It's like trying to explain why babies cry. Many things may be wrong, but knowing what the causes are and what the solutions and options available are complex. The current plant theory is that plant health starts with healthy roots. A strong, large root system provides the nutrients and moisture that allows vigorous growth. So what are some of the reasons for poor roots?
Soil Contaminants: Poor roots may result from soil contaminants (like the painter's brush cleaner, or the neighbor's dog), or perhaps too much compost or improperly cured compost which actually takes nutrients’ away from the soil that the new plant desperately needs for new
growth. New plant roots may also be compromised if planted under trees belonging to the legume family (Fabaceae). In the desert that includes Mesquite, Palo Verde and Acacia, whose roots actually extend well beyond the leaf canopy and can steal life giving support from plants near them.
Over Watering: The long term effect of over watering is sick roots. Water is the controlling factor in plant growth but with desert adapted plants too much of it will weaken the plant. The longer or more often periods of over watering occur, the weaker some plants become. However, over-watering itself is usually not the actual cause of death. The actual cause of death, which occurs later, is often a secondary invader - Insect or disease - which follow the weakness brought on by water stress, and inability to fight the invader.
Wrong Choice of 
Plants: poor genes in plants not suited to their ecosystem may cause poor growth. The bottom line is: the right plant in the right place will be happier and healthier. Last of all is
Barren Soil. Many areas in our desert environment simply lack the ability to support plant life no matter what. The solution to this problem can be extensive given our rocky conditions and the fact that one can only guess as to how much soil will need to be replaced without an extensive soil analysis of all the areas where your trying to grow your ornamental garden.
Other causes of plant failure include:
The summer Heat. Our hot desert climate is extremely brutal on plant life. Even plants that have adaptations to the hot desert sometime cannot take the heat any longer. Did you know that even a Cactus plant can get sunburn? Plants that are planted near pavement and walls need to be very hardy and adapted to the conditions because heat will reflect from the surface directly on top of them and fry them like an egg. Citrus trees are especially sensitive to Sun and Heat. Take special care to cover them up the first summer they are planted in your garden.
The winter Cold: Frost in the desert is common and unless the plants are protected they run the risk of frost bite. Plants will dye from frost because water in them will actually be sucked out from inside them. Would you ever think that plants
could suffer from drought even when water is readily available? Believe it because it’s true and this death of a plant can happen overnight. This past year we had uncommonly cold temperatures, dipping down to the low 20’s and even into teens in some places, when you tie that with the extremely hot and dry months of May, June, and July, many plants didn’t stand a chance.
How to Plant for Success: One last thing that will help keep your plants form dying is to let you know the proper way to install a plant in the ground. The planting hole should be twice the width of the plant container and no deeper than the top of the taper of the trunk. In fact, planting your plant level, or slightly higher with the final grade is optimal and will help keep the base of the plant from being soaked from over watering. If necessary the hole can be broader to permit safe handling and planting without injury or breakage of the root ball or root system. Planting containers should be removed in such a manner that the plant root is not injured. Carefully cut them off if you need to. The back fill around the plant should be a combination of composted nitrogen stabilized wood shavings (60% minimum Fir
bark, 40% Douglas fir bark) containing >/=1% nitrogen. I like to use Miracle Gro garden soil because it has even more nutrients included to help plants get a good start. If care is not taken to insure a healthy start your plants will probably die.
After a couple of weeks go by, start to feed your new plants. For all plants I recommend Miracle Gro with the hose end feeder. For many of our clients who don’t have the time, or desire to garden we recommend letting us install an in line fertilizer system that injects a small amount of nutrients every time the irrigation system operates. Pricing for this system is around $500.00 installed. Good Luck and happy planting!