Mulch is anything placed on the surface of the soil to keep soil temperatures moderate, decrease evaporation of soil water, improve the appearance of a garden or landscape, and decrease the population of weeds. It is generally broken down into three types:
- Living (rarely used)
Organic mulch is composed of plant materials that have died and dried. These materials include dried leaves, chipped bark or wood, straw or hay, pine needles, cobs, nutshells, even shredded paper. See figures J (straw) & L (chipped bark). They can hold water, keeping plants moist during times of low rainfall. Over time, it decomposes, becoming a dilute source plant nutrients. Because it tends to slow down water loss and degrade, thus enriching soil, it is best for plants that are not desert adapted. Such plants as fruit trees, roses, many landscape shrubs and vegetables benefit from organic mulch.
Plants that are desert natives or desert adapted benefit most from some inorganic mulches. Desert pavement is the natural layer of rocks and gravel found in undisturbed areas. In the desert southwest, rock in various colors is a frequently used mulch, meant to imitate desert pavement. See figures D & E (small brown or red rock). Small rocks are easy to walk on and serve the same function as other mulches. They are often mixed with somewhat larger rocks (figure b) for visual interest. Large rocks, as in figure C, F, & H are usually meant to prevent people from walking on a surface. Rounded river rocks are usually not for mulch, but in a dry creek bed. See figures G & I.
Although rock mulch is commonly used, it can stress plants that are not native or desert adapted. Light & heat reflect off the surface of the rock, making the air around plants hotter.