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.
Various factors can help you decide which mulch to choose. If you live in a fire-prone area, it’s best not to use organic mulches near your house; gravel is the better choice in that instance. If you’re concerned about cost and environmental impact, consider using a mulch that is readily available in our area. Some people obtain pecan shells from the pecan orchards surrounding Tucson, or from Acme Sand & Gravel.
To get mulch for free, you can contact local tree-trimming companies to see if they have spare wood chips you can use. Romeo Tree Service and Finest Tree Service both have a form you can fill out to get free wood chip mulch delivered to you.
If you have trees on your property, you can use small branches you've trimmed off as a loose mulch around larger plants. Shredded paper can also be used as mulch, but will need to be weighed down with a heavier mulch to keep it from blowing away. Another way to make your own mulch is to use a wood chipper. You can rent these at heavy equipment rental companies such as Sunbelt Rental or Ahern for about $500 per day. You can also sign up at Tucson Toolbox and rent a wood chipper for free.
Whichever mulch you choose, get the most out of it by applying it to a depth of a few inches — at least 2 inches for inorganic mulch and 3-4 inches for organic mulch. Keep the mulch about 6 to 8 inches away from the main trunks of your trees and bushes to avoid damage and rot to the bark. Check your mulch depth every few months and replace as needed.
The costs of mulches vary, but in general it is cheaper to buy it in bulk than by the bag. Most mulches are sold in bulk by the cubic yard, which covers about 100 square feet to a depth of three inches. To figure out how many cubic yards you need, multiply the number of square feet you need to cover by the depth in inches (usually 3). Then divide that number by 324 to get the number of cubic yards you need. You can also use this handy mulch calculator.
Pros and cons of organic mulches: shredded bark, sawdust, pecan shells, shredded paper, wood chips
- Put nutrients into the soil as they degrade
- Wildlife friendly
- Keep soil cooler
- Reduce reflected heat
- Need to be replaced more frequently
- Not a good choice near the house in fire-prone areas
- Cost: Average of $30-50 per cubic yard unless you are purchasing something unique, like cedar. If you are using your own yard waste, it's free!
Pros and cons of inorganic mulches: gravel, decomposed granite, pebbles
- Last longer
- Neat, tidy look
- Safer for fire-prone areas
- Heavy for DIY gardeners
- Reflect heat back into your yard
- More expensive
- Not friendly to ground-burrowing bees
- Cost: average of $150 per cubic yard; about $30-60 per ton
Most nurseries and home improvement stores carry mulches in bags. For less expensive and more environmentally-friendly mulch, purchase in bulk from local sources like Tank's Green Stuff for organic mulch or Arizona Trucking & Materials for gravel.
For more information on mulch choices, you can read about "Using Mulch in the Landscape" from the University of Arizona Cooperative Extension.