The best thing you can do when planting an avocado tree in poor soil is to plant on a mound. How? Why? We’ll get there, but first: What is poor soil?

For avocados, poor soil is not deep or has slow drainage or has high clay content.

Not deep means less than about two feet before you hit something hard — rock or a layer of clay, usually.

Slow drainage means that if you dig a hole and fill it with water, it takes more than a handful of hours to drain out of the bottom of the hole.

High clay content means soils that are classified as clay types (“clay loam,” e.g.).

(What is your soil type? Find out in the USDA’s Web Soil Survey.) 

Why plant on a mound

What do these poor soil qualities do that prevents an avocado tree from thriving when planted within them? For shallow soil, the problems are that the tree’s root system will not be able to reach deeply to anchor itself, and the rock or clay barrier below is sometimes not highly permeable, which causes the soil just above it to remain soggy. (More on why soggy soil is dangerous for avocados later.)

So building a mound increases soil depth, but it also means that the tree has that much more aerated, quality soil that the roots can easily penetrate and grow in (unlike through an impervious layer down below).

For the problem of slow drainage, the mound elevates the avocado roots above the soil that drains slowly. When it rains in the winter for a week straight and you have puddles standing in parts of your yard, the avocado up on the mound will dry out first and get oxygen back into its roots. Researchers have estimated that while most plant species need the soil to have about 10 percent air content, avocados need the soil to have about 30 percent air content in order to thrive. (See “Root Asphyxia and Irrigation Management in Avocado Orchards.”)

Mounds for avocados have been tried and studied for decades. It has long been observed that they provide benefits. Here is an early report, “Mounds Aid Root Rot Replants” by Goodall, Ohr, and Zentmyer from the 1987 Yearbook of the California Avocado Society.

Because all other types of fruit trees are less sensitive to slow drainage compared to avocados — I’ve seen many yards where citrus, stone fruits, apples, and pomegranates were growing fine but the adjacent avocados were struggling — don’t make the mistake of thinking that if your dirt is fine for your orange tree it should be fine for your avocado. That’s not always true.

How to plant on a mound

If any one of these poor-soil conditions is present at all, I would plant on a slight mound at least — just a couple inches helps. I would plant up higher and higher the more of these conditions are present and the poorer that each individual condition is.

For example, in my own yard I have sandy loam soil and the soil has fast drainage — no problems there. However, there are some shallow spots here and there. When I dig a hole to plant an avocado, if I find that the good dirt in that area is less than about two shovel heads’ depth, then I plant high accordingly. In other words, if I run into a lot of rocks or some harder, different colored subsoil, then I plant up. How high? As high as is needed to make sure there’s at least two feet of good dirt below the newly planted tree.

Nowels avocado tree planted on slight mound in my yard.

Some yards get scraped by earthmoving equipment when the house is built, and there is almost no good dirt left in certain parts of the yard. In such a yard, you would find the soil shallow for sure, but you also might find it additionally compacted and with poor drainage. This kind of extreme situation certainly calls for an avocado tree being planted on the highest of mounds. A mound that is at least two feet above the surrounding grade might be necessary.

How to build a mound 

On avocado farms they sometimes use heavy machinery to form berms that are two or more feet high.

Avocado grove in Redlands, trees planted on berms.

(See a video and discussion of such a berm planting at Cal Poly San Luis Obispo at 49 minutes into the April 2021 Seminar of the California Avocado Society.)

You can imitate this by scraping up dirt from around the planting site or from another part of your yard to build your mound. Sometimes it may be necessary buy some soil.

Do use primarily real dirt to make your mound; do not use exclusively a bagged potting mix, as those products are so porous that you would have to water your tree very frequently.

I actually made a form of this mistake the first time I built a mound for an avocado tree that I planted over shallow soil. I built the mound with side walls that were too vertical. Rather than a mound, it was more like a pyramid or a volcano. I had a hard time keeping mulch on the mound’s sides and the tree dried out faster than any of my other trees so it was difficult to keep the tree watered sufficiently during summer. 

The point is, the sides of your mound should not be too steep. The higher your mound, the wider your mound so that the sides are not too steep.

Examples of dimensions that work are: 1 foot high by 3 feet wide, and 2 feet high by 5 feet wide.

And use primarily real dirt to form your mound, but it is fine to mix some compost and wood chips into the medium also. I’ve done this and I’ve seen others do it successfully too.

Avocado trees planted on mounds made of soil and wood chips on a farm in Santa Paula.

After planting on a mound: mulch

To protect your mound from erosion, you should cover it with mulch. The problem is that this mulch tends to follow gravity and slide down the sides of the mound. The best prevention of this is to erect some netting around the mound. Chicken wire works perfectly.

You almost can’t add too much mulch. Make the netting at least a few inches higher than the top of the mound (the top of the mound being the same height as the base of the trunk of the tree) so that it can contain all of the mulch that you add.

What kind of mulch? Wood chips, leaves, compost, and chopped up tree trimmings are all good.

Chicken wire around mound to contain mulch under Hass avocado tree in my yard.

Case study: start to finish

Recently, I helped a friend plant some avocados in his yard in Point Loma, San Diego, and here was our process. We first dug a hole for a drainage test. About a foot down we ran into a layer of clay. So right there we knew we would plant on a mound of some height, but we still had to fill the hole with water to test the drainage.

Drainage test for avocado planting hole.

The water disappeared after 1-2 hours so drainage was acceptable. We refilled the hole with dirt almost to the top and placed the tree over it. We scraped soil from the surrounding area to form the base of the mound and then added a bag of “E.B. Stone Top Soil Plus” purchased from a nursery, which is primarily sandy loam dirt.

Mound partially built. Made of native dirt and sandy loam soil from bag.

We then completed the mound with compost and wood chip mulch. The tree’s rootball at the top of the mound ended up at just over a foot above grade.

After this, we enclosed the mound with chicken wire and added more mulch. Then we watered the tree in thoroughly.

After planting on a mound: watering

The good thing about a mound can be that it drains faster and dries out faster than the soil below it, but this fast drainage and drying has to be kept in mind and your watering has to be tailored to it.

In the beginning, for about the first year after planting, you must water the tree right next to the trunk. This is where the roots are. There is no need to water the entire mound.

Also, you must water more often compared to a tree that is planted on flat land or at the same level as the surrounding dirt.

Two final considerations: gypsum and clonal rootstocks

One of the dangers with slow-draining soil for avocados is that it invites infection by Phytophthora cinnamomi, which is a microorganism that causes a disease called Root Rot. In essence, avocado roots in constantly soggy soil send out a signal to Phytophthora to attack.

Beyond planting on a mound (and irrigating properly), a few things have been found to further combat this Phytophthora attack. One is the addition of gypsum (calcium sulfate). Adding about 15 pounds of gypsum on top of the dirt under a newly planted avocado tree is sufficient. 

(See “Effects of Gypsum Soil Amendments on Avocado Growth, Soil Drainage, and Resistance to Phytophthora cinnamomi by Messenger, Menge, and Pond. And see “The Effects of Calcium on Avocado Growth and Root Health” by Messenger, Menge, Amrhein, and Faber.)

The second thing is to use the best genetics available for your avocado tree, specifically by buying a tree that has a clonal rootstock. These clonal rootstocks have shown tolerance to infection by Phytophthora. Avocado trees on clonal rootstocks are not widely available to home growers, but for the latest in availability see my post about where to buy an avocado tree.

You might also like to ready my post, “Avocado rootstocks: What do they matter?”

Additionally, keeping a thick layer of mulch on the mound not only prevents erosion but also gives the tree’s roots a hospitable environment in which to grow and increases the tree’s resistance to Phytophthora. (Read more on that in “Factors Affecting Root Rot Control in Mulched Avocado Orchards” by Downer, Faber, and Menge.) 

GEM avocado trees planted on mulched berm in Ventura County.

Even if you don’t have ideal soil conditions, you can probably still grow a fruitful avocado tree through modifying your style of planting and the care that you later give to the tree. It takes extra work, but it does work.

For an inspirational example, see the results of Rich’s trees in this video:

For my general post on how to plant avocado trees, see HERE

All of my Yard Posts are listed HERE

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