The desert is not like many other places that have a decent amount of rainfall per year and can go over 100 days without seeing a drop. It is necessary to provide some water to your plants and can depend on several factors such as temperature and your soil conditions. Know your soil and if you have to water more frequently due to more rocky or sandy conditions. If there is not a lot of sediment or soil, the water will drain through and not give the plant enough time to absorb the water it needs. On the contrary, if there is too much sediment, clay, caliche, then the soil has to be amended by using gypsum to break up that hard soil for better water drainage and less root restriction. If your plant looks droopy or drying up, extra water might be necessary. Keep in mind that all plants need oxygen as much as it needs water. You can drown your plants and kill them just as easy as you can let them dry up.
Bushes/Shrubs/Vines/Ground Covers/Roses - Daily in the Summer, much less in the winter, a gallon or so (depending on soil conditions)
Trees/Citrus/Palms -Once a week in the Summer, 25-35 gallons, very slow rate of water flow
Cactus/Agave/Aloe - Once a week in the Summer, a good soaking, 1-4 gallons depending on cactus size.
Do not fertilize when you first plant into the ground, this could burn the roots and possibly send the plant into shock. Fertilizing is not necessary for trees as we live in the desert and there is a lot of heat for them to grow. If you would like to promote a faster growth at the beginning, it is ok to fertilize for the first couple years. Citrus is the exception and fertilizing should be done 3 times per year starting in mid February. A good way to remember the times of the year to fertilize are Valentines Day, Mother's Day, and Halloween.