So, , , , you haven't gotten around to mulching your plants?
I read other writer's articles from time to time where they mention that it is important to mulch the desert soil. I want to tell you that I think the word important is not delivered to you in strong enough terms by the authors.
I would like to rewrite that sentence here and offer an explanation.
It is absolutely necessary to mulch plants in the desert.
You are wasting your money buying plants if you think they can do well, or in many cases even survive, if you are not mulching them immediately after planting, especially in the drying heat of summer. After you have transplanted and watered your new plants the sun is right behind you to heat-up the bare soil and pull out the water within a couple of hours. Go stand in the yard naked after a shower and tell me how long it takes you to dry-out and heat-up. Lets get real here . . . you HAVE to cover the soil out as wide as the mature canopy of the plant or tree will grow.
Mulch can be anything that covers the soil. I've seen people use carpet under the canopy of their trees, some use rocks and wood such as shredded cedar or redwood (shredded because it doesn't tend to blow away like wood chips or bark). In flower beds I use alyssum and intense planting. In veggie beds I use silver reflective plastic. By the way, it is a very bad idea to use wood or bark chips in veggies beds.
What is mulch? If you lay down on the soil you are mulch, simply because you are covering the soil. You are protecting it from drying out and heating up and protecting the roots from the same disaster. Don't lay there to long or you will become compost.
I know you think that the roots will go way down to get moisture and nutrients, but this is not usually the case in our desert gardens. Some trees will develop a tap root but, if the tree was grown in a nursery can it is unlikely it will ever grow a tap root. Many feeder roots grow in the top few inches of the soil and grow in a wide circle out from the tree. This is why it's important to keep the moisture from evaporating by mulching the soil.
Another kind of Mulch
Some bags in the nursery are labeled Composted Mulch. This stuff is meant to be dug into and mixed with native dirt to provide tilth, nutrients and microbes to the soil. Plant roots like this environment, they can move easily to get the nutrients. It's important to use this while it is fresh and new and moist to take advantage of its full potential.
Don't use wood chips/bark or shredded wood to mulch flower or veggie beds. The reason for this is that it can get mixed into the soil where it will start to decompose. This process of decomposing takes the nitrogen away from your plants and they will have great difficulty growing. They just sort of sit there, staying little, week after week, because the nitrogen they need is being used by the wood.
On veggie beds I use plastic 'silver reflective mulch' with drip irrigation underneath. In flower beds I plant lots of herbs and use a 'living mulch', like alyssum, to draw the pollinating insects I need for my fruit trees and veggies. Also, planting a variety of 'annuals' intensely (real close together) acts like a mulch as they will grow to shade the soil. This is good under the canopy of trees and it's also very pretty. I planted my petunias 6 inches apart. Confession: As I began to run out of petunias they went in 8 - 12" apart, but the newly sprouting alyssum will fill in the spaces, and it is becoming very fragrant around here. (Alyssum re-seeds itself every year and mostly comes back white.)
When you purchase composted mulch to mix with your soil, make sure there are no pieces of wood in it. If there are, it is not fully composted and it can (probably will) rob your plants of needed nitrogen.