When you see comparisons between avocado varieties, often discussed are the flower type, size of the tree and its fruit, peel color, and cold sensitivity. Rarely, however, do you get a clue as to a variety’s sensitivity to heat.

(See, for example, Table 5 on page 30 of Book 1, Chapter 2 of “Avocado Production in California.”)

But in the last few years, I’ve noticed more avocado growers — commercial and backyard — inquiring about the heat tolerance of different varieties. Of course, the heat of July 6 and 7, 2018, which broke historical records across Southern California, especially fired up this concern.

Max temp was 113 on my shaded porch both July 6 and 7, 2018.

I grow a few dozen varieties of avocado trees in my yard, and I’ll share observations of how certain trees fared during the July 2018 heat wave, as well as during a somewhat less extreme heat wave in 2016. I’ll add my observations of how other avocado trees of different varieties responded to the July 2018 heat event in other locations in Southern California. In addition, I’ll share the observations that other people have made of different avocado varieties dealing with high heat in Southern California and the San Joaquin Valley. Finally, I discuss how the idea of heat tolerance is not as straight forward as it might seem.

Before I go on, let me note that this post is not about helping avocado trees cope with high heat. That post is here: “Protecting avocado trees from heat.”

What is heat or high heat?

Of all the avocado varieties that I grow, none get significant damage on their leaves unless the temperature reaches over about 105 degrees, as long as the tree is well watered, healthy, and over about knee height. For example, my yard got up to 104 degrees last week (on July 11). The worst damage on a healthy avocado tree in my yard (in full sun, no protection) was taken by the Pinkerton.

Pinkerton tree after 104 degrees.
Just minor sunburn on Pinkerton after 104 degrees.

Such damage isn’t pretty but neither is it significant, in my view. (No fruit is dropping, no twigs are dying back, etc.)

Therefore, if you live in a place that never reaches over 105 degrees, don’t bother reading the rest of this post except out of pure curiosity (and perhaps pity for those of us who do deal with such heat).

However, temperatures below 105 can be dangerous to avocado trees if they hit at specific times, specifically, in the late spring (think: June). This is because some avocado varieties bloom late, and temperatures around 100 can cause these flowers to not set fruit or recently set fruitlets to drop. More about this in my notes on the Reed variety below. But remember that if you don’t get up to 105 in summer, then you almost surely don’t get up to near 100 in late spring either.

So what is “heat” for the purposes of this post? Heat is around 100 in late spring or over 105 anytime.

My yard: my observations

I’m located in Ramona, San Diego County. Over the last eight summers that I’ve been growing avocados in this location, there have been many days over 105 degrees, but there have only been two heat waves where my avocado trees had to deal with temperatures over 110 degrees. Those two events have especially revealed different responses to high heat among my different avocado varieties.

June 20, 2016

On June 20, 2016, the thermometer on my shaded front porch reached 109 degrees. It was surely a few degrees hotter out among the trees. The humidity dropped to a low of 7% during this heat.

Nevertheless, no fruit dropped. I had mature fruit on my Reed and Lamb trees. In addition, I had newly set fruit on my Reed (which was also still blooming at the time), Lamb, and Hass trees. None of this fruit dropped during or in the weeks after the high heat.

(Prior to that day, I had irrigated in anticipation of the heat. On that day, I intermittently ran the micro-sprinklers under my trees.)

Some foliage was damaged, however. The worst burn on leaves and stems was incurred by my Hass and Fuerte trees.

Hass after 109-plus degrees.

The Sir-Prize, Sharwil, and Pinkerton were somewhat less burned. And the Reed and Lamb trees appeared almost undamaged.

Reed after 109-plus degrees.

An important lesson from that heat event was that Reed, Lamb, and Hass can hold their young fruit through a hot spell that reaches around 110 on even the longest (and therefore potentially hottest in terms of duration) day of the year.

I wasn’t surprised that the mature fruit held on the Reed and Lamb, but it was remarkable that the tiny new fruit held since young fruit is more vulnerable to dropping because of high heat than mature fruit.

July 6 and 7, 2018

The heat of July 2018 was the highest on record for most of Southern California. Santa Ana hit 114 and Riverside hit 118. Avocado trees looked torched all over the Southland.

In my own yard, the worst burn of foliage was seen on my Pinkerton and Nimlioh trees.

Hass and Fuerte were in the middle.

Hass damage after heat of July 2018.

The Lamb canopy looked slightly damaged, and the Reed canopy looked least damaged.

Lamb after heat of July 2018.
Reed after heat of July 2018.

(See video of the damage on some of my trees on July 8, 2018.)

The more important damage was to fruit, not foliage. Immediately after the heat subsided, it appeared that most of the fruit held. But over the next few weeks, much fruit dropped.

(Again, I had irrigated prior to the heat. But this time it was four days before — on July 2. I then irrigated during the heat on July 6. I also pulsed micro-sprinklers intermittently throughout the heat and even hosed down canopies a few times.)

By the end of July, I could judge how each variety had held onto its fruit.

From worst to best:

The Hass dropped almost all of its young fruit. There was no mature fruit, as it had all been harvested by late June. 

The Reed dropped almost all of its young fruit. But it held onto all of its mature fruit.

The Lamb dropped most of its young fruit but held onto some of it. The Lamb was in an “off” year in 2018 and was holding no mature fruit.

The Pinkerton dropped half of its young fruit. It was not carrying any mature fruit.

My other trees were too young or carrying too few fruit at the time to be worth mentioning.

One important lesson I took from this heat event was that different varieties respond to high heat in different ways. While the Pinkerton tree’s foliage looked terrible, it tenaciously held onto more of its young fruit than any other variety.

Heat damage to Pinkerton tree in July 2018.

On the other hand, the Reed tree’s foliage looked relatively unfazed but then it couldn’t hold onto its young fruit.

Which is preferable: tough leaves or tough fruit? Dropped fruit is a total loss. The Reed had basically no crop to harvest in 2019. Leaves grow back, as you can see in the photo of the Pinkerton below.

Regrowth on Pinkerton tree after heat damage. Photo taken August 23, 2018, six weeks after heat.

However, avocados bloom mostly on foliage that has grown during the previous summer. If much of that foliage gets damaged, then the following spring’s bloom is diminished, and that year’s fruitset is diminished.

My Pinkerton had a weak bloom in the spring of 2019 whereas my Reed had a heavy bloom in the spring of 2019 and set so much fruit that branches had to be propped for support.

To me, there isn’t a clear winner. Each tree essentially lost a year’s crop although a different year’s crop. But for my location and others with similar levels of heat (that is, not significantly higher levels of heat), I’d give the slight advantage to Reed considering that it showed that it can hold its young fruit through temps around 110 (in the 2016 event), just not through temps around 117. (Though my shaded porch registered 113, I estimate the temperature out among my trees to have been about 117 since that is what the nearest official weather station recorded.) It must be remembered that the July 2018 heat of 117 was an all-time record event, not a frequent, let alone yearly, occurrence.

Other locations: my observations

I visited other yards and avocado groves in the days, weeks, and months after that record heat of July 2018. Mostly, but not always, varieties responded the same as they did in my yard.

At a friend’s grove nearby that is larger than mine but has many of the same varieties, I noted many striking similarities: 

-Pinkerton trees’ foliage terrible, the worst, even though they held their young fruit. 

-Hass dropped almost all fruit and foliage looks really bad. 

-Fuerte foliage almost as bad as Hass. (Fruit drop? Didn’t note.)

-Reed foliage looks good, but dropped all young fruit. 

-Lamb looks pretty good and is holding some young fruit. 

-Holiday foliage is OK and is holding some young fruit.

-Bacon foliage is OK, holding much young fruit. 

-Stewart same as Bacon. 

-Ardith foliage excellent, maybe best of all, but hadn’t bloomed that spring.

Down the road from my house there are commercial groves of Hass avocados. Strangely, some groves had trees that looked similar to mine after the July 2018 heat.

Avocado grove on Highland Valley Road with moderate heat damage after July 2018.

Yet, other groves of Hass had trees that looked half dead.

Another avocado grove on Highland Valley Road with extreme heat damage after July 2018.

Later, on July 23, I drove through the Santa Clara Valley in Ventura County (Route 126) and I noticed similar contrasts. All of the groves appeared to be Hass, if my eyes served me well, but some groups of trees looked hardly harmed while others right next door looked set on fire.

Avocado grove between Fillmore and Piru with little heat damage after July 2018.
Another avocado grove between Fillmore and Piru with severe heat damage after July 2018.

Difficulties in assessing relative heat tolerance of avocado varieties

So we have the same variety, Hass, experiencing similar heat, but with a spectrum of responses. Explain that. (And if you can’t, then how can you validly compare different varieties?)

There are many contributing factors to a particular avocado tree’s (or entire grove of trees’) response to heat. First, there’s the overall care of the tree and health of the tree. Water, soil fertility, soil salinity, diseases, pests, etc. Some of these factors are invisible (e.g. soil salinity), and some are probably unknown (e.g. low-level fungal infection).

Second, what about rootstock? These trees may have the same scion variety on top but many have different types of rootstock underneath. This is particularly relevant to backyard growers, whose trees are usually on seedling rootstocks and are therefore variable. But even avocado farms had trees on seedling rootstocks back in 1917 when a severe heat wave hit Southern California. Thomas Shedden of Monrovia made this observation of his grove then: “In rows of same variety, age, planting and care, here and there trees stood the burning heat remarkably well, while the next several would have to be heavily trimmed, or cut back to stumps.” Was this rootstock influence?

These days, many avocado groves have trees on clonal rootstocks so there is little to no variation of that factor. And it has been documented that the same scion variety performed differently on different clonal rootstocks during that July 2018 heat. You can see some unpublished data from a trial of 30 rootstocks on Hass scions in Santa Paula, Ventura County at about 1:40 in this video. Also see 1:48 for heat damage results from another similar trial in Temecula. These are University of California trials.

Third, all varieties are on different phenological schedules. In other words, they are growing new leaves and blooming at slightly different times. This is critical because younger leaves are more susceptible to heat, as are younger fruit. Reed blooms late so if there’s an early heat wave, such as in late June or early July, the Reed fruitlets are very small compared to the size/maturity of fruitlets of all other varieties, making Reed fruitlets more vulnerable to the heat. So the mere timing of a heat wave can affect different varieties differently, making comparisons between varieties challenging.

Other locations: others’ observations

In the Summer 2020 issue of the Avocado Quarterly, a publication of the California Avocado Society, Sonia Rios, a farm advisor with the University of California Cooperative Extension, wrote an article titled “Beat the Heat” where she said that the most sensitive varieties to heat are “Hass and Fuerte, whose new leaves shrivel like plastic held to fire. Less affected are the varieties Sir-Prize and Sharwil, which show blackening and curling on some new leaves. Some of the toughest are the Pinkerton, Lamb Hass, and Reed. They show nearly zero evidence of having been uncomfortable under a terribly strong sun.”

Rios doesn’t detail where or when she observed this, and it sounds like she’s only commenting on the heat tolerance of the varieties’ foliage (not fruit), but it’s interesting to note all the same.

Mary Lu Arpaia, also of the University of California, has made comments on the heat tolerance and heat responses of certain avocado varieties. Arpaia has observed that GEM is more “heat hardy” than Hass, and that GEM fruit grown in the hot Central Valley keeps its size a bit better than Hass but sometimes has a more elongated shape than when grown in cooler locations down south and along the coast.

Arpaia has also said that Lamb is more heat tolerant than Hass. Unfortunately, I haven’t heard her elaborate on how she has seen Lamb reveal better heat tolerance than Hass — tougher foliage, fruit, both, other? 

One of the few commercial avocado growers in the Central Valley, located near Exeter, who mainly grows Hass and Lamb, once told me that his Hass avocados tend to be smaller than those grown in cooler locations. (See examples of avocados grown in the Central Valley compared to a cooler location in Orange County at 52 minutes in this video.)

This farmer also mentioned that he isn’t so happy with his Lamb trees because of their habit of dropping fruit as soon as it is barely mature in about May. Hass doesn’t have this early-drop habit for him. (This is possibly related to the fact that Hass is harvested before May, before heat arrives, and it’s possible that heat exaggerates the early-drop habit of Lamb.)

Eric Focht, a researcher for the University of California, has said that a variety called Harvest did particularly well in the heat of the Coachella Valley, where the University did some trial plantings. But Focht noted that this observation was anecdotal; no formal study on heat tolerance has been done.

An important side note is that heat tolerance does not necessarily come together with tolerance to the other end of the temperature spectrum: cold. Harvest performed worst in a cold spell at a University of California planting in the Central Valley. This is important to keep in mind because often locations that are extreme on the hot end are also extreme on the cold end. Tough trees in the heat aren’t necessarily tough all around.

Conclusions

Where does all this leave us? General conclusions:

-No formal study of heat tolerance of avocado varieties exists to my knowledge;

-All we have are anecdotal observations;

-These anecdotal observations sometimes conflict, which is not surprising because . . .

-One, claims about heat tolerance are sometimes not well defined (toughness of foliage or fruit holding capacity or what?);

-And two, we’re not always comparing apples to apples (different rootstocks, size and ages of trees, timing of heat in relation to the different varieties’ phenological schedules, irrigation and other care for the trees)

Tentative suggestion

I’d love to be able to land on a short list of avocado varieties that have been shown to be superior in heat overall. Inversely, I’d love to be able to list some varieties that tank in the heat and shouldn’t be bothered with in certain locations. But what I’m actually left with is the feeling that there’s a lot left to learn about the heat tolerance of avocado varieties, and every variety that I know that is worth growing in other respects is worth trying in a location with heat.

I suggest this because of the results I’ve seen in my yard along with all of the other evidence gathered and discussed above. And one final piece of the puzzle that informs this is the visit I made to a grove in Redlands in February of 2019. (I mentioned this visit in my post about protecting avocado trees from heat.) On that day I observed excellent crop loads of mature or nearly mature fruit on trees of the following varieties: Hass, Fuerte, Carmen, Bacon, and Pinkerton.

Pinkerton tree in Redlands after July 2018 heat, carrying a bench-bending crop. Photo taken February 2019.

The Pinkerton and Bacon I wasn’t too surprised about. But especially the Hass and Fuerte crops were curious. These avocados would have been small fruitlets during the extreme heat of July 6 and 7, 2018. I don’t know the high temperatures that this grove reached on those days, but I know that the general area is usually as hot or even hotter than nearby Riverside where it hit 118.

These Hass and Fuerte trees carried young fruit through a couple days of heat around 118 degrees. I know of many other Fuerte trees in the area that dropped all or much of their young fruit in July 2018. I know of many Hass trees that did the same, of course. Moreover, the Hass trees that held their young fruit in the Redlands grove were young and old alike, from two years in the ground to more than two decades in the ground.

I’m reminded of something a Chilean avocado farmer told me when I asked him about his opinion of the difference in heat tolerance between the varieties he grows: Hass and Reed. He said, “It’s not about the variety; it’s about the management.”

I don’t know if he’s all right, but I suspect he’s at least partly right. So why not try any avocado variety anywhere?

Note: I published a thin, first version of this post in 2016. And I’ve left all of the comments from that original post below.

All of my other Yard Posts are HERE

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