Planting Avocados

It seems like the simplest thing is the hardest. Recently, I was called out to evaluate why newly planted trees were failing at two sites and they both had a common problem. In one case, the trees had been planted too deeply at the beginning. At another, a large amount of planting amendment had been incorporated, and over a year's time, the trees had settled, so that they too had their graft unions covered with soil. In the latter case, the trees' unions were 4-8 inches below grade. It seems appropriate to review basic planting practices. In the best-case scenario, trees are planted from February to May, but depending on the area, they can be planted at other times, as well. So, I just got a call about planting, so it's probably time for a refresher.

Often times the grower harkens to the old adage: “dig a $5 hole for a 50-cent plant.” And so a lot of time and money and energy are put into that hole. Nothing costs $5 anymore. Trees cost closer to $50, so there might be a greater urge to do it all right. So the first thing first is forget the planting mix and those mycorrhizal inoculums. They either don't work or they might just damage your expensive tree.

Adding organic matter to a planting hole appears to be a promising step towards achieving that five-dollar hole. It seems logical that steer manure, peat moss, compost, etc. would improve poor soils by increasing aeration, nutritional value, and water holding capacity. And it does - in the immediate vicinity of the planting hole. Eventually, amended planting holes will have negative consequences to plant health.

The initial results are positive; roots grow vigorously in this ideal environment as long as irrigation is provided. But what happens when these roots encounter the interface between the planting hole and the native soil? Native soil contains fewer available nutrients, is more finely textured and is less aerated. Roots react much in the same way as they do in containers: they circle the edge of the interface and grow back into that more hospitable environment of the planting hole. The roots do not establish in the native soil, eventually resulting in reduced growth rates.

Soil water movement is problematic as well. Amended backfill has markedly different characteristics than surrounding native soil; it is more porous and water will wick away to the finer-textured native soil. In the summer, moisture within the planting hole will be depleted by the plant but not replaced by water held more tightly in the native soil. When irrigating, water will move quickly through the amended soil only to be held back by the more slowly draining native soil. The resulting bathtub effect, where water accumulates in the planting hole, floods the roots and eventually kills the plant.

Finally, all organic material eventually decomposes. If you've incorporated organic matter, within a few years that organic matter will have become ash and the bulk has turned into carbon dioxide. The soil will have settled and if you have followed the rule of thumb of 25% by volume, the trunk stem probably will have sunk below grade and the bud union will be buried. The buried union only exacerbates the flooding problem during irrigation or wet conditions. This was the problem that I went out to see.

Basic planting steps

1)    Dig a hole somewhat wider, but no deeper than the sleeve that the tree comes in. Making the hole wider (18 inches) allows room to manipulate the tree by hand and remove the sleeve once it is in the hole. Making the hole deeper than the sleeve allows for soil to accumulate around the graft union. Even if the hole is backfilled to the “appropriate” depth, because of subsidence of the loose earth, the tree can become buried. Do not put gravel in the bottom of the hole. This is commonly thought to improve drainage. It does not, it makes it worse.

2)    Gently tamp loose earth around the tree. Do not back fill with a planting mix. This creates a textural discontinuity which interferes with water movement both to and anyway from the tree. The fill soil should be free of clods to avoid air gaps and poor contact between roots and soil. Do not cover the root ball with soil; the irrigation water needs to come into direct contact with the root ball.

3)    The trees should be watered as soon as is practical after planting.

4)    Using drip irrigation the, the emitter should be near the trunk, so that water goes directly onto the root ball. Shrinking and swelling of the polyethylene tubing can move the emitter off the ball.

5)    After about 4-6 months the drip emitters can be moved from the trunk to 6-8 inches from the tree. Moving the emitters avoids keeping the trunks wet and reduces the likelihood of crown rot.

6)    In most situations, newly planted trees should be irrigated every 5-10 days with 2-5 gallons of water for the first 2-4 months until the roots get out into the bulk soil. Depending on what the weather is like, they still might require frequent irrigations, because the rooted volume holding water is still small. After the first year in the ground, another dripper can be installed on the opposite side of the tree. As the tree grows the number of drippers should be increased or the system converted to fan or microsprinklers.

And in the case of mycorrhizae, they are wonderful. They are nature's gift to all of us. They aid plants in their uptake of nutrients, improve plant health and may actively transmit information from one plant to another. They are a diverse range of fungi associated with plant roots and are everywhere – even Antarctica. And that's the point, introducing them to the planting is not going to help. They are there already, a sea of them. Putting a few nursery-grown spores into a planting hole when there are already highly adapted fungi present just does not happen easily. So not using a planting mix and not adding mycorrhizal inoculum is going to make that hole a little bit cheaper.

In root rot conditions

Planting in ground that has had root rot can add some new steps to the planting process. On relatively flat ground (<15 degree slope) trees will benefit from being planted on a berm or mound. This creates better aeration and drainage for the roots. It also means that the trees tend to dry out faster, so more frequent irrigation may be necessary. Where machinery can be employed, creating berms is usually less expensive. Surrounding soil should scraped to the planting site, and little incorporated with the soil surface where the berm or mound is to be built. In bringing surrounding native soil to the planting site, it is important that an interface between the imported soil and the soil surface is not created. Just mounding a different soil on top of a surface alters water flow through the mound into the bulk soil. The berms can be built 1.5 to 2 feet high with a 4:1 slope. The raised planting position should be irrigated to settle the soil. The soil should then be allowed to dry out prior to planting to avoid mucky soil. Only clonal rootstocks should be replanted into root rot soil. Applying gypsum (15 pounds per tree), a thick layer of mulch around the base of the tree (3-6 inches deep, but not immediately on the stem of the tree) and finally application of fungicides will help. Application on the berm or mound also protects the soil from eroding away with rains.

The key to root rot has always been dependent on irrigation management. There is nothing more important than getting the right amount on at the right time. If you are doing interplanting into an existing orchard where trees have died, it is imperative that the new trees be put on their separate irrigation line so that they can be irrigated according to their needs. Simply putting a smaller emitter on the young trees compared to the older trees means that they will still be irrigated on a cycle that is not optimum for their survival. It doesn't matter if you are using clonals; they will die just as easily with poor water management as a seedling.

The following is a pictorial guide to proper tree planting ( click on "ATTACHED FILES" planting holes.