Answer: 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. To determine if they have received sufficient water, 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. Track how much water you apply, or how long your system runs and then you will know how much you need to reach the appropriate depth. 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. If you use drip, 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. The same with a hose trickle.
A combination of wind, inefficient watering and high temperatures before the plant has a chance to establish a strong root system will combine for "wind throw" as it is known, so start watering your sissoo deeply as described here.
Use the 1-2-3 Rule as an easy method to remember 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 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 deep. 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.
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, browning along leaf edges, and leaf drop. Deep watering?or leaching?prevents this by flushing the salts past the root zone. Always water slowly, deeply and as infrequently as possible.
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. Note these are guidelines, which will vary depending on your soil type, microclimate, etc. and they apply to desert-adapted plants. Desert adapted trees 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).
If your sissoo is in a lawn situation, that is problematic because the lawns get watered so frequently for short periods of time, which encourages tree roots to stay at the surface. Deep water on schedule and the situation should improve. Good luck!
Q&A Library Searching Tips