Hibiscus leaves turn yellow in response to stress or as part of the plant's natural life cycle. Old leaves turn yellow and drop off naturally, but sometimes yellow leaves are due to stress caused by factors that must be identified and eliminated.
Too much or too little water can cause yellow leaves. A hibiscus plant needs plentiful water during hot, sunny days and may need water more than once a day during these conditions. The soil should be kept evenly moist but not soggy.
Excessive wind may also stress the plant. It is important to keep it sheltered from drafts and water it more frequently in windy conditions. If the temperature drops below freezing, one should bring the plant indoors.
Light is also important. Hibiscus thrives in full sunlight, and too much shade can turn the leaves yellow. A plant with leaves that are partly green and yellow may suffer from a nutrient deficiency called chlorosis and need fertilizing. A soil pH that is too high or too low can also stress the plant.
If the yellowing leaves look misshapen or dotted, insects may be the problem. Hibiscus is especially susceptible to mites, which are detected by inspecting the undersides of the leaves. Mites also leave a web-like white substance on the stems. Mites can be washed off with mild soapy water. Afterward, the plant should be sprayed gently to remove all soap.