Hibiscus will do this when the plant is over or under watered. Container grown plants are especially susceptible. The older leaves (those farthest down the stem) are usually the first to go. The symptoms are often delayed a bit after the water stress conditions. So you may have recently watered but still see the problem. Overwatering contributes to root loss from lack of air and is often complicated by an ensuing root rot. This situation can result in a long term setback for the plants.
Lack of nitrogen can also cause a gradually loss of the green color in older leaves, but such symptoms are different from the sudden yellowing and dropping most often encountered.
Basically, hibiscus needs a significant amount of potassium (the 3rd number on a fertilizer label), a small amount of phosphorus (the 2nd number), and a moderate amount of nitrogen (the 1st number).
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