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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 problem in the desert for non-native plants, such as gardenia. 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.
Soil with a high pH (alkalinity) also inhibits iron absorption. If you are using correct irrigation methods and iron deficiency symptoms are still present, apply iron chelates or ferrous sulphate to the soil. Both are readily absorbed by a plant?s roots.
Although gardenias are sold here, they are not particularly well-adapted to desert growing conditions. They prefer rich, acidic soil, regular moisture and are susceptible to salt burn. Desert soil is alkaline (no matter what amendments are added, it is not possible to drastically change that), with limited organic matter, and of course, are conditions are extremely arid. Our water and soil is high in salt. Water slowly and deeply, and let the water leach salts past the root zone. "Sprinkling" with water lightly and frequently allows salts to accumulate in the soil, burning roots. Salt burn shows up as yellowing and browning along the edges of the leaves. Good luck with your gardenia!
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