Mulch, the covering you put over the soil in your garden, has many advantages in our hot desert climate — it protects the soil from being swept away by wind and rain, reduces water runoff, cools soil and roots down, and keeps moisture in the soil by reducing water evaporation. If applied 2-4 inches thick, it can also reduce weeds. Some common types of mulch include bark chips, straw, gravel, shredded paper, and even sawdust.
Mulch falls into two categories: organic and inorganic. Organic mulch is anything that has been alive before — bark, wood shavings, leaf litter, straw, pecan shells, or paper. Inorganic mulch is essentially rock, usually in the form of some type of gravel; crushed rocks or decomposed granite are the most common.
Artificial materials, such as plastic and landscape fabric, are also available. Plastic mulches are used in agriculture to heat up soil and lengthen the growing season. Plastic and fabric are not recommended in our climate, as they break down quickly in our sunlight and do not offer any advantages to our plants.
In Tucson, many people choose gravel to cover the soil in their gardens. However, gravel doesn’t keep soil cool and moist in our hot climate as well as organic mulches do — in fact, it tends to reflect heat back up into your yard and onto your plants. Another advantage of organic mulches is they decompose and enrich the soil underneath with nutrients. Some may think this is a disadvantage, since organic mulches need to be replaced more frequently, but gravel mulch also needs replacing because it tends to get pressed down into the soil. Organic mulches are friendlier to wildlife, particularly ground-burrowing solitary bees. However, even with gravel mulch you can still have a pollinator-friendly yard by leaving some spots of bare ground for them.