Rhododendrons are spring-flowering shrubs with big, thick leaves. They grow best in partial sun with moist, well-drained acid soil. Failure to provide adequate growing conditions may cause health problems in rhododendrons, some of which result in yellow, drooping leaves.
When rhododendrons lack sufficient amounts of iron, their leaf tissue turns yellow and the veins stay green. Check the soil pH to ensure an acid environment, and add sulphur to lower the pH if needed. Apply a peat moss and bark mulch mixture 2 to 4 inches deep around the roots of the plant to improve drainage and air circulation.
In rhododendrons, a fungus called Phytophthora causes root rot in excessively wet growing conditions. Stunted growth occurs and drooping, yellow leaves appear. The wood becomes discoloured from the base upwards as the disease progresses.
Take immediate action to keep your rhododendron from dying. If your rhododendron suffers from root rot, lower moisture levels by replanting a young shrub or reducing the watering of an older specimen. Spray iron sulphate on rhododendron leaves or add iron to the soil to correct iron chlorosis. Preventive measures include keeping mulch at a depth of at least 2 inches and using an iron-containing fertiliser in late spring or early summer.