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.
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 three times 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 after the third time it is filled, 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 or third 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.
Finished holes should be two times as wide as the root ball. This allows roots an opportunity to expand before running into compacted soil.
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." The soil should slope gently away from the trunk to the drip line. Water should never accumulate at the trunk.
Do not amend the soil for any plant except tropical fruit trees and shrubs. Plants need to accommodate to native soil, which their roots will encounter when they reach beyond the planting hole.
Stake newly-planted trees to minimize trunk movement. A slight sway is necessary to develop a strong trunk and roots. The stakes must not be next to the trunk. Use two stakes, on opposite sides, two or three feet from the trunk. Use broad straps to fasten the trunk to each stake. After one year, remove the stakes.
The distance any plant should be kept away from a wall or fence equals one-half of the mature width of the plant plus one foot. For example, a given variety of orange tree may grow 20' wide. It must be kept 11' from a wall. For the distance between two plants, add the mature widths, divide by two, and add one foot. When the new plant is small, this distance will seem large, but the space will fill quickly in just a few years.
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.
Deep water established trees to a depth of 3' at least 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, will need more frequent watering depending upon the temperature, time in ground, and species.
A few plants have shallow root systems and do not benefit from deep watering. They will need more frequent irrigation than plants that can grow deep roots. Barbados Cherry and Guava fall into this category, needing daily 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 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 ensure that the water has reached 3' deep.
Use 6" or more of organic mulch, such as straw, for newly-planted shrubs and trees to conserve moisture. Keep the mulch 8-12" away from the trunk to avoid trapping moisture on the bark. Moist bark can develop fungal, bacterial and viral diseases.
To deep water a shrub or 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 few weeks from mid-spring until harvest for fruit plants. 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 over several hours, deep soaking the soil to an appropriate depth. The difference between this method and deep watering is that it allows wider distribution of water over the roots. 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 for several hours, 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. This method can better distribute water at the drip line and beyond where tree roots must be enticed for wind stability.
Bubbler irrigation is similar to drip irrigation except more water is supplied at one time with bubblers. It consists of 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.