Answer: Unfortunately there isn't an easy answer to this question, but understanding effective watering is the most important thing anyone can do to grow healthy plants. How often and how much to water depends on many factors including plant type, age, size, soil conditions, weather conditions and the specific microclimate where the plant grows. And, the type of watering system (drip, hose, bubblers) is also a factor. Without more detail, I can't give you a specific answer, but I'll provide some basic details below that will help you understand how to water any plants you might have.
Desert soil and water both contain lots of salts, which can accumulate in the root zone over time. Salt burn shows up as yellowing and browning along leaf edges, and leaf drop. Deep watering?or leaching?prevents this by flushing the salts past the root zone. Here's what's happening. Salts dissolve in water. Salt buildup forms where the water stops penetrating. For example, if over time water soaks 6 inches deep, the salts will be deposited where the water stops at the 6-inch mark. If you water plants lightly and frequently, salts will build up in the top layers of soil and damage or kill your plant over time. We see this happen a lot with drip irrigation because it doesn't supply sufficient water for deep watering. For example, a drip emitter that puts out one gallon per hour would only put a quart of water on the ground in 15 minutes. Think about dumping a Big Gulp on a tree, and you can visualize how ineffective this would be. Always water slowly, deeply and as infrequently as possible.
Roots also need oxygen to survive and soil that is continually wet doesn't provide it. Use a soil probe (any long, pointed piece of metal 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. For mature trees, water should reach 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 the information above to determine how moist the soil is before automatically applying more water. It's essential that you allow your drip system (or hose or bubblers) to run long enough for water to penetrate the appropriate depth. Depending on the size and number of emitters, soil type, etc. this might take several hours or 10 hours or many more. You can reduce the time you run the system by putting on extra emitters or changing to emitters with higher gallon/hour flow rates.
As a tree grows, its new roots tips, where nutrients are being absorbed, spread out laterally. If you are watering only within a few-foot area at the base of the tree, it's not really being watered effectively. Expand your watering zone out PAST the tree's canopy. 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.
Use the 1-2-3 Rule as an easy method to figure out how much water to apply. Small plants with shallow root systems, such as perennials, veggies, herbs, cacti, succulents have roots that reach about 1 foot deep, so water needs to penetrate that far. When the top 1 inch of soil dries out, it's usually time to water again. Shrubs (like your lilacs) have root systems that are 2 feet deep so water needs to soak 2 feet deep. When the top 2 inches of soil dries out, it's time to water. Trees are 3 feet, etc. As plants establish root systems, the time between waterings can be lengthened, but it is always essential to water to the same depth. So you are applying the same amount of water with each irrigation regardless of the time of year, but the frequency changes. As warm weather arrives, you need to water more frequently than during winter. For veggies and small plants, it may be necessary to water daily.
You didn't say how old your tree was or when it was planted, but here are some watering guidelines for establishing desert-adapted plants from Desert Landscaping for Beginners, published by Arizona Master Gardener Press. Weeks Since Planting 1-2, water every 1-2 days; Weeks 3-4, water every 3-4 days; weeks 5-6, water every 4-6 days; weeks 7-8, water every 7 days. Gradually extend the watering as plants establish. Desert adapted trees like mesquite that are already established take watering every 7-21 days in summer (May-Oct); 14-30 days in fall (Oct-Dec); 30-60 days in winter (Dec-Mar); 14-30 days in spring (Mar-May).Note these are guidelines, which will vary depending on your soil type, microclimate, etc.
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