Answer: 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, a cold snap can injure the leaves; they will wilt suddenly and then drop off the plant.
Hibiscus is sometimes host to aphids (Insecticidal Soap is effective). Spray according to label directions if you find aphids feeding on the leaves of your plant.
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