Answer: I really think you're overwatering. It's also possible that your plants are simply trying to adjust to the heat while they are getting their roots established. Here are some basics of watering: 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. If you ?sprinkle? plants lightly and frequently or run drip irrigation for short periods, the root ball doesn?t get moistened. Salts will build up in the top layers of soil and damage or kill your plant. Salt burn often shows up first as browning leaves. Deep watering?or leaching?prevents this by flushing the salts past the root zone. It's essential that you allow your drip system (or hose) to run long enough for water to penetrate the appropriate depth. Depending on the size emitters, soil type, etc. this might take several hours or even 10 hours. You can reduce the time you run the system by putting on extra emitters or changing to emitters with higher gallon/hour flow rates. For trees, water should soak 3 feet deep, for shrubs 2 feet, and for annuals, perennials, cacti and succulents, 1 foot. Are you sure how far the water is soaking? 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.
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.
Hopbush is not susceptible to much in the way of disease or insect problems, but you might want to take a sample back to the nursery to see if they can diagnose something in particular. Good luck!
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