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Sunday, June 22, 2014

Wood Mulches for Desert Soils are Remarkable

Desert soil typical of the Las Vegas valley with caliche
If you don't think wood mulch is important for tree growth in desert soils then you need to study this picture of fruit trees in their second year of growth. These are bareroot fruit trees, supplied all the same size, from Dave Wilson Nursery. Fruit and nut trees include plum, pluot, almond, peach and apple. All planted in the same manner in a typical desert soil in Las Vegas. The only difference was the application of wood mulch to the surface of the soil at the time of planting. Not bark mulch, wood mulch that was diverted from landfills and used as a mulch instead and supplied by a tree service company, First Choice Tree Service.

This is their second year of growth. The soil is classified as a sandy loam but hard as concrete because it lacked soil organic matter. The pH of the soil was right around 8. The salinity of the soil was over 40 mmhos or dS/m. Boron levels varied but about 8 ppm on an average. This soil was about as bad as any soil can get and still grow something. This is very typical of raw desert soils in the Las Vegas Valley.
Wood mulch in the orchard and improved soils

How it was done

Irrigation was installed using 2 gpm bubblers. Irrigation was bubbler and basin, with a six foot basin surrounding each tree. Holes were dug with picks and shovels four feet in diameter and deep enough only to accomodate the roots of the bareroot trees at planting time. All rocks larger than a baseball were removed and the soil taken from the hole was amended with an equal amount of dairy compost. Before planting all holes were leached to remove some salts with about 20 inches of water applied to each basin.

Trees were planted in late January into the basins and watered in as they were backfilled with amended soil. A starter fertilizer, about three pounds of 16-20-0, was mixed with the backfill at the time of planting. All trees were watered thoroughly as they were being planted to remove air pockets. Trees were staked and rabbit protection was provided to the trunks as either paper guards or wire screening. A basin was constructed around each tree and included the 2 gpm bubbler. Basins were about four feet wide.
Soil under mulch after three years of tree growth and irrigations

Finally a coarse wood mulch, coming from freshly chipped landscape trees in the valley, was applied to a depth of four inches in the basin of half of the trees. Orchard aisles were also covered in wood mulch between trees that were mulched. The trees grew for one complete season with and without wood mulch. This picture was taken two months into the growing season of the second year.


  1. I have picked up wood mulch to use around my fruit trees, in the past, but have been afraid that it's a fire hazard in my suburban "postage-stamp" yard. How far should it be kept from the house? Any additional thoughts on this? Thanks....

  2. You do make a good point. It is possible, but unlikely, that a cigarette thrown into the mulch will make it catch fire. This is a problem with wood mulches used at the resorts in Las Vegas as well. Ammonium sulfate (21-0-0) dissolved into water and sprayed on the mulch will increase its fire retardant properties and encourage its decomposition, aiding the soil beneath it.I will post the document in the blog next week. Please watch for it.

    1. Thanks--I look forward to it.

  3. I applaud this continuing research.

    The LV MG Orchard is one of the SW USA's garden wonders. I still remember the chill I received watching Tom Spellman, of Dave Wilson Nursery, on a 2 part video talking about the exposed caliche layer in the desert around Las Vegas. Seeing the stacked inches thick layer of caliche (from where the MG's bulldozed it up and piled it out side their soon to be orchard). Then the camera swung around and we see the magnificent miracle orchard in the desert. Wow!

    In fairness, you can't do this without water. But watching and researching their original tilling in manures, and their heavy use of wood chip mulch producing a verdant fruit tree orchard and incredible soil in a short time. They quickly showed if you have a source of water you can quickly rehab really bad desert with good practices, planning and maintenance.

    Sadly the two part video is now unavailable due to copyright issues with the opening and closing music.

  4. SP–11–04 The Combustibility of Landscape Mulches
    Nevada study

    The question on fire is an excellent one. The wood chip mulch doesn't appear to be very flammable, but it is still flammable. And one might expect spontaneous ignition stories if it were likely from decomposing piles of wood chips that get very hot on the interior.

    I have seen government or insurance recommendations of five to fifteen feet the distance to keep combustible mulch away from the home ("defensible space"). Distance depends on area home is in. The more decomposed wood chips are the more water they will hold and the less likely to burn.