Organic and Inorganic Mulches in
Desert Landscapes
Landscape horticulture in the desert southwest is unique in far too many ways to possibly list in a single newsletter. One example is our approach to the application of surface mulches in the landscape, primarily the widespread use of inorganic or rock mulches.
Mulching, especially around trees, has several beneficial properties: reduces moisture evaporation from the soil surface; helps suppress weed seed germination; insulating soil; improving soil biology (microbiology); reduces the likelihood of trees being damaged by mowers or weeding tools; gives the understory an attractive, uniform appearance; to name a few.
Most of the associated benefits of organic mulches, enjoyed in less arid regions of the country, are almost entirely absent in desert conditions. The combination of low humidity and low levels of soil microbial activity severely restricts the decomposition process that allows organic mulch to release nutrients into the soil and build soil texture. This decomposition process has been associated with enriching and diversifying the population of soil microbes and is thought to help suppress plant pathogens. In typical desert landscape designs these biological processes are not highly active. It’s important to add that the failure of organic amendments to decompose in our desert soil is precisely why they are NOT recommended as a planting backfill amendment for trees.
Inorganic mulches are popular in the desert as they are highly durable, not easily dislodged by wind and typical landscape traffic, are available in colors and textures that blend with our native surroundings and, once applied, require little maintenance. Rock mulches are highly permeable, allowing seasonal rain to penetrate to the soil surface. To a limited extent, these mulches can reduce erosion but heavy, monsoon downpours on mildly sloped terrain can erode rock mulch and soil. Where applicable, these erosion conditions can be reduced by using larger size rock mulches that are often called rip-rap. Like organic mulches, inorganics suppress surface evaporation of soil moisture and weed seed germination. Care should be exercised to make sure that liquid fertilizers and weed control chemicals are applied with sufficient water to allow them to penetrate the surface layer of mulch.
Appreciate that all surface applied mulches, organic and inorganic, can serve to mask actual soil surface conditions. This may be cause for concern as moisture rings associated with drip irrigation application may be obscured and plugged, broken or leaking emitters can go unnoticed and uncorrected. Similarly, tree settling, with the crown moving below the original planting depth (crown level should be at or ideally slightly above the natural soil level) could go undetected, leading to serious consequences to tree health, vigor and longevity.