Answer: Yellow leaves can be caused by many things including lack of nitrogen, insufficient light, water-logged soil (plant roots need oxygen to thrive), dry soil, or iron deficiency. If the older bottom leaves are yellow, but new growth is green, it's usually a lack of nitrogen. If new leaves are yellow, with green veins, it's usually a lack of iron. Lack of nitrogen is a more common problem than lack of iron. However, here in the low desert, iron chlorosis if fairly common at certain times of the year for landscape plants. (It wasn't clear to me if your plants were in the ground or in containers.) Iron is present in the soil but when soil is wet (during rainy seasons) it is often unavailable for uptake by plant roots. If your plants are in containers they will need regular fertilizing as the roots will quickly use up what is available and they can't expand into surrounding soil to seek more.
I suggest you examine the leaves using the above info to determine if it is a nitrogen or iron deficiency. (It could be different for the different plants.) Next, make sure you are watering properly. The majority of plant problems in the low desert are due to inefficient watering. If your plants are in the ground, water should be reaching to a depth of 2 feet for shrubs. 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. Allow water to soak slowly and deeply through the root zone. Don't "sprinkle" water or let drip emitters run for just a short time. Often times with drip systems, they are not timed to run long enough to provide a slow, deep watering. When leaves brown around the edges, the problem is often salt burn. This is common in our area with low rainfall, alkaline soil and water high in salts. Browning usually occurs on the old leaves first. This excess salt accumulates in the leaf edges, where it kills the tissue and the leaf dries out and turns brown. It's important to water deeply, slowly and usually infrequently. At least once a month, water deeply enough to "leach" or push salts well below the root zone. Frequent, light "sprinklings" or short time spans with drip irrigation, allow salts to accumulate in the top layers of soil, where the roots are, which is bad news.
Similar symptoms occur when too much fertilizer has been applied. It's generally not advisable to fertilize landscape plants in our summer heat. Finally, although the nurseries sell them, hibiscus is not well-suited to the desert environment. It is usually a constant struggle to keep them healthy as they are poorly adapted to our conditions.
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