One of the many signs of the fall season in Southern California is the browning of avocado leaves, also called “tip burn” because the browning starts at the leaf tips. The photo above shows leaves on one of my Hass avocado trees here in late fall. Why do they look like this? And what can be done about it?
There is one primary culprit of the typical tip burn that develops in the fall, and that’s chloride in our irrigation water. In Southern California, much of our water is imported from the Colorado River, a source which is salty. (What is a salt in this context? See Wikipedia’s salt page.) One of the salts that the Colorado River contains in unfortunately high proportions is chloride.
How high is the chloride level? Usually from 80 to 100 parts per million. You can look at your particular water district’s annual quality report for more precise numbers, but most of Southern California is similar because we import from mostly the same sources. According to researchers at the University of California, in a study titled “Adoption of Water-Related Technology and Management Practices by the California Avocado Industry”, avocados suffer if the chloride level is over 75 parts per million.
What happens is that all spring and summer we water our yards with this high-chloride water and most of our plants don’t show much dislike, but avocados are especially sensitive to it. Avocado roots take up the high-chloride water and move it up their tree’s trunk, out the branches, and into the leaves. Leaves breathe out this water, but while the water evaporates off the leaf surface, the chloride remains within. It’s like when you take a swim in the ocean and the sun dries the water off your skin but leaves a crust of salt.
Over the irrigation season, the chloride level builds up in the avocado leaf until it reaches a toxic level and the leaf tissue begins dying, starting at the tip. That’s the brown: chloride-induced death.
The phenomenon of chloride leaf burn has been understood by avocado farmers for a long time, at least 75 years. (Here’s an article from 1951 titled “Leaf Burn of Avocado”.) But I like to test things for myself if I can, so one year I watered two of my avocado trees entirely with rainwater that I’d stored in tanks. Rainwater contains almost no chloride. Sure enough, those two trees had perfect foliage while my others showed typical leaf burn that fall. Not a conclusive trial by any means, but it helped me believe what I’d always read.
What can we do about it?
Let me first note that minor tip burn on avocados is no big deal. More specifically, if less than about ten percent of a tree’s canopy is brown (dead), there should be no reduction in fruit yield. In other words, if a tree has only a little tip burn, it will still produce as much fruit as if it had perfect foliage. I’ve seen this play out over and over on my trees.
Why does more than about ten percent leaf burn cause a reduction in yield? It’s because when the season for flowering begins (usually in late winter), the tree will drop those damaged leaves and grow new ones rather than do much flowering. Few flowers = few fruit.
The question then is how to keep the fall tip-burn to a minimum. The answer is, in a phrase: water a lot.
The one good thing about chloride is that it moves with water. Because of this, commercial farmers primarily manage it through leaching, as can we home gardeners. By leaching, I mean that they overwater on purpose so that the extra water will carry the chloride that has built up in the soil down deeper into the soil, so deep that it is below the reach of the tree’s roots — and therefore no longer able to affect the tree.
(During the summer, the chloride level increases in the soil for the same reason that it increases in the leaf tips: evaporation. The sun evaporates water from the upper handful of inches of soil while leaving the chloride behind, thereby increasing its concentration.)
There are two main ways to leach. You can water a lot at one particular time. For example, farmers who use micro-sprinklers will run the sprinklers for up to 24 hours straight, and they’ll do this once a month in July, August, and September. Alternatively, you can water a little extra every time you irrigate. This is called adding a “leaching fraction”, and the fraction is usually 10 to 20 percent on top of what the tree needs to grow well otherwise.
For myself, I’ve done it both ways and seen slightly better results with a leaching fraction as opposed to monthly leachings. So, these days, I deal with chloride by watering a little extra every time I water.
By the way
Look at the south side of your avocado tree. Notice the amount of leaf burn.
Now compare it to the north side.
Always, the leaves on the north side are a deeper green and have less browning. And this is related to the fact that the sun shines on the south side more intensely, extracting more water from those leaves every single day of the year, which means more chloride is collecting in those leaves compared to the north side. (Sides would be reversed in the Southern Hemisphere.)
Also note that an avocado leaf that dies a natural death — a healthy death, you could say — a death from old age (called senescence by botanists), does not die starting at the tip nor does it turn brown before it falls. It turns uniformly yellow.
And by the way, certain avocado varieties (not to mention rootstocks) are less tolerant of chloride, and therefore get worse tip burn. Hass is one of these. Reed and Fuerte, on the other hand, are two which show more tolerance of chloride. I say this both from observing trees in my yard as well as from observing trees in other places.
May your avocado leaves be mostly green this fall, and may they be flowery and fruitful come spring. If not, more water ought to solve your lot.
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