Hibiscus (Hibiscus rosa-sinsensis) is a popular cultivated plant native to South Asia. There are hundreds of cultivars with different colored flowers, although the original plant has red blooms. Yellow leaves on a hibiscus plant are often a sign that the plant is stressed in some way and is shedding leaves. Diagnosing the cause of the stress involves analyzing the conditions your hibiscus plant is growing in.
After a period of intense flowering during spring and early summer, hibiscus plants shed leaves before briefly becoming dormant. If your plant has yellow leaves during the hottest summer months, they are probably being shed naturally by the plant and are nothing to worry about.
Hibiscus plants need abundant water during the summer and will drop leaves to prevent water loss if under-watered. Water your plant as soon as the surface of the soil around it gets dry and provide enough water to reach the root ball. A raised, circular moat around the main stem of garden hibiscus plants will allow water to soak in rather than running off.
Hibiscus plants suffer if their roots get waterlogged. Reduce watering in the colder months of the year if your plant has yellow leaves. Allow the soil to dry out completely to a depth of 2 inches before watering in winter. Consider pruning back surrounding vegetation shading your plant so that it gets as much sun as possible.
Nursery-grown hibiscus plants planted outdoors without a period of acclimatization often suffer from sun damage to the leaves. This occurs in the form of yellow or whitish patches on young leaves. Sun damage will not kill a hibiscus plant and the damaged leaves will eventually drop and be replaced by others.
Check your hibiscus plant for signs of spider mite infestation such as fine webbing and yellow mottles on the undersides of leaves. Left untreated, a serious infestation will turn the whole plant yellow and even kill it. Keep plants with spider mites humid by spraying them with water twice a day. Treat spider mite infestations with a commercial miticide sprayed thoroughly onto every leaf, or with an organic preparation containing neem oil from a plant nursery.
If your hibiscus plant has yellowish leaves that do not drop off, it could be suffering from chlorosis caused by a lack of micro nutrients such as iron and magnesium. Fertilize the plant with a commercial fertilizer enriched with trace elements.