your hibiscus tree may be suffering a number of different problems. Lack of water in the soil for even a brief period of time can result in yellowing and loss of the leaves, especially older leaves. When we see leaf curl another consideration would be insects feeding on the undersides leaves. This would include aphids, whiteflies, or scale insects. I suggest you take some of the leaves in a plastic bag to your local county extension office or them to look over and identify any possible culprits. Then an appropriate control recommendation can be made, depending on what they find.
Q&A Library Searching Tips