Answer: Hibiscus are sold here, but they are not really well-adapted. They are grown in perfect conditions and then transported to a nursery and then to our landscapes, so the changes are a shock to the plant. 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.) Soil should be kept moderately moist (but not wet). Finally, transplant shock can contribute to yellowing. If new growth shows up as green, that might be the problem. Try to isolate each of these possibilities one at a time to determine the problem. Finally, ineffective watering is the number one cause of plant problems we see in the low desert. I've included some info on watering below. I hope it helps!
Salt Burn and Watering
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 alot with drip irrigation because it doesn't supply sufficient water for deep watering. For example, an 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. For trees, water should soak 3 feet deep, as this is where most of the roots are located. 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. Use a soil probe (any long, pointed piece of metal, such as sharpened rebar 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. When the water has penetrated past 3 feet deep, you can stop the irrigation. Time the system so you'll know how long to run it. 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.
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 edge, or dripline, 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.
With any plant in the desert, your goal is to water slowly, deeply and as infrequently as possible for that plant's needs. Leaching is the only effective treatment to prevent salt burn. Finally, note that similar symptoms to salt burn will appear with overfertilization.
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