Caring for Rhododendrons

Cherie Moyer
Adams County Master Gardener

Rhododendrons are among my favorite shrubs. There are more than 700 species throughout the world, which have given rise to thousands of varieties. Many botanists classify azaleas and rhododendrons together thus giving us more choices.

Some appealing aspects of rhododendrons are their great diversity in size, their beautiful evergreen foliage, their staggered flowering periods from early spring through early summer, and their wide range of colors. Rhododendrons are hardy; they tolerate shade, poor climate and city pollution. They are easy to grow and do not require annual pruning.

Rhododendrons do, however, require a humus-rich soil and sufficient moisture because of their shallow roots. Don't allow them to dry out and protect them with a generous bed of leaves or mulch. Rhododendrons like acidic soil from 4.5 to 6.0 pH.

Sun requirements vary for rhododendrons. For the most part rhododendrons require partial shade. If their area is too shady, the plants will not bloom. I have mine planted in full sun next to the house, and they have grown quite well for the past 20 years.

Rhododendrons are susceptible to various diseases. In fact, I don't know many plants that aren't. The number one cause of diseased plants is the wrong plant growing in the wrong place. If you keep this rule in mind, you will have fewer problems with pests and diseases. Rhododendron diseases include chlorosis, leaf spot, borer damage, salt burn, die back, and root rot. A few common pests that rhododendrons attract are spider mites, lacebugs and black vine weevil.

Spider mites are a major pest of many garden plants. They cause damage by sucking sap from buds and the undersides of leaves. As a result, the green leaf pigment disappears, producing a yellow or bronze stippled appearance. There may be silken webbing on the lower surfaces of the leaves. To diagnose this problem, hold a sheet of white paper underneath an affected leaf or branch and tap sharply. Minute green, red, or yellow specks the size of pepper grains will drop to the paper and begin to crawl around. The pests are easily seen against the white background.

A second major pest is lacebug. The upper sides of the leaves are mottled or speckled yellow and green. The mottling may be confused with mite or leafhopper damage. It can be distinguished from other insect damage by the hard, black, shiny droplets that are found on the undersides of damaged leaves. The lacebug is a very small (1/8"), spiny insect with clear lacy wings. Damage occurs in spring and summer. Populations of lacebugs are highest when rhododendrons are grown in a sunny rather than a shady location. The wingless, immature insects and the lacy-winged adults suck sap from the undersides of leaves. As they feed, droplets of black excrement accumulate around them. If the infestation is heavy, the lacebugs can be controlled with a number of insecticides.

Another damaging insect is the black vine weevil. The adult weevil feeds on the leaves producing a C-shaped notching around the leaf margin. To determine if the plant is infested with the vine weevil, inspect the foliage after dark. Using a flashlight, look for a black or grayish insect about 1/5 to 2/5 inches long. It has an elephant like snout and rows of tiny round depressions on its back. It is present from May or June to September. The notched leaves are unsightly, but the damage caused by the weevil is usually not severe. I find that a magnifying lens goes a long way in helping to locate and identify any insects!

A fairly common ailment of rhododendrons is chlorosis. It is really an iron deficiency of the plant. The leaves turn pale green to yellow. The newest leaves may be completely yellow with only the veins and the tissue right next to the veins, remaining green.

Rhododendrons are acid-loving plants. They prefer soil with a pH between 4.5 and 6.0. The yellowing is due to a deficiency of iron in the plant. The soil is seldom deficient in iron. If the pH of the soil is 7 or higher, the iron is in an insoluble form so the plant is unable to absorb the iron from the ground. A high soil pH can come from overliming, or from lime leached from cement or brick. Plants use iron in the formation of the green pigment in the leaves. When the iron is lacking, the new leaves are yellow.

For treating chlorosis, spray the foliage with a chelated iron fertilizer and apply it to the soil around the plant to correct the iron deficiency. Correct the pH of the soil by amending it with sulfur, iron sulfate or ammonium sulfate. Work the amendment into the root area to lower the pH. To help maintain an acid pH, use fertilizer that is specially formulated for acid-loving plants.

Rhododendrons can be susceptible to winter burn. Leaf drying and browning can occur as a result of winter exposure. When the leaves on the rhododendrons roll up, the plant is trying to protect itself from the dry winter air that causes the leaves to lose their moisture more rapidly than it can be replaced. The leaf edges dry out and die. If the soil is frozen, then the plant can't absorb any water either. You can help protect your plants by planting them behind buildings or other plants that can serve as windscreens. Mulching is so important in helping to prevent this winter injury. Make sure your plants are well watered before winter sets in.

Another prevalent disease is dieback. The leaves and the terminal portion of a branch wilt and die. The leaves may turn reddish brown and remain attached to the plant. The leaves may also be rolled and have spots that look water soaked. At the base of the wilted branch may be a sunken, brownish, dead area. Several different fungi cause dieback. Infected soil and tools, rain, and splashing water spread the fungi. The fungi enter the plant through a weakened area. You may see a sunken dead area or a canker on the twigs which cuts off the flow of nutrients and water to that side.

To manage this disease, cut out the affected branches a few inches below the affected area and then destroy the branches. You can then spray with a basic copper sulfate fungicide after blooming. Repeat 2 or more times at intervals of 14 days.

Although rhododendrons can be susceptible to various insects and diseases, they are still wonderful plants that will reward you many times over with their bold springtime beauty if they are well cared for. Plant some and see if you don't agree. And if you run into a problem with their care, the Penn State extension office has a "hot line" to answer all of your gardening questions and further assist you. Better yet, bring in a picture or a sample of the infected plant and let us help you diagnose the problem.

Read other articles on shrubs and vines

Read other articles by Cherie Moyer