Answer: I'm sorry the lilacs are not performing to expectations. The National Gardening Association answers gardening questions for Monrovia on the website, but are not involved with their toll-free number that you tried. I will try to help you with your lilac question.
Watering is the #1 problem with just about every plant problem in the arid Southwest. Once a plant has been in the ground for a couple months and established roots, watering every other day is not beneficial. Many people think because it is so hot in the desert, they must water that often, but in reality, it is better to water deeply, but as infrequently as possible. Without knowing exactly how much water is being applied with your system, I can't give a specific answer, but overwatering drowns roots, as they need oxygen to survive and constantly wet soil forces out the oxygen molecules. Or, more often, the problem is insufficient water is not penetrating the root system deeply, perhaps just getting the top of the soil wet. This is very common with drip systems, which are programmed to turn on frequently for short times when they are first set up, but then are not reprogrammed as plants mature. For example, an emitter that puts out one gallon per hour (very common) would only put a quart of water on the ground in 15 minutes. Think about taking a quart jar of water out to a plant and pouring it on the ground. It wouldn't make a dent in the plant's needs.
I can't tell you exactly how long to run your system because I don't know how much water it puts out, such as how many emitters there are and how much water each puts out. 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. A soil probe will help you determine how far water has soaked. It moves easily through wet soil but stops when it hits hard soil. I?d suggest you let your drip run for 1 hour, then wait an hour or so (for the water to continue penetrating), then use a sharp stick or pointy thing as a soil probe to determine how far the water penetrated in your soil. For most areas, it's necessary to run irrigation much longer than people would think. The majority of the plant problems we see are because drip isn't running long enough. In improved soil garden beds, such as for veggies, it will soak more readily through the soil than it will in landscape settings.
As plants mature, emitters must be moved outward to keep pace with the expanding root system. I wasn't quite sure what you meant by 4" and 12" across, but if that is your watering well, it's likely too close to the stem. Feeder roots that absorb water are spreading out past the dripline (canopy edge), so apply water just beyond that. Add a layer of organic mulch around the base to help maintain moisture and reduce soil temperatures.
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.
Finally, lilacs are not the most well-adapted plant for our arid growing conditions. This particular type is bred to do better in warm areas, but they are still marginal, in my opinion. Even in cold climates that are more favorable, "lilacs not blooming" is one of our frequent questions. So, it may take awhile for your plants to establish. I hope this info helps. Good luck!
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