The best time to plant shrubs and trees in desert areas with hot summers is fall, winter, and spring. Shrubs and trees planted in the summer tend to have low survival rates.
When choosing trees, it is worth knowing 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.
Digging a hole in dry, hard desert soil is best done with an electric jackhammer which is easily purchased online. If a jackhammer is not available, dig a hole, several inches deep, of the proper width using any tool available, then pour in water. Wait until the water is absorbed, then dig out the mud and add more water. Repeat until the hole is the proper depth. Digging by hand is best done after a long rain when the ground is soft.
Caliche or caliche clay is a compacted layer of soil and accumulated salts found in valleys that is nearly impenetrable. A jackhammer is recommended for digging into caliche, but it can be accomplished after a heavy rain with a pickaxe and lots of patience. When you find caliche underneath your yard, it must be dug out underneath every hole. Removing all caliche in an entire back yard is the best choice, but very time consuming.
Finished holes should be one to two inches shallower than the root ball and three times as wide. This allows roots an opportunity to grow before they run into rock-hard dirt. A wide hole also allows more rainwater to accumulate around the roots in our hot, dry climate and may reduce the need for frequent watering. The sides of the hole should be jagged or form a square, rather than being smooth and round. Holes that have smooth, round, hard dirt sides may encourage roots to circle around and not push outward.
The most important aspect of digging a hole for a tree, or any plant, is drainage. All holes must be dug deep to remove underlying caliche. After the hole is dug, and refilled to a level needed by the rootball, fill it with water to check how rapidly it drains. One-half to one hour is an acceptable time for a hole to completely drain. If it takes longer than one hour, dig another hole somewhere else.
Mountain sites may have an underlying granite layer, rather than caliche, preventing drainage. Granite layers are too deep to dig through, so a location with deeper soil, or a slope must be selected instead. Never plant a tree in a flat area where underlying granite is close to the surface. Any hole you dig will seem to drain properly the first time you fill it, but the second time, the water will not drain at all. The surrounding dry soil absorbs the first amount of water, becomes water logged clay, and can absorb no more. Root rot follows.
The crown of the plants's root, and in some cases the root flare below the crown, must be one to two inches above the surrounding soil, and the root ball must not settle so that the root crown, or root flare, is below the soil line. "Plant it high, it won't die. Plant it low, it won't grow."
Stake newly-planted trees to minimize trunk movement. A slight sway is necessary to develop a strong trunk and roots. After one year, remove the stakes.
To determine how far apart shrubs and trees should be planted, look up the maximum width a given plant species is likely to attain. The distance any two plants should be kept apart equals the sum of the maximum widths of the two plants divided by two. For example, a 'Desert Museum' Palo Verde can grow to 20 feet wide, and a 'Panachée' Fig to 12 feet. Twenty plus 12 divided by two equals 16 feet separation. However, in a residential setting, this can be reduced by one-third, to 11 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 if desired.
As a firewise precaution, the closest tip of any tree branch should be at least 30 feet from a 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 a house. All tree trunks should be at least 40 to 60 feet away from any structure.
During the warm season, deep-soak established ornamental trees once a month to keep them healthy. Many insect and disease problems are caused by insufficient water. Trees in place less than three years, and fruit trees, may need weekly watering depending upon the species.
A few plants have shallow root systems that do not respond to deep watering. These will need more frequent irrigation than plants with deep roots. Some tropical fruit plants such as banana and guava have this characteristic and respond well to bubbler irrigation.
Keep track of soaking rains on your watering calendar. You can omit watering within a week 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. 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 organic mulch for newly-planted shrubs and trees to conserve moisture. Keep the mulch 8-12" away from the trunk.
To deep water a shrub or small tree, allow a hose to run water slowly between the trunk and drip line for 1-6 hours depending on the size of the plant. The hose may need to be moved a couple of times to ensure coverage of the entire root area. Irrigation should happen every week from mid-spring until harvest for plants producing fruit. Deep watering results in water penetrating far under the plant. As the soil on top dries, the plant's roots will follow the moisture deep into the ground, protecting themselves from the heat of the sun at the surface, and resulting in a larger root volume to better survive low water conditions.
Basin irrigation involves building a circular dirt dike just outside the drip line, and flooding the area with water, deep soaking the soil to an appropriate depth. Many fruit trees will need soaking in this manner once a week while they are fruiting. There are some trees, however, that are intolerant of flooding, and the amount of water used should drain quickly. This method is not used for trees which cannot tolerate water on their trunk or graft union.
Double ring basin irrigation involves (1) building a circular dirt dike one or two feet outside the drip line; (2) building a second dirt dike circle inside the first, one or two feet away from the trunk; and (3) flooding the area between the dikes with water, deep soaking the soil to a depth appropriate to the species of fruit tree. The inner dike avoids getting water on the graft union and causing disease. Most fruit trees will need soaking in this manner once a week while they are fruiting. There are some fruit trees, however, that are intolerant of flooding, and the amount of water used should drain quickly.
Bubbler Irrigation is a system of pipes installed underground that uses above-ground bubblers to surround a tree in a circle or square and provide water on a timed basis. This is often used in commercial tree groves. The bubblers circle the tree at the drip line, not at the trunk. It is necessary to encourage roots to grow away from the trunk for wind stability. The bubblers are placed under mulch, such as straw, to shield the plant's roots from the sun and high summer temperatures, and to reduce evaporation. The bubblers are on a daily timer so the amount of water the plant receives can be regulated, with less water at certain seasons of the year.
There is a way to provide extra water to trees over an extended period without irrigation. Create 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 slowly into the ground, providing water to nearby roots for weeks after a good rain.