Inorganic (e.g., decomposed granite, gravel) and organic (e.g., compost, peat moss, bark) mulch, when applied around and beneath trees and shrubs have several beneficial properties, like reducing soil surface moisture evaporation; suppressing weed seed germination; insulating topsoil to protect shallow from heat damage; improving soil microflora, reducing the likelihood of tree and shrub damage from mowers or weeding tools, and giving the understory an attractive, consistent appearance.
Most of the nutritional benefits associated with organic mulches in wet, temperate regions of the country, are inaccessible in most desert settings. The combination of low humidity and limited soil microflora populations and activity, restrict the decomposition process required for organic mulches to improve soil structure and release essential plant nutrients into the soil. The byproducts of these decomposition processes enrich and diversifying the population of soil microbes, improve soil water penetration and water retention, and are thought to help suppress plant pathogens. In natural and built desert landscapes, these biological processes are limited or nonexistent. The failure of organic amendments to decompose in desert soils is precisely why they are not recommended as a planting backfill amendment for desert native and desert-adapted landscape trees and shrubs.
The broad popularity of inorganic mulches, in desert landscapes is a product of their durable, stability and resistance to wind, light rains, and pedestrian traffic. They are available in an assortment of colors, sizes, and textures that blend with the native desert surroundings, and once installed, require little maintenance. Rock mulches are permeable, allowing seasonal rain to penetrate to the soil surface and into the plant root zone. To a limited extent, these mulches can reduce surface soil erosion but the combination of heavy, monsoon downpours and even mildly sloping terrain can lead to extensive rock mulch and soil erosion. Where applicable, these erosion concerns can be addressed by using larger size rock mulches called riprap.
Like organic mulches, inorganics suppress evaporation of surface soil moisture and inhibit weed seed germination. While these mulches are permeable, applications of dry or quid fertilizers and weed control chemicals to trees and shrubs planted in setting with a layer of inorganic mulch, should be supplemented with follow up surface applied water to allow these products to penetrate the surface mulch in order to reach to soil beneath.
All surface applied mulches, organic and inorganic, can serve to mask adverse cultural and maintenance issues. The absence of observable soil moisture rings, associated with plugged, broken, or defective drip irrigation emitters may be hidden by mulch layers and go unnoticed and uncorrected. Tree settling, when the crown sinks below the original planting depth (2 to 4” above the soil level) could go undetected, leading to crown and trunk rots that compromise tree health, vigor, and longevity.
Recent research suggests that inorganic mulches, particularly when applied over large expanses of land, like freeway easements, street medians, and retention basins, may contribute to urban heat island conditions. Radiant energy from the sun is absorbed and accumulates, during daylight hours, in both living and non-living components of the built environment. The release of this heat back into the surrounding atmosphere contributes to the severity of the urban heat island, particularly in desert communities. One study reported that with an average ambient air temperature of 105 F, the surface temperatures of dry/dead grass, paved hiking paths, bare dirt, asphalt roadways, and decomposed granite were, 134, 135, 142, 142, and 148 F, respectfully. These extreme all returned to the ambient air temperature when these same surfaces were within the shade canopy of landscape trees.
