{"id":4392,"date":"2018-11-09T11:15:15","date_gmt":"2018-11-09T19:15:15","guid":{"rendered":"http:\/\/gregalder.com\/yardposts\/?p=4392"},"modified":"2022-12-23T20:58:09","modified_gmt":"2022-12-24T04:58:09","slug":"protecting-avocado-trees-from-cold","status":"publish","type":"post","link":"https:\/\/gregalder.com\/yardposts\/protecting-avocado-trees-from-cold\/","title":{"rendered":"Protecting avocado trees from cold"},"content":{"rendered":"\n

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, \u201cWell, if the tree can\u2019t survive, it doesn\u2019t belong here. I\u2019m not going to pamper it.\u201d <\/span><\/p>\n\n\n\n

I now realize the shortcomings of that philosophy. In inland Southern California, and many parts of the rest of California, you may just need to help a young avocado tree through its first winter or two until it can withstand all but the rarest and coldest arctic blasts we receive.<\/span><\/p>\n\n\n\n

This post is about when avocado trees need cold protection, my experiences using various protective measures, and a few methods that others have used.<\/p>\n\n\n\n

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At what temperatures do you need to protect avocado trees?<\/b><\/p>\n\n\n\n

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. T<\/em>his is the threshold for observed damage to avocado trees in my yard.<\/i><\/span><\/p>\n\n\n\n

On\u00a0the day after Christmas in 2015, my 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.<\/span><\/p>\n\n\n\n

In late February of this year, 2018, my 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.<\/span><\/p>\n\n\n\n

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\u2019s Freeze of 2015<\/a>, and it remains <\/span>the lowest temperature reached in my yard during the five winters I\u2019ve spent here. (See video update from winter 2020 at bottom<\/em>.) Damage to my avocados was widespread although not serious except to young trees.<\/p>\n\n\n\n

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\u2019t protected the Pinkerton in any way.)<\/span><\/p>\n\n\n\n

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The Pinkerton, after suffering many hours in the mid to upper 20s.<\/figcaption><\/figure>\n\n\n\n

(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.<\/a>)<\/span><\/p>\n\n\n\n

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In summary:<\/p>\n\n\n\n

–Consider protecting young and small or unhealthy avocado trees when it’s predicted to drop to about 32 degrees<\/p>\n\n\n\n

–Consider protecting mature and healthy avocado trees from about 30 degrees<\/p>\n\n\n\n

–Don’t anticipate significant damage to mature and healthy avocado trees (fruit and limb damage) until below 27<\/p>\n<\/blockquote>\n\n\n\n

Preparing an avocado tree for imminent cold: <\/b>Irrigate<\/b><\/p>\n\n\n\n

If there\u2019s 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, <\/span>\u201cPassive Frost Protection of Trees and Vines.\u201d<\/span><\/a>)<\/span><\/p>\n\n\n\n

Remove mulch?<\/b><\/p>\n\n\n\n

Related to the soil’s ability to store heat and radiate it later, I\u2019ve read advice to remove mulch from beneath the tree since mulch blocks soil from warming as much as possible. This might make sense. I\u2019ve 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. <\/span><\/p>\n\n\n\n

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.<\/span><\/p>\n\n\n\n

Methods of protecting avocado trees from frost and freeze: <\/b>Overhead coverage<\/b><\/p>\n\n\n\n

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.<\/span><\/p>\n\n\n\n

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I\u2019ve used umbrellas.<\/figcaption><\/figure>\n\n\n\n
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I’ve used blankets and rugs.<\/figcaption><\/figure>\n\n\n\n
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I’ve used wood boards.<\/figcaption><\/figure>\n\n\n\n

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.<\/span><\/p>\n\n\n\n

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.<\/p>\n\n\n\n

Higher by how many degrees? I\u2019ve 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.<\/p>\n\n\n\n

(See an article from 1951 by Don Gustafson called \u201cFrost Protection of Young Avocado Trees,\u201d <\/a>showing growers in Orange County using burlap covers.)<\/p>\n\n\n\n

Wrapping sides<\/b><\/p>\n\n\n\n

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. <\/span><\/p>\n\n\n\n

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The above photo shows a young avocado tree wrapped in Reflectix bubble insulation. I happen to have some of this material on hand so I’ve used it to protect trees in winters past.<\/p>\n\n\n\n

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.)<\/span><\/p>\n\n\n\n

Frost blankets<\/strong><\/p>\n\n\n\n

Another material that is deliberately made for insulation, but this time specifically for insulating plants, is a frost blanket such as this one.<\/p>\n\n\n\n

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N-Sulate frost protection blanket.<\/figcaption><\/figure>\n\n\n\n

I bought this N-Sulate frost blanket at a nursery for $13. It’s also available online.<\/p>\n\n\n\n

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. In my experience, such claims are exaggerated, however. I would only count on about half that amount of protection. (See the videos at the end of this post for more explanation.)<\/p>\n\n\n\n

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.<\/p>\n\n\n\n

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I secured the frost blanket to small branches using clothespins.<\/figcaption><\/figure>\n\n\n\n
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View inside the frost blanket.<\/figcaption><\/figure>\n\n\n\n

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.)<\/p>\n\n\n\n

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)<\/a>.<\/p>\n\n\n\n

Lights<\/b><\/p>\n\n\n\n

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. <\/span>Leave the lights on all night, especially through the early morning when it\u2019s usually coldest, to add a few degrees of warming. <\/span><\/p>\n\n\n\n

Here are some lights that would work for this purpose: Brightown Globe String Lights.<\/span><\/p>\n\n\n\n

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Note that they are incandescent and not LED, which wouldn’t give off the desired heat.<\/figcaption><\/figure>\n\n\n\n
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I set them up for a show on this Nimlioh tree.<\/figcaption><\/figure>\n\n\n\n

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.<\/span><\/p>\n\n\n\n

Buckets of water?<\/b><\/p>\n\n\n\n

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\u2019m not convinced it is effective, especially compared to other techniques.<\/span><\/p>\n\n\n\n

Sprinklers?<\/b><\/p>\n\n\n\n

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\u2019ve 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<\/a>.)<\/span><\/p>\n\n\n\n

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In summary:<\/span><\/p>\n\n\n\n

–Any overhead coverage raises the temperature around the tree a few degrees. <\/span><\/p>\n\n\n\n

–Overhead plus side coverage keeps a tree even warmer.<\/span><\/p>\n\n\n\n

–Lights strung in a tree add warmth.<\/span><\/p>\n\n\n\n

–Lights or other heat within a covering would add maximum heat and protection of an avocado tree in the cold.<\/span><\/p>\n<\/blockquote>\n\n\n\n

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.<\/span><\/p>\n\n\n\n

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.<\/span><\/p>\n\n\n\n

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Up the hill we plant.<\/figcaption><\/figure>\n\n\n\n

Update March 19, 2019:<\/strong><\/em><\/p>\n\n\n\n

The winter certainly was rainy but it was not warm. The coldest night in my yard was February 19 when the thermometer dropped to 27. Fortunately, I didn’t lose even the tiniest of avocado trees.<\/p>\n\n\n\n

Mostly, I protected them with frost blankets (Agribon AG-50, as seen here<\/a>). A few trees I helped with only umbrellas, and for a couple young trees I added heat with lights.<\/p>\n\n\n\n

Update February 2020:<\/strong><\/em><\/p>\n\n\n\n

A night of 25 degrees caused a spectrum of damage to different trees:<\/p>\n\n\n\n

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