It is never to late to plant, it is a question of how much time and effort you are willing to make sure the plant survives. The hotter and sunnier it gets the more time and effort is required. I would not plant from mid June through mid September.

I have the summers off, so I do quite a bit during the summer. Of course, you have to make sure the plants are full sun or shade tolerate. I believe you can plant anything you want as long as you mulch and water accordingly. I don't fertilize until it cools off.

I know this is a year old but I thought I'd add my 2 cents' since watering is always relevant in AZ!

I bought 6 mesquite trees 2 years ago in JUNE when our nursery was practically giving away the 5 gallon size. DH and I went out to the "back 40" with pickaxe, caliche bar and shovel and dug holes (probably 3-4 feet wide, half again as deep as the root ball so that I could add amended soil). I made small pyramids at the bottom of each hole and surrounded them with small rocks (I think George Brookbank said to do this - we have very clay-ey and caliche-ridden soil).

Put the root ball (after carefully pulling the roots away from each other) on the pyramid and one of us held the trunk while the other filled the hole and tamped the earth very well.

We used 2 8-foot green metal stakes to support each tree and then made basins a bit wider than the canopy. Watered very deeply (several hours at slow drip) twice a week for several weeks. Then backed off and watered deeply once a week, then once every two weeks. Watered very deeply once a week the next summer. This year, they are all over 10 foot (with the exception of one very mean-looking Chilean, who seems determined to be a wide, bushy, spiny, evil thing) and sprouting out new leaves and branches with gusto.

It's a lot of work, and you MUST be obsessive about your watering schedule, but it can be done!