and pollination

In addition to the links I’ve included above, these are some of my favorite resources on avocado flowering and pollination, from Bob Bergh and others:

“The Remarkable Avocado Flower” by B. Bergh

“Avocado Pollination Basics” (video) by M.L. Arpaia

“What Pollinates the Avocado Flower?” (video) by R. Hofshi

“Pollinators of Avocado” by G. Ish-Am

“Stingless Bees Can Serve as Efficient Avocado Pollinators” by S. Gazit and G. Ish-Am

“Native Pollinators of California Avocado as Affected by Introduced Pollinator Gardens” by G. Frankie, B. Faber, J. Pawelek, R. Thorp, R. Coville, and C. Jadallah

“Avocado Pollination and Bees” by O.I. Clark

“Avocado Pollination Tests” by O.I. Clark

“A Study in Cross-Pollination of Avocados in Southern California” by A.B. Stout

“History and Review of Studies on Cross-Pollination of Avocados” by C. Gustafson and B. Bergh

“Avocado Tree Arrangement and Thinning in Relation to Cross-Pollination” by B. Bergh

“Interplanting Complementary Avocado Varieties Aids Fruit Production” by D. Gustafson

“Pollen Production in Avocado” by C.A. Schroeder

I’ve also written some other posts related to avocado flowering and pollination:

“Grafting a pollenizer branch into your fruit tree”

“Hand pollinating avocados”

“Cross-pollination of avocados, or Why I planted a Hass next to a Fuerte”

HERE is a list of links to all of my Yard Posts

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