With new trees and shrubs, the objective is to water enough to moisten the surrounding soil a bit, NOT just the rootball. In fact our soils will draw water out of the potting medium if they are dry.

Use the rule of 1, 2, 3. Water to a depth of 1 foot for turf and groundcovers, 2 feet for shrubs and three feet for trees. Think of it as 'water banking'. When you apply water it soaks in to a certain depth. Then evaporation from the soil surface pulls the moisture back up to the soil surface. The longer you can keep the moisture in the root zone, the better off your plants will be and you won't need to water as often.

Usually the length of time you irrigate should be set to about 2 -3 hours because most drip systems are set up with one gallon emitters. Having multiple emitters for your trees will help deliver a larger quantity of water (assuming they are on the same valve as your other plants). One gallon of water will moisten about 1 cubic foot of soil - this is about minimum for 1 gallon size plants.

For your new plants -
Weeks 1 & 2 - Water every 1 - 2 days in summer
Weeks 3 & 4 - Water every 3 - 4 days in summer
Weeks 5 & 6 - Water every 5 - 6 days in summer
You will probably need to stick to this last schedule throughout summer for your plant selections.
When temperatures cool in the fall (October), extend the interval between waterings. By December you should only need to water once every two weeks.

Hand watering

One of the common mistakes made by gardeners new to the desert is standing over their plants with a hose, spraying leaves until they drip. Unfortunately, its the roots that take up the water, not the leaves. If you are hand-watering, the best thing you can do with that hose is lay it down near the plant not right next to the stem/ trunk, but out near the tips of the roots. In trees, at least, that means near the ends of the branches. You want the plant to form a wide and deep root system. Shallow roots collect too much heat from the surface (which is one reason mulch is so important, but thats another FAQ!) and can contribute to trees blowing over in monsoon storms.

When you lay the hose down, slow the flow to a trickle and let it soak in over a long period of time, following the guidelines mentioned above. Water gushing out will just run off instead of soaking in.

You may want to pick up a copy of Landscape Watering By The Numbers. It's free and available at many nurseries or by calling your local water conservation office.

Happy Gardening!

Thanks to AZTREELVR and Pagancat for their expert advice!