Keeping your rhododendrons in optimum health will go a long way in fighting off insects and diseases. Every cultural requirement will need to be met for good bloom and shiny green foliage. Once established and healthy, your rhododendron will bloom reliably each year, and provide an evergreen presence in the garden.
Root weevils are a major problem for the rhododendron. The adult beetles come out at night and leave notches along the edges of the leaves. If the damage were not so obvious, you would never know they had been there. Remove the unsightly leaves, and new ones will grow. Spraying the rhododendron foliage with neem oil is effective against root weevil. The weevil larvae hatch in the fall, overwinter in the soil and feed on plant roots in the spring. If there are many larvae, they can do substantial damage to the rhododendron. Neem oil can be poured into the soil around the rhododendron roots to reach the larvae.
Soil pH is important to rhododendrons. If they are planted in alkaline soil, the plants will suffer nutritional deficiencies. This is called iron chlorosis. It is a condition in which low iron conditions prevent the plant from absorbing nutrients. The mature leaves will have yellow veining, and the new foliage will emerge yellow. To correct the problem, the soil pH must be changed. Adding iron chelate to the soil can be helpful. You can also try a foliar application of iron chelate, or ferrous sulfate. For long-term improvement, use acidic mulches, soil amendments and fertilizers. If the rhododendrons are planted near concrete, correction will be impossible. Concrete continually leaches into the soil creating an alkaline environment. If this is happening, you will need to move your rhododendron.
Rhododendrons are misrepresented far too often by being placed in full sun situations. Their general health will suffer, and their foliage will burn. There are some varieties that are more sun tolerant, but most prefer to have protection from the afternoon sun. Rhododendrons have shallow roots and need regular water. Too much sun will keep them in a constant state of dehydration. The only remedy is to move the rhododendron to a protected location.
For good bloom, you must meet all the cultural requirements of your rhododendron. A fall application of fertilizer or compost is important to replace energy exerted during the bloom cycle. Rhododendrons need just the right amount of sunlight to bloom properly. In nature, they are found in dappled sunlight, or under trees where they receive morning sun, and afternoon shade. This is ideal to keep the foliage vibrant, and have successful flower production. Deadheading spent blooms will reserve energy for next years flowers. Rhododendrons begin forming next years buds immediately after the flowers have withered. Deadheading must be done before these new buds become visible. If the buds are removed by accident, there will be no flower in that spot the following year.