Hibiscus, simply put, are temperamental plants. Seasoned growers are used to sudden yellowing or periodic dropping of leaves and tend not to panic, but novice growers can become quite frustrated at trying to diagnose the cause. While the potential causes are many, sometimes there is no cause at all.
Hibiscus like to be kept moist, but not wet. Keeping a hibiscus too wet or dry will cause yellow leaves. Since hibiscus are very sensitive to changes in soil moisture, do not let a plant dry out completely between waterings, especially container grown plants. A good rule of thumb is to stick a finger into the soil, and if it is dry to the second knuckle, water. Never plant a hibiscus in an area that is prone to flooding or standing water.
There are several Hibiscus diseases that can cause yellowing leaves and leaf drop.
Canker, leaf spot, and mushroom root rot are common diseases that cause yellow or dying leaves on hibiscus.
Canker fungus causes branch and twig death, and can kill an entire plant. All diseased wood should be pruned off and destroyed.
Mushroom root rot causes a sudden wilt and death. Remove dead and dying plants with as much of the rootball as possible. Sterilize the soil thoroughly before replanting in that spot.
Leaf spot is caused by various fungi and bacteria. Damage is usually minor. the primary control is to remove and destroy all diseased leaves.
Nematodes are microscopic worms that can invade and damage the roots of hibiscus, causing yellow and dropping leaves. There is no available treatment for nematodes other than soil sterilization prior to planting.
Insects that can cause yellow leaves on hibiscus include aphids, spider mites, mealybugs, and whiteflies. Ants always indicate the presence of aphids. Fungus gnats in too moist soil can damage roots and cause leaf drop. Hibiscus scale can suck the juice out of the plant and cause yellow leaves.
Always check all parts of the hibiscus plant carefully for pests, including the bark and underside of leaves.
Hibiscus are sensitive to any abrupt change in environment. Sudden cold, heat, water and even a cold draft can cause yellow leaves on hibiscus. While outdoor plants are at the mercy of nature, providing consistent environmental conditions for indoor plants will help eliminate leaf drop.
Hibiscus are heavy feeders, but overfertilization can burn roots and cause yellow leaves. Underfertilization, especially a lack of minor elements such as iron, can cause leaves to yellow. Using a balanced fertilizer with minor elements is recommended.
Hibiscus are sensitive to chemicals, and some stringent rules apply to their use. Use oil sprays at half strength and never in the winter. Never mix insecticide and fungicide when treating hibiscus. Never spray a hibiscus during the heat of the day. Avoid using herbicides around hibiscus.
Hibiscus regularly drop leaves during the change of seasons in spring and fall. Being such sensitive plants, hibiscus may develop yellow leaves for no reason that can be determined. Experienced growers accept the fact that hibiscus are rather temperamental, and unless there is an obvious cause, do not worry much about yellowing leaves. Luckily, hibiscus grow back lost leaves very quickly in most instances.