Answer: The perennial hibiscus, or rose mallows, as a group prefer a very rich and very moist soil in the garden; this reflects their native wetlands habitat. In my experience, a hot dry spell can cause them to turn yellow in protest very quickly. You might try watering them deeply and regularly and try a sidedressing of either compost or a balanced fertilizer according to the label instructions. A mulch will also help retain soil moisture.
There is also a minor possibility of insect attack; you might look very carefully along the stem and the undersides of the leaves for evidence of insects.
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