Rather than a whimper, summer in Southern California often goes out with a bang, with a heat wave around Labor Day. I’m glad this one is over, as at my place it was 100-degrees or higher for nine days straight. But before putting it in the back of my mind, I want to note some observations and lessons in order to help interpret the results we see in our food gardens and manage next summer’s heat waves, especially September ones.
September vs. June

A couple characteristics distinguish heat waves in September (the end of summer) from those in June or early July (the beginning of summer). June or early July heat waves are felt mostly inland. The beaches usually remain hidden under fog while valleys are blazing at triple digits. In contrast, September heat waves affect our whole region from desert, mountain, valley, to coast.

Also, the days are longer during June and early July, and so the same temperature will cause more damage there in early summer compared to September.

And plants are in different phases of growth in June and early July compared to September. Many fruit trees have just finished flowering and have small, new fruit growing in June and early July; small, new fruit are more sensitive to heat than bigger, mature fruit. Therefore, on the whole, fruit trees need more protection from heat in June and early July than they do in September, given the same heat conditions.

Finally, whereas a hot spell in June or early July marks only the beginning of the season of more potential high heat, September heat waves often finish with precipitous drops in temperatures and the beginning of our fall season. This year, the final day of heat was September 10 at 102, and then September 11 dropped to 89, September 12 was 84, September 13 was 85, and the meteorological forecasts are for temperatures in the 70s and 80s with possible rain showers for the next ten days.

This is not unusual. After the heat wave of September 3-7, 2020, the temperature dropped from 99 on September 7 to 76 on September 8. Yes, a 23-degree drop in one day!

Nevertheless, our entire months of September and October in Southern California can continue to be warm, even with hot spells beyond mid-September; however, after that time the sun is weaker and incapable of causing the damage it can in summer.

Fruit vs. foliage

The high temperature in this September 2024 heat wave was 114 in my yard, and it was 105 or higher for six days in a row. What I saw was that this damaged exposed fruit of tomatoes, peppers, avocados, apples, pears, pomegranates, and mangos.

Where I shaded fruit, the damage was null.

Shade vs. no shade

With some shade cloth, no damage was seen on healthy avocado trees.

But without shade cloth, even healthy avocado trees showed some sunburn, especially to new leaves.

Tropical plants vs. temperate plants

We might think that plants from tropical or subtropical climates would be tougher in heat compared to plants from temperate climates, but that is not always the case. For example, during this heat wave, banana leaves showed some sun bleaching; in contrast, apple leaves did not.

Then again, guavas showed themselves to be very tough in heat.

Mango leaves also showed themselves to be tough, tougher than avocado, banana, and litchi leaves. Even new (red) leaves on mango trees in my yard and elsewhere had almost no damage.

Heavy crop vs. light crop

Fruit trees with heavy crops incurred more damage than those with light crops, on average. I noticed this with avocados and litchis and pluots in particular. This happened mostly because the weight of the fruit bent down branches, which exposed the branches (and sometimes the fruit) to sunburn.

Trees with heavy crops need more of our attention going into a heat wave. Long before the heat, we should thin fruit or cut back the ends of laden branches that are bending to horizontal. Paint those branches. Shade them. Spray water on them to cool them during the heat. Whatever is practical in the situation.

What I would do the same

I did a few things right and a few things wrong related to this heat wave. One thing I did right was not shade my zucchini plant. Shading it would have been a waste of time and resources.

The most important thing I did right was pay attention to the weather forecasts, notice the heat predicted, and irrigate all of my plants before it arrived.

I can’t stress enough how effective this practice is: Irrigate before the heat starts.

In past heat waves I’ve made the mistake of waiting to water some plants during the heat, that is, after the heat has arrived and become high enough for plants to show stress. It allows for damage that a “pre” irrigation can prevent.

If you’re close to the beach, then heat will never get high enough to cause the kind of damage seen on the Hass avocado tree above, but your trees can still be stressed to a lesser extent by September heat waves, and pre-irrigation can prevent this.

What I would do differently

One thing I messed up during this heat wave was my placement of shade and overhead sprinklers on certain plants. I forgot how southern the angle of the sun had already become at this time of year. I placed my protection too overhead whereas I should have placed it more on the south sides.

When is a heat wave done?

The heat wave is over, right? In my yard, twenty miles inland, the days are down to the mid-80s. I was at the beach yesterday, and the onshore breeze was strong; the ocean water was even warmer than the air.

But damage from a heat wave continues to reveal itself for weeks after the air temperatures have dropped. Don’t count your chickens just yet.

Fruit from certain trees might still drop, for example. After the July 2018 heat wave was finished and the temperature dropped on July 9, I saw no fruit drop on avocados. I thought I had pulled my trees through without fruit loss. But by July 25, lots of fruit was on the ground. In fact, my large Hass tree had shed a few hundred young avocados and was left holding zero.

I’ll be watching my corn over these next few weeks. Some were tasseling during the heat, and I’m pretty sure their pollen was killed. Will I still get some ears from the later bloomers?

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