Featured Article: Mulch Magic and Desert Soils

Mulch Magic and Desert Soils
Bob Morris, Consultant for Viragrow, Inc.

Using organic mulch, and its importance when applied to desert soils, is frequently misunderstood and overlooked. I remember 20 years ago approaching woody plant professionals in the Las Vegas Valley if they would participate in a mulching study with fruit trees. The proposal was to divert green waste from landscapes, all of it from our urban forest, away from landfills and applied to the soil surface of a newly established University Orchard.

The mulch idea gelled from discussions with Tom Spellman and Ed Laivo from Dave Wilson Nursery in California. When local professionals were approached, all but one passed on participating because of fears that insect and disease outbreaks would occur in the Orchard and spread into the community.

Dave Wilson Nursery on Mulch

The University Orchard was developed as a demonstration facility, in a partnership with Dave Wilson Nursery, focused on evaluating fruit tree variety performance under Mojave Desert conditions. All of the fruit trees grew well in their early years but there was a general consensus they might perform better if organic surface mulches were used.

Organic mulches were difficult to find in desert communities back then. When you did find wood mulch it was expensive and typically imported as a forestry byproduct. The idea of using community green waste was not popular at that time. Instead, green waste was buried in landfills.

One local professional, Tony Valenti owner of First Choice Tree Service, contacted me privately and said he was interested in participating. The first loads of chipped trees that came from Tony gave us lots of flat tires. Tony worked with us to eliminate Mesquite from the mix. We could live with the pine cones but not Mesquite thorns.

The next trees eliminated were palms. Chipped palm, particularly the petioles of the fronds, decomposed too slowly under our desert conditions. Trees containing borers, our worst insect fear, were sent through the chipper and the borers, as well as their homes, were totally destroyed.

We intentionally chipped wood containing active borers. They were demolished during the chipping process.

We have fewer plant disease problems in desert communities than plants grown in communities from wetter climates. In 20 years we have never experienced disease problems coming directly from mulch applied to the Orchard floor. However, collar rot can be a problem with young trees if wood mulch is in direct contact with their trunks. Tree collars, used to protect young trees from rabbit damage, were very effective at keeping wet mulch away from tree trunks.

Irrigation at the Orchard is either by bubbler with a basin around the tree or drip irrigation. In either case, if wet mulch was in direct contact with the immature trunk of fruit trees, collar rot, a deadly disease for young trees, can be expected. For this reason, wood mulch is kept away from the trunk of all fruit trees until they are in their fifth or sixth year. At that age, it seems there is enough maturity to the trunk to hold up against contact with occasionally wet surface mulch.

The amount of mulch applied to the soil surface has varied over the years from as little as 3 inches to as much as 6 inches or more. We have never had a problem with coarse wood mulch 6 inches deep covering the Orchard floor. In fact, I prefer to use coarse mulch applied deeply.

Orchard mulched with composted urban tree mulch, 4 to 6 inches deep.

What have we noticed when we apply wood mulch to desert soils? Wood mulches “dissolve” into desert soils at the rate of about one half inch per year. This required us to renew surface mulch about every three years. Fruit trees surrounded by wood mulches outgrew trees not mulched by at least 100% and in some cases 200%.

The top couple of inches of desert soils covered in mulch changes color from grey or light tan to a chocolate brown in a matter of one to two years. This is because of the increase in organic content as the mulches decompose. Soil texture changes. Surface soils that required digging with a pick, now could be scooped up with your hands. Earthworms appeared where none survived before.

Soil chemistry improved. Salinity was easier to manage because of improved water percolation through the soil and the flushing of soil salts. Boron, a common element in our desert soils and particularly troublesome to trees at very low levels, was bound by the increased organic content of the soil.

The need for weed control, for all intents and purposes, was eliminated. Mushrooms would appear throughout the Orchard after our infrequent rains. Some were concerned about the appearance of mushrooms was a sign of mulch decomposition and soil enrichment. We could stretch irrigation intervals an extra day during the heat of the summer because soil moisture was conserved and surface evaporation was slowed.

Mushrooms like this is a good indicator sources of organic matter are decomposing. That's a good thing!

Problems? Outside of collar rot on young trees, not very many. We noticed an abundance of what I called as a kid, “boxelder bugs”. They would on occasion attack soft fruit and become a general nuisance due to their numbers. In the same regard, but not as much of a nuisance, there were roaches near irrigation boxes in the mulch litter.

Some places have reported concern over termites. We do have termites in the Mojave desert but they never became problem for us.

Organic mulches conserve soil moisture, moderate soil temperature and reduce competition from neighboring plants. They modify the root and soil environment thus facilitating the penetration of water into the soil root zone. Organic mulches add organic matter to the soil as well as its breakdown products, humic and fulvic acids. Improvements to the soil chemistry interact favorably with plant nutrients found in the soil, lower soil alkalinity, encourage the development of earthworms and arthropods that thrive in these environments, reduce potential erosion, prevent soil compaction, along with a myriad of other benefits.

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When mulch is placed in a landscape, more feeder roots tend to develop closer to the soil surface due to an improved soil and plant environment. This is partly due to more favorable root growth conditions due to mulch decomposition and partly because of suppression of deeper roots because of higher soil water content in the underlying soil.

Inorganic mulches, such as rock mulch, provide some of the same benefits as wood mulch but do not contribute soil improvement since their breakdown products are not organic in nature. Rock mulches play a key role in reducing water losses from the soil due to the evaporation, controlling weeds, modifying soil temperatures, reducing soil erosion and reducing water runoff.

But rock mulches contribute to soil mineralization and the disappearance of soil organic matter. Even though soil amendments may have been mixed with the backfill at the time of planting, this soil organic content disappears in 3 to 4 years.

As soil organic matter disappears, the physical and chemical properties of the soil changes. Naturally occurring soil chelates, responsible for making minor elements such as iron available to the plants, disappear. Soil salts begin to accumulate because they cannot be flushed from the soil as easily. In short, non-desert plants intolerant of desert soils begin to yellow, leaves begin to scorch, plant canopies thin due to leaf drop and slower growth rates and plants are less tolerant to diseases and the harsh desert environment.

Organic mulches play a vital role in landscape plant health, particularly in mixed landscapes where desert and non-desert trees are grown together.

Bob Morris, Consultant for Viragrow, Inc.

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