There's been a lot of avocado and citrus planting going on and this is a good time for a reminder about how to dig a hole. This is by our colleague Jim Downer in Ventura County, Horticulture Advisor and also past president of the International Society of Arboriculture, Western Chapter.  In the text, where you see Fraxinus or some other tree name you don't recognize, just slip in avocado or citrus and keep reading.  Also, check out the references.

Green side up! Oh, and do not sink the rootball below grade!

I have always been amazed at how the simplest of procedures or practices can go so wrong. For the green industry, the best example of this is planting.  The act of putting green in the ground is our business.  We do this.  The problem is, we often do it wrong, carelessly, or without regard for the outcome—dead trees!  A consultant friend often expressed how deep planting and covering the root ball with native fill are the most common mistakes he sees. I have to agree--landscape plants die at the hand of man more than from all the diseases and insects combined.  There are various incorrect ways to plant a tree, such as adding too much organic matter to the backfill, installing a dry root ball and then not irrigating after planting, or adding too much fertilizer to the backfill.  The practice I want to cover in this article is planting too deeply.  The problem continues despite research about planting that recommends correct planting depths.

   Planting depth is often ignored when plants are installed in landscapes.

Deep planting can result in death of woody and non-woody or herbaceous plants either because they rot (in moisture-saturated soils) or because they dry out.   In either case, the symptoms are similar: wilting, sunscald or burnt leaves (necrotic tissues in the middle of the leaf), lack of growth, leaf drop, and eventually, necrosis of leaves, shoots and branches (all above ground parts).   Irrigation usually does not improve symptoms because by the time they are noticed the plant has already been harmed beyond repair. 

Root balls placed below grade cause several problems during establishment. Since native soil surrounds the root ball, there is an immediate problem with an interface between the two soil textures.   Most container media are “light” to promote drainage characteristics necessary for container culture.  When these soil-free media are planted in soil which is of a much finer texture, the resulting interface does not allow water to enter the root ball.  Water must completely saturate the surrounding soil before it will cross the interface (Harris et al., 1999). As the plant draws down its container media moisture, the root ball desiccates beyond the permanent wilting point and the plant dies.  This process is extreme in plants that are grown in peat-based media because the peat moss can become quite hydrophobic as it dries and then the interface issues are exacerbated.   Special care should be taken with citrus and avocados to plant them at or above grade so the media itself is exposed to irrigations.   

Acid plants are however, no exception to the above suggestion. Installing the plant at or above grade (if only ½-1 inch) will prevent excessive drying of the root ball due to interface smothering.  It is however, very important that the root ball itself is irrigated in the first month of establishment not just the surrounding soil.  Newly planted nursery stock does not absorb water from landscape soil, only from its own rootball.  Until roots grow into the native soil, the plant must be irrigated to keep its rootball moist.  The surface of the rootball can be protected with a coarse wood chip mulch.

Not all installers get planting depths wrong at the start. When the plants are first installed, everything looks good.  The problem is sometimes related to the amount of digging used to make the planting hole.  If the hole is dug too deep, and soil added back to bring the final grade to level, the plant can slump as water settles it.  Digging destroys soil structure, so backfill under the rootball always settles - the plant sinks.Soil will wash in from the sides covering the root ball and sealing it from future irrigations.

Deeply planted woody plants are subject to diseases. The area where the roots of a plant join its main stem is the root collar. This area is very metabolically active and requires oxygen.  In some cases, the stem above the root collar is green and photosynthesizes. Acer japonicum the Japanese maple has a clearly demarcated root collar region.  Soil goes on the brown part and the green part should remain above ground.  When the main stem is buried, the plant is predisposed to attack from canker forming fungi or other plant pathogens that can girdle the stem, killing it and all that grows above it.  

It is quite clear from the literature that there is a strong species effect to the tolerance (or lack of tolerance) to deep planting. In a study of red maple and Yoshino cherry, only 50% of cherries survived deep planting, while there were no significant losses of maple to deep planting practices (Wells, et al., 2006).  Arnold and others, 2007, found that green ash (Fraxinus pennsylvanica) was more tolerant to below-grade installation than golden rain tree (Koelreuteria bipinnata).  In the same paper by Arnold et al., they showed that mulching can make deep planting worse.  When trees planted below grade were mulched, mortality levels increased.  

If plants survive deep planting, there can be other consequences. Wells and others 2006, showed that red maple (Acer rubrum) had increased numbers of girdling roots the deeper they were planted.  When planted 6 inches below grade trees had 48% of their trunk encircled by girdling roots, when planted 12 inches below grade 71% of the trunk was affected. 

Not all researchers found that soil over the root ball is detrimental. Gilman and Grabosky, 2004, found that if irrigation is plentiful (over an inch of applied water), trees survived and were less stressed three months later.  Although planting depth did not impact growth of Southern live oaks, the study was relatively short term (7 months).  I have also found in my own study of landscape shrubs that deep planting of five different genera of shrubs were not affected by planting depths of up to 4 inches below grade.   The limitation of these studies is that they are short term.  Over longer periods, disease and greater periods of hypoxia during high rainfall seasons may have cumulative detrimental effects not seen in the establishment phase of growth.   When studied for three years, Arnold and others (2007), found that planting slightly above grade (3 in) improved growth of oleander and sycamore, while planting slightly below grade (3in) was harmful to all tested plants. 

References

Broschatt, T. 1995. Planting depth affects survival, root growth, nutrient content of transplanted pygmy date palms.  HortScience 30:1031-1032.

Arnold, M.A., G.V. McDonald, and D. Bryan. 2005.  Planting depth and mulch thickness affect establishment of green ash (Fraxinus pennsylvanica and Bougainvillea goldenraintree (Koelreuteria bipinnata).  J. Arboric. and Urban Forestry 31:163-170.

Arnold, M.A. G.V. McDonald, D.L. Bryan, G.C. Denny, W.T. Watson and L. Lombardini.  2007.  Below-grade planting adversely affects survival and growth of tree species from five different families.  J. Arboric. and Urban Forestry  33:64-69

Gillman, E. and J. Grabosky. 2004.  Mulch and planting depth affect live oak (Quercus virginiana Mill.) establishment.  J. Arboric. and Urban Forestry 30:311-317

Harris, R.W., J.R. Clark, and N.P. Matheny. 1999.  Arboriculture: Integrated Management of Landscape Trees, Shrubs, and Vines. 3^rd ed.   Prentice Hall, Upper Saddle River, NJ.

MacDonald, J.D., L.R. Costello, J.M. Lichter, and D. Quickert. 2004.  Fill soil effects on soil aeration and tree growth.  J. Arboriculture 30:19-27.

Wells C., K. Townsend, J. Caldwell, D. Ham, E.T. Smiley and M. Sherwood. 2006.  Effects of planting depth on landscape tree survival and girdling root formation.  J. Arboriculture and Urban Forestry 32:305-311.

A recent letter which followed a farm visit to another grower and finally a phone call about planting an avocado tree from another grower.  The basics of planting are often overlooked and we should revisit them to make sure we know how to do the basics.

The letter:

I am going to plant some avocado trees on a slope. I will be drilling a hole with an 18 inch auger about three feet deep.

I have lots of wood chips from avocado trimmings. Should I mix some avocado wood chips with the soil to insure good drainage when I plant the new trees?

Or should I mix in some compost to help good drainage? Some areas here have poor drainage because of clay soil.

The response:

DO NOT put anything in the planting hole.  It makes drainage worse.  And if its not fresh the decomposing gases will kill the roots. And gradually over time, the organic matter decomposes and the tree sinks deeper into the ground, covering the graft union and the tree dies.  I[m glad you asked.  Someone is spreading the word that this is how to plant.  It's a disaster if you do this.  DON'T.  You think you are doing everything right, but it is wrong.  There's physics and biology involved and a good horticulturist knows better because of experience.  Mulch applied to the soil surface is not the same as organic matter – planting mix put in the hole.

And watch out for the auger. Especially in heavy soils, the auger can seal the sides of the hole, making an impenetrable surface to the roots. The sides need to be scrapped or scratched to make sure the seal hasn't been created. And don't go 3 feet deep. Go to the depth of the planting sleeve. When you go deeper, the refill soil will compact and the root ball sinks, burying the crown.

 From the UCR Avocado website:

http://ucavo.ucr.edu/general/answers.html#anchor1423493

Planting

The avocado is a shallow rooted tree (most of the feeder roots are in the top 6" of soil) which needs good aeration. They do well if mulched with a coarse yard mulch. Current recommendation is to put approx 1/3 cu yd per tree when planting. When applying the mulch, be sure to stay about 6-8 inches away from the trunk of the tree. They like the soil pH around 6 - 6.5.  If you can, plant your tree in a spot protected from wind and frost. Also, avocado trees typically do not do well planted in lawns so try to plant your tree in a non-lawn area.

• When should I plant my avocado tree? Avocado trees like warm ground. Ideally, they should go into the ground from March through June. If they go in during the summer there is always the risk of sun damage because the trees can't take up water very well when young.
• How big a hole should I dig? As deep as the current root ball and just as wide as the width plus a little extra so you can get your hands into the hole to plant it. Don't drop the tree into the hole, the roots don't like that, ease it into the hole. The avocado root system is very sensitive and great care should be taken not to disturb the root system when transplanting. If the tree is root bound, however, loosen up the soil around the edges and clip the roots that are going in circles.
• Should I put some gravel, crushed rock or planting mix at the bottom of the hole? No. Do not put gravel or anything else like planting media in the hole. The sooner the roots get out into the bulk soil, the better the tree will do. Planting mix creates a textural difference between the root ball and the bulk soil and causes water movement problems. Remember, there are 5 million acres of tree crops in California planted without planting mix.
• I have a heavy clay soil. Should I elevate the tree in a mound for better drainage? Yes, good idea. Make the mound 1 to 2 feet high and 3 to 5 feet around. Put down 20 pounds of gypsum spread around the base of the tree and mulch the area with 6 inches of woody mulch keeping the material about 6-8 inches away from the tree trunk.
• What do you mean by a "coarse yard mulch" and where can I get some? Redwood bark will work and maybe cocoa bean husks and shredded tree bark. Need something that is woody and about 2 inches in diameter. Coarse yard mulch is available at some garden supply centers. Be sure it is COARSE, not fine, yard mulch - and disease-free to prevent introducing diseases to your tree (like root rot). Another source of coarse mulch would be a tree trimming operation, like Asplundh or Davy. They usually have material that has been pruned from the tops of trees and doesn't contain any diseased roots. Just go through the yellow pages looking for tree services.

These directions hold true of all trees, citrus, avocado, mango..............And for a nice discussion of avocado planting and root rot, go to:

http://www.avocadosource.com/journals/avoresearch/avoresearch_02_01_2002.pdf