I'm sorry your having trouble with your trees. Let's start with their growing zones. USDA hardiness zone numbers are based on average annual minimum temperatures, i.e., how cold tolerant or "hardy" will any plant be. Monrovia categorizes those trees as adapted to zones 3 to 7. Las Vegas is in zone 8.
There is also a heat index, based upon number of days over 86 degrees. At that point, plants begin to experience stress and damage to their cellular proteins. Las Vegas is in zone 9 (121-150 days) or 10 (151-180 days). That tree is rated for zone 7 (60-90 days) so your hot weather is also a factor stressing these trees.
For future reference, don't buy trees with damage to the trunk and if installers damage it, then don't accept the tree, even at a discount. Large bark wounds allow entry for insects and disease. However, I don't agree with treating for insect control if there is no sign of pests. Nor should anything be applied to the wound to "seal" it. Trees have their own healing mechanisms, and applying something interferes with it, and more often seals in bacteria.
As for watering, about 90 percent of the plant problems we see are because of ineffective watering, either over or under. I'm going to include some details on that below.
However, I am curious as to why the trees were left in their pots. The root systems will never be able to expand out into surrounding soil to obtain nutrients and water, and provide a strong anchoring system for the trees. Contrary to popular belief, most roots are not vertical taproots. The majority of a tree's root system spreads horizontally in the top 3 feet of soil, reaching up to 4 times the diameter of the above-ground canopy. Only highly invasive plants are planted within a container with an open bottom, such as running bamboo or mint. Over time, these trees will not be able to thrive.
Without more info on how you watered, I can't say much about that. Here is info on appropriate water technique. Running drip frequently but for short time periods, even several times a day, is not effective because the root ball doesn?t get moistened. For example, an emitter that puts out one gallon per hour would only put a quart of water on the ground in 15 minutes. For mature trees, water should soak 3 feet deep; for newly planted trees, about 2 to 2.5, depending on the size/depth of the rootball when it was planted.
Use a soil probe (any long, pointed piece of metal or wood to poke into the soil) to check how far water has penetrated. The probe moves easily through moist soil, but stops when it hits hard dry soil. There are numerous variables involved for watering schedules, such as type of soil, how fast or slow it drains, sun and wind exposure at your site, temperature, age and condition of the plants and much more. Use the information above to determine how moist the soil is before automatically applying more water. It's essential that you allow your drip system to run long enough for water to penetrate the appropriate depth. Depending on the size emitters, soil type, etc. this might take several or many hours.
Desert soil and water both contain salts, which can accumulate in the root zone over time. This salt buildup forms where the water stops penetrating. Short periods of watering cause salts to build up in the top layers of soil and damage or kill your plant. Salt burn shows up as yellowing. Deep watering?or leaching?prevents this by flushing the salts past the root zone. Always water slowly, deeply and as infrequently as possible.
As a tree grows, its new roots tips, where nutrients are being absorbed, spread out laterally. Expand your watering zone out PAST the tree's canopy as it grows. As the tree grows, continue expanding that water zone. If you have an irrigation system, you need to move the emitters out. If you use a hose, just drag it out further. In any case, water slowly and deeply to ensure water penetration and to leach salts below the root zone.
I hope this info helps.
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