Answer: Yellowing leaves can be caused by numerous things, including overwatering, underwatering, lack of nitrogen, and lack of iron. Although hibiscus are sold in the low desert, as a tropical plant they are not ideally suited to our conditions, so our harsh environment can take a toll on them. I assume that you purchased a variety that is bred to do better in the desert. Typically, you should fertilize in February/March at the start of the growing season with a nitrogen fertilizer. Hibiscus also prefer acid soil conditions and our desert soils are alkaline, so you might want to use a product such as Miracid. Read instructions and follow carefully. Water deeply before and after applying any fertilizer to reduce chance of burn. Nitrogen deficiency shows up as older leaves turning yellow, while new growth is green. If your hibiscus is lacking in nitrogen, you could fertilize lightly at this time. Summer is considered a dormant period for many of the non-native plants that grow here, such as roses and hibiscus. It's best not to "force" them to produce at this time with fertilizer. It stresses the plant and may cause fertilizer burn. Provide protection from the hot afternoon sun and strong winds. 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. Fertilize again just as weather cools in the fall.
Iron chlorosis is also a problem in the desert. Overwatering, combined with heavy summer rains, can promote root rot, fungal diseases and iron chlorosis. 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 may be present in the soil, it is not always in a form that plants can use. 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 symptoms are still present, apply iron chelates or ferrous sulphate to the soil. Both are readily absorbed by a plant?s roots. I hope this info helps.
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