Answer: There are a variety of possible explanations when non-native plants such as gardenias are involved, so I'm going to provide some basic info on desert growing conditions. Plants that are native to regions where the soil is acidic, contains alot of organic matter (i.e., is dark brown/black), nutrients and moisture often struggle when transplanted in the desert. These non-native root systems haven't adapted to obtain nutrients from our salty, alkaline soil. In addition, our intense heat and aridity is not to their liking. However, your eastern exposure is the best location for them.
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. 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.
Iron chlorosis is a common problem in the desert for non-native plants. Chlorosis is recognized by new leaves that are yellow, while the veins remain green. If the condition is severe, the entire leaf may be yellow. Although iron is present in the soil, it is not always in a form that non-native plants can use. (Native plants seldom suffer from iron chlorosis.) Overly wet soils are depleted of oxygen. (As water fills in the minute spaces between soil particles, air moves out.) Plant roots need oxygen to absorb iron in the soil. To help prevent chlorosis, always water slowly, deeply and infrequently.
Another possibility is salt burn. 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 (such as running drip irrigation daily for 15 minutes, etc.) 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. As stated earlier, always water slowly, deeply and as infrequently as possible.
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 frequently is not beneficial. Many people think because it is so hot in the desert, they must water daily or every other day, but in reality, it is better to water deeply, but as infrequently as possible. 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.
The bud drop is likely caused by the sudden fluctuations to 100+ degrees we've been experiencing. Without knowing what the spray is, I can't comment on that. I hope this info is helpful.
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