Hibiscus leaves turn yellow because of pest infestations, diseases and environmental changes such as too little fertilizer or insufficient water. In some cases, yellowing leaves are a perfectly natural response to cooling temperatures.
Pests of both the insect or pathogen variety can harm a hibiscus. The harm is visible when leaves turn yellow and drop from the plant. Scale insects are drawn to the plant's vibrant flowers, using their mouth stylets to pierce the plant's soft tissue and consume the sap. This keeps vital nutrients from getting to the leaves. Nematodes feed on the roots, causing sores to form that make it more difficult for the plant to absorb water and nutrients. Nematodes infestations occur most often in soil that isn't well-drained.
Humidity can also encourage the growth and spread of fungal diseases. The leaves on a hibiscus with a fungal infection can either turn completely yellow, turn yellow around the edges or get yellow discoloration. Well-drained soil helps to keep fungal infections from spreading. However, a hibiscus needs plenty of water to thrive, a lack of which can cause leaves to yellow and drop prematurely. If the soil contains too much fertilizer or not enough nutrients, the leaves can also be affected.