My attitude toward protecting avocado trees during winter cold snaps has evolved. I once lived close to the beach where I never thought about protecting my avocados in winter. Then I moved inland and watched the temperatures dive at night but shrugged at first, “Well, if the tree can’t survive, it doesn’t belong here. I’m not going to pamper it.”
I now realize the shortcomings of that philosophy. In Southern California, you just need to get a young avocado tree through its first couple winters and then it can withstand all but the rarest and coldest arctic blasts we receive. And adding a couple degrees of warmth on a couple nights each winter is usually all it takes.
This post is about when avocado trees need cold protection, what protective measures I’ve tried in past winters, and new methods I will test this winter, if necessary.
At what temperatures do you need to protect avocado trees?
I have a thermometer on my porch where I estimate that the temperature stays around 4 degrees warmer than out in the open parts of my yard. Over the past handful of winters, I’ve recorded some of the low temperatures on that thermometer along with damage caused to avocado trees.
Damage to young and small avocado trees, roughly younger than three years in the ground and under five feet tall, or unhealthy avocado trees can happen once the temperature drops below freezing, 32 degrees. But I’ve never seen damage to mature and healthy avocado trees until the temperature drops below 30. This is the threshold for observed avocado damage in my yard.
On the day after Christmas in 2015, the yard got down to 29 and I noticed a couple of newly emerged leaves on various avocado trees partially burned. There was frost on the ground throughout the yard that morning.
In late February of this year, 2018, the yard got down to 27 and new leaves on avocado trees were damaged, plus some older leaves showed brown mottling, and most flowers on all of my avocado trees were killed (except some deep inside the canopy). The frosty photo at the top of the page was taken during this event.
During the last days of 2014 and the first days of 2015, the yard got down to 26. I refer to this event as the New Year’s Freeze of 2015, and it remains the lowest temperature reached in my yard during the five winters I’ve spent here. Damage to my avocados was widespread although not serious except to young trees.
There was not only cold burning of avocado leaves on all varieties, including Fuerte and Sir-Prize, but also branch tips killed back up to six inches in a couple parts of the canopy of my Reed avocado tree. As far as young trees went, a Pinkerton that had been in the ground for less than a year had all of its leaves and all but one of its limbs killed back to the trunk. (I hadn’t protected the Pinkerton in any way.)
(This was an unusually cold event, when snow fell in parts of southern avocado country: Escondido, Valley Center, and Temecula. See photos of that snow in avocado groves at this blog post by Reuben Hofshi.)
–Consider protecting young and small or unhealthy avocado trees when it’s predicted to drop to about 32 degrees
–Consider protecting mature and healthy avocado trees from about 30 degrees
–Don’t anticipate significant damage to mature and healthy avocado trees (fruit and limb damage) until below 27
Preparing an avocado tree for imminent cold: Irrigate
If there’s a weather alert that temperatures will drop below freezing, step one is to be sure your avocado tree is anchored in soil that is moist. Water as necessary. There are two reasons for this: One, an avocado tree in moist soil will be hydrated and best able to handle the stress of low temperatures. Two, moist soil is able to store more heat from the sun during the day, which means it can also radiate that heat at night. (See this interesting article from the University of California, “Passive Frost Protection of Trees and Vines.”)
Related to the soil’s ability to store heat and radiate it later, I’ve read advice to remove mulch from beneath the tree since mulch blocks soil from warming as much as possible. This might make sense. I’ve observed the pattern innumerable times in my yard and neighborhood where, during light frosts, mulched (with wood chips or compost) areas are covered by frost while adjacent bare soil is not.
Even so, last winter I experimented with removing mulch from below some of my trees and found unclear results. During the frosty nights of late February 2018, some mulched trees were damaged, some were not. Some unmulched trees were damaged, some were not. I don’t know what conclusions to draw from this, honestly.
Methods of protecting avocado trees from frost and freeze: Overhead coverage
Now for what to do to protect your avocado tree during a frigid night. A small tree is easily protected by putting a roof over its head. Any overhead covering is warmer than nothing.
I’ve also used nylon tarps on top of ladders set over the tree, trash cans, cardboard boxes, and more. I’ve seen that they all keep the tree at least a few degrees warmer, in part as judged by the frost forming on ground near such protected trees but not under them, in addition to their sustaining no damage.
How does overhead protection work, and how to use it? The way this method works is by trapping radiation that emanates from both the tree and the soil below the tree. At night, the tree and soil are releasing a certain amount of heat into the air, and if you can block that radiation you can keep the temperature around the tree higher than it otherwise would be.
Higher by how many degrees? I’ve read estimates of 4-8 degrees. It seems to depend on the materials used. Regardless, even if it’s only two degrees higher, a young tree kept at 31 compared to 29 can mean the difference between no damage and all leaves lost.
(See an article from 1951 by Don Gustafson called “Frost Protection of Young Avocado Trees,” showing growers in Orange County using burlap covers.)
Will you trap even more heat by wrapping the sides too? Sure thing. It’s like the difference between sleeping under a mere tarp and sleeping in an enclosed tent.
The above photo shows a young avocado tree wrapped in Reflectix bubble insulation (see it on Amazon here). I happen to have some of this material on hand so I’ve used it to protect trees in winters past.
The one thing to keep in mind about using both overhead protection and wrapping the tree entirely is that they should be put in place before sunset to be most effective. You want to begin trapping the radiation before it gets too cold. Also, you want to remove them in the morning once sun is on the tree, unless it’s a structure like my wood board above, which lets light onto the tree and soil. That structure could actually be left in place all winter. (Great option for frost protection if you’re going on vacation.)
Another material that is deliberately made for insulation, but this time specifically for insulating plants, is a frost blanket such as this one.
I bought this N-Sulate frost blanket at a nursery for $13. It’s also available online, such as on Amazon here.
Frost blankets can entirely wrap a small tree, and if they do, blankets of this weight (1.5 ounces per square yard) claim to be able to give 6-8 degrees of protection. But they can also give somewhat less protection if used to partially cover a larger tree. I’ve got a good crop of fruit on my Lamb avocado tree that I don’t want to lose, so if temperatures threaten to damage this fruit I’ll throw the blanket over it like this.
This frost blanket allows a lot of light to enter, as you can see in the photo above. Because of this, you could safely leave it on a tree during the day. (Another good option for frost protection if you’re going out of town during winter.)
By the way, if you had a couple of smaller avocado trees that you wanted to protect, you could completely wrap them with a pack of smaller frost blankets, such as these 7’x7′ ones made by Agribon, available through Peaceful Valley Farm Supply (Grow Organic.com).
An additional way to add warmth to a larger tree is the classic Christmas lights approach. Run an extension cord out to the tree, then string Christmas lights (or similar lights) through the tree. Leave the lights on all night, especially through the early morning when it’s usually coldest, to add a few degrees of warming.
Here are some lights that would work for this purpose: Brightown Globe String Lights (here they are on Amazon).
Of course, you can use lights on small trees as well. In fact, both covering a tree and turning lights on under the covering adds maximum heat. You can also use other sources of heat placed below the tree, such as a shop light, a table lamp, or a space heater.
Buckets of water?
Some people claim that you can get some protection by leaving buckets of water under or next to the tree. When water freezes it releases a small amount of heat. I tried this one night a long time ago and found no effect compared to nearby trees that spent the night bucketless. I’m not convinced it is effective, especially compared to other techniques.
Taking advantage of this same principle is the idea of running sprinklers under a tree throughout the night. As the water freezes it releases some heat. In order for this to work, the sprinklers must be run continuously all night. I’ve never tried this because other methods seem better suited for a home garden situation. Large groves do make use of this method though. (See this article by Ben Faber of the University of California which briefly notes how to use sprinklers on avocados for frost protection.)
–Any overhead coverage raises the temperature around the tree a few degrees.
–Overhead plus side coverage keeps a tree even warmer.
–Lights strung in a tree add warmth.
–Lights or other heat within a covering would add maximum heat and protection of an avocado tree in the cold.
When a winter night gets cruel and it’s below 40 already at dinner time and protecting your avocado tree is called for, is it worth the effort? Yes. I’ve lost young trees in the past that I might be eating from now if only I’d thrown a little protection over them on one or two nights.
Prepare now. Have the materials on hand. This advice is directed at myself as much as anyone, for I’m especially exposed to the cold this winter as I’ve got more small young avocado trees to shepherd than usual. Can we please get a warm and rainy winter like 2016/2017? Wish me luck, I wish you luck.
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