The best time to plant trees in desert areas with hot summers is fall, winter, and spring. Trees planted in the summer tend to have lower survival rates.
When choosing trees, it is important to know that the cute four-foot sapling you purchased will grow to the size of a mature tree over time, although not necessarily to the size of specimens in irrigated city parks. Look around your town, and you can find tree monsters that have overtaken houses and yards.
When digging holes for trees, you must learn if the species of tree you are planting grows a taproot. If it does not, dig the hole just deep enough so that the crown of the root ball is one to two inches above the surrounding soil and 2-3 times as wide as the root ball. A wider hole will hold more of our sparse rainwater and may reduce irrigation frequency. Refill the hole with unamended native soil unless it needs a soil type very different than our neutral to alkaline desert soil.
If the tree does have a taproot, the hole must be dug deep to remove decomposing granite and caliche that may thwart deep roots. At the same time, the root ball must not settle so that the root crown is below the soil line. Water thoroughly.
Fill your newly dug hole with water to make sure it drains well. If it does not, a hole may have to be dug in another location.
Check stakes on newly-planted trees for trunk movement. A slight sway is necessary to develop a strong trunk and roots. After one year, remove the stakes. Some trees will not need stakes at all.
To determine how far apart trees should be planted, look up the maximum width a given tree species is likely to attain. The distance any two trees should be kept apart equals the sum of the maximum widths of the two trees divided by two. For example, a 'Desert Museum' Palo Verde can grow to 20 feet wide, and a Western Redbud to 18 feet. Twenty plus 18 divided by two equals 19 feet separation. However, in a residential setting, this can be reduced by one-third, to 12 feet, but more frequent watering will be needed. When the saplings are small, this distance will seem too far. Plant small shrubs which can be later removed, not trees, to fill in the space.
As a firewise precaution, the closest tip of any tree branch should be at least 30 feet from your house. Add the maximum width the tree can grow, divided by two, to 30 feet. That is the distance the tree trunk should be planted from your house. All tree trunks should be at least 40 to 60 feet away from any structure.
During the warm season, deep-soak established trees once a month to keep them healthy. Many insect and disease problems are caused by insufficient water. Trees in place less than two years may need weekly watering depending upon the species. Keep track of soaking rains on your calendar. You can omit watering within two weeks of a soaking rain that lasts many hours. On sloping ground, however, many heavy rains are over too quickly to soak in, and will not deeply water the soil.
Water landscape trees out around the drip line, not at the trunk. This will encourage roots to grow out and better support the trunk against strong winds. Also, water will soften up the ground and allow the roots to penetrate more easily. One way to water trees is with a soaker hose placed around the tree at the drip line. But a tree with a 14' diameter will require a 50' long soaker hose. Another way to deep water is basin irrigation. In either case, run the water for one to three hours until the ground is soaked three feet down. Poke an iron rod with a handle into the ground to measure water penetration depth. Allow the soil to dry out between waterings.
Use 2" of mulch for newly-planted trees to conserve moisture. Keep the mulch one foot away from the trunk.
There are two ways to provide extra water to trees over an extended period without irrigation. The first is an open dry pond in the center of a group of trees where the "pond" is deeper than its surroundings. Rainwater will accumulate in the pond and soak into the ground, providing water to nearby roots for weeks after a good rain. The second is to dig a deep pit in the center of a group of trees, fill it with brush and unwanted plant debris, then cover it with dirt so that the top is slightly lower than surrounding soil and water can drain to it. This becomes an underground reservoir not affected by evaporation. Eventual rotting of the brush fill will cause the top to sink after several years, leaving a shallow depression where rainwater can accumulate.
Trees with invasive roots such as Acacia and Mesquite should be watered beyond the drip line after the first year. After two years in the ground, it is better not to water them. Their invasive roots will travel far beyond the drip line to find rainwater wherever it settles. Keep these trees more than 50 feet from septic systems, sewer and water pipes.
Some trees will grow slowly with little water but faster when watered weekly or every two weeks. Desert Willow grows quickly with extra water to 30' high. Arizona Rosewood does not seem to respond to extra water, but its faster-growing evergreen cousin, Vauquelinia Corymbosa, may. When a tree is close to its desired height, taper off watering to once a month during the warm season.
Finally, trees that have established themselves in the wild probably do not need your help. Even though they may seem too crowded, or in the wrong place, let them be. They may be near a low-lying area where rainwater collects. If they seem drought stressed and are desirable trees, however, consider deep soaking in April or May to break the spring drought.