Too Hot To Handle: California Avocado Growers Must Adapt To Higher Temps

Scorching heat across interior Ventura County this July spelled trouble for avocado growers. When the mercury climbs over 100 degrees for days, it can mean total crop loss. Growers across California’s avocado growing region who are vulnerable to high heat have applied old-fashioned farming ingenuity and novel techniques to combat the worsening threat caused by extreme heat. 

“It’s definitely getting hotter,” said Dr. Ben Faber, a farm adviser with UC Cooperative Extension. “Just 10 years ago, 110 degrees was unheard of, and since then we’ve had three summers in Santa Paula and Fillmore when it’s gotten 110 degrees and hotter. It’s gotten to 120. It’s not supposed to happen, but it’s happening.” 

The avocado tree is sensitive to temperature fluctuations in both directions, and the aftermath of heat damage looks a lot like the aftermath of frost. After extreme heat, the orchard needs to be evaluated for damage. Fruit can drop — in severe cases, most of it — and if the leaves start to get crunchy from heat damage, the first step is to reduce irrigation and fertilization. The damaged tree cannot process the irrigation like a healthy tree, so excess water, even from regular irrigation, could lead to root rot on impacted trees. Another tell-tale sign of heat damage is browning of the stem where it connects to the fruit. Avocados with browning stems will eventually drop.  

“Leaves are crispy. The fruit is on the ground. You’ll know heat damage when you see it. It’s really obvious,” Faber said. In the event that the leaves drop, apply whitewash to exposed branches to prevent sunburn. Otherwise, avoid pruning until there is new growth in order to protect the tree from sunburn. 

It could take six months to a year to observe the extent of the damage to each tree, whether it’s a matter of temporary fruit and leaf damage for the season or if the tree is severely impacted and might never recover. 

Beware Of Forecasts Over 90 Degrees 

Finicky avocado trees react to temperatures over 90 degrees. The stomata on the leaves, which open to allow transpiration under normal circumstances, close to retain moisture, but a side effect is causing the tree’s internal temperature to accelerate upward. If the forecast predicts that the temperature at your orchard will exceed 90 degrees throughout the day, consider that as being in the avocado tree heat advisory zone. 

“We’ve had to advise Index Fresh growers on recognizing and preparing for ever-more-frequent extreme heat,” said Keith Blanchard, California Field Manager for Index Fresh. “The good news is there are measures to take that can protect the crop and trees from extreme heat.” 

Faber, a specialist in subtropical horticulture, pointed out that avocado trees are at home in tropical weather, but the tree evolved to live under larger trees in the forest. Cultivators have taken avocados out of their natural shaded setting, so we must apply farming tools to protect them. “Avocado trees were naturally accustomed to hot weather, but also being shaded,” he said. 

Preparing For Extreme Heat 

As they have adapted to extreme heat, growers have developed game plans with mainly two proven techniques, advanced irrigation and overhead sprinklers. 

Overhead sprinklers can reduce ambient temperatures by 10 degrees through evaporative cooling in the orchard. If the forecast calls for 100 degrees, it is possible to bring temps down to about 90 degrees in the orchard, or right at the threshold before the trees close their stomata and enter the danger zone where internal temps spike. 

Even before it becomes time to switch on the overhead sprinklers, growers should prepare by irrigating ahead of time. Trees that are hydrated will withstand greater heat and suffer less damage, even if temps climb over 90 degrees. “Sometimes, the grower says, ‘Oh, it’s hot. I’m going to turn the irrigation on.’ But by then it’s too late,” Faber said. “If you can anticipate it and hydrate the tree, that’s when you are helping, because when it’s too hot, water isn’t going anywhere.”

In addition to overhead sprinklers, growers can reduce ambient temperature through adding moisture and evaporation to ambient air. The added overhead shower is most helpful when the water settles on the leaves or other surfaces, because that water will cool the tree down immediately, Faber said. 

Anecdotally, growers in hot temp zones have been able to install overhead sprinklers for about $1,200 per acre by laying them over existing irrigation systems. Growers will use overhead sprinklers in blocks during heat events, and it is important to monitor and rotate sprinkler locations frequently enough to keep trees moist throughout hot weather. 

Different Results On Different Trees 

Orchards in the same climate can have wildly different outcomes from extreme heat events. A recent event in Santa Paula caused nearly 90% fruit loss at one avocado orchard but others that were nearby withstood temperatures with far less damage. 

“Really, it’s a carpet of fruit on the ground,” Faber said of the badly hit orchards. 

Heartier trees where the fruit sets inside the canopy seem to handle extreme temperatures better. Also, some growers are finding that GEM trees, shaped so fruit sets inside the canopy, tend to fare better in hot or cold temps. The added protection from the elements was a primary reason for selecting the GEM to commercially cultivate. 

Note Of Caution 

Faber advises against using products marketed as Antistress 550/2000 on avocado trees. The products, which are applied to the canopy, can do more harm than good on avocados.