How to Grow and Care for Rhododendron

Rhododendron is a genus of more than 1,000 species and more than 25,000 cultivars and hybrids. They grow as native plants in many parts of the world, including North America. By far, the largest number of wild rhododendrons are native to Asia, and it is those species that many of the cultivated rhododendrons are derived. 

Because rhododendron species are so numerous, the genus has been divided into subgenera, and they have also been grouped into sections and subsections according to similar features. Rhododendrons can be evergreen or deciduous, and large-leaved and small-leaved. They come in a wide range of sizes and flower colors and shapes. The tubular-, funnel-, or bell-shaped flowers can be fragrant or non-fragrant. Some rhododendrons have tiny scales on the leaves (lepidote), or they don’t (elepidote). The bloom time ranges from late winter through early fall. 

The growth rate of the different species varies widely. Always choose a variety that fits your climate, which will also determine the best planting time.

All rhododendron are toxic to humans and pets.

Rhododendron Plant Care 

While rhododendron species vary in their requirements, and many grow in a wide range of conditions, there are a few common denominators that apply to all types. These include naturally acidic soil, adequate moisture, and partial or dappled shade. 

Once the plants are established, they do not require a lot of maintenance other than light pruning and the yearly application of two to three inches of organic mulch around the base of the plant to keep the soil moist and suppress weeds. 

Light

In their native habitats, rhododendron grows in dappled woodland shade and under trees. Choose a location in partial shade where the plant is not exposed to the hot afternoon sun, which is especially important in a warmer climate. 

Soil

Rhododendrons need moist soil, however, they are just as sensitive to soggy, heavy, or compacted soil with poor drainage, in which their fine, hair-like roots get damaged. They prefer acidic soils with a pH between 4.5 to 6.0. The soil should be rich in organic matter. 

Water

The plants need adequate moisture. Especially during the first year after planting, consistent irrigation is crucial. Water the plant slowly and deeply twice a week during the first growing season. Established shrubs need watering during dry and hot weather without rain because the roots are very shallow and the soil around them dries out quickly. Even if the plant does not show any sign of drought stress yet, water it every two to three weeks during dry spells. 

Temperature and Humidity

The temperature range very much depends on the rhododendron variety. Some varieties tolerate heat and humidity, and others are cold-hardy. Most varieties do not like strong winds and are sensitive to temperature extremes. 

Fertilizer

If you have planted your rhododendron in rich soil with plenty of organic matter, there is usually no need to fertilize it. In less fertile soil, use a special fertilizer for acid-loving plants in late winter or early spring and follow the label instructions for rhododendrons. 

Types of Rhododendrons

Most rhododendrons sold in the nursery trade today are cultivars and hybrids but here are a few popular species native to North America: 

• R. catawbiense (Catawba rosebay, Catawba rhododendron) has dark, glossy, evergreen foliage and deep pink flowers. It can grow up to 20 feet tall. It is native to the moist mountain woodlands of the Appalachian states and suitable for USDA zones 4-8. 
• R. maximum (Rosebay rhododendron) is an evergreen shrub with white flowers in the summer. It is native to Eastern North America and especially hardy, being suitable for USDA zones 3-7. It grows 5 to 15 feet tall and about as wide. 
• R. macrophyllum (Pacific rhododendron, California rosebay) is an evergreen shrub or small tree that grows 6 to 12 feet tall. It is native to the Pacific Northwest; its pale pink to purple flowers are the state flower of Washington. 
• R. periclymenoides (Pinxterbloom azalea) is a bushy azalea that is native to the Eastern United States. Unlike most other varieties, it tolerates dry, nutrient-poor soil. It has lightly fragrant white to pink flowers in the spring and grows 3 to 8 feet tall and 3 to 5 feet wide. 

Cultivars and Hybrids

From the thousands of rhododendron cultivars and hybrids, bred both from native and non-native species, here are some popular ones:

• P.J.M. Rhododendron (Rhododendron x ‘P.J.M.’) is a hybrid with bright lavender-purple blooms and dark evergreen leaves that turn mahogany brown in the winter. It is both heat- and cold-tolerant and hardy in USDA zones 4-8. It grows 3 to 5 feet tall and spreads more than 5 feet.
• September Song Rhododendron (Rhododendron x ‘September Song’) is an evergreen hybrid for regions with milder winters. It grows 4 to 5 feet tall and spreads more than 5 feet. The flowers in the spring are pink, yellow, and orange.
• Nova Zembla Rhododendron (Rhododendron x ‘Nova Zembla’) is a hybrid evergreen with bright red flowers on a shrub that grows up to 5 feet tall and wide. It is suitable for USDA zones 4-8.
• Roseum Elegans Rhododendron (Rhododendron x ‘Roseum Elegans’) is a hybrid that grows 6 to 8 feet in height and width. The large pink-rose flowers resemble lilac. It is suitable for USDA zones 4-8.
• Pink Treasure Rhododendron (Rhododendron kaempferi ‘Pink Treasure’) is an evergreen cultivar when grown at the warmer end of its range (USDA zones 4-8), where its leaves turn red in the fall. The shrub, reaching 6 feet in height and 5 feet in width, has pink pendulous flowers in the spring.

Pruning

The main reason for pruning rhododendrons is to remove dead, diseased, or broken branches. This can be done any time.

Pruning to shape it, thin it for better airflow, or rejuvenate a mature shrub should be done soon after the bloom, or no later than early summer because by mid or late summer, the plant has formed next year’s flowers.

Propagating Rhododendron

Before you set out to propagate a rhododendron, identify whether it is a straight species, a cultivar, or a hybrid. It only makes sense to propagate a straight species, as the outcome of propagating a cultivar or hybrid is unpredictable—the new plant does not have the desired features. Plus, some cultivars or hybrids are trademarked and propagation is prohibited.

1. If you have a suitable rhododendron for propagation, take these steps to propagate it from cuttings. Be aware that this process takes four months or more.
2. In the early fall, take several 4-inch green softwood cuttings below a leaf node. The branches should be slim and have one whirl of leaves but no flower buds. If there are more than four leaves, remove them. Cut the leaves in half to make the leaf surface smaller.
3. Have small pots with sterile damp potting mix ready and make a 1-inch hole with a pencil. You can put more than one cutting in the container, as not all will root.
4. At the bottom end of the cuttings, slice off a thin layer of bark on either side of the stem, 1 inch long.
5. Dip the cutting in rooting hormone and immediately insert the cutting into the hole. Gently firm the soil around the cuttings.
6. Place the pots with the cuttings in a warm, bright location but away from direct sunlight. Keep the soil evenly moist at all times but not soggy. It takes three to four months for large-leaf rhododendron to root.

Potting and Repotting

Smaller compact rhododendrons with a maximum height and width of 3 to 5 feet at maturity can be grown in containers. Choose a container that is at least one-third larger than the nursery container the plant came in, and with large drainage holes. Fill it with potting mix for acid-loving plants. Place the container in a location where it gets shade in the afternoon.

When the plant has outgrown its container, and the roots reach the sides and grow out of the drainage holes, repot it in a larger container.

Overwintering

If you planted a variety in the landscape that is suitable for your climate, and it is in a location where it’s shielded from chilly winter winds, there is no need for any winter protection. Potted plants, on the other hand, need to be winterized because the roots are exposed and vulnerable to winter cold.

Extreme winter cold might damage the buds. If the plant is small enough, loosely wrap it with burlap to protect it temporarily. Take special care not to break any buds.

Common Pests & Plant Diseases

Rhododendrons are susceptible to a wide range of pests and diseases. The plant attracts aphids, mites, borers, lacebugs, caterpillars, leafhoppers, mealybugs, nematodes, scale, thrips, and whiteflies. Possible diseases include powdery mildew, blights, canker, crown rot, leaf gall, root rot, leaf spot, and, rust. he healthier the plant, the better equipped it is to fend off any of these pests and diseases. 

How to Get Rhododendron to Bloom

There are several possibilities for why your rhododendron isn’t blooming. It could be damage to the flower buds from a severe freeze or late frost, or you could have accidentally removed the flower buds when pruning too late in the season. In those cases, there is nothing to be done than wait until the next year. Or, the plant might not get enough sunlight. Rhododendrons need shade but too much shade affects their bloom. Pruning other plants around often helps to let more light in. Drought stress is another possible reason so make sure to water the plant in dry spells. Imbalance in the soil can also affect the bloom. If there is excess nitrogen in the soil, you get lush foliage but no flowers. The source of nitrogen runoff might be an over-fertilized lawn nearby. Or, the pH of the soil is too high. A soil test is required to confirm this and you’ll need to lower the pH as described below. 

Common Problems with Rhododendron

Rhododendron leaves turning yellow can be a sign of the pH being too high. This can happen when the plant is near a concrete sidewalk, driveway, or foundation which leaches lime and raises the soil pH. Test the soil pH first before lowering the pH. Amend the soil with wettable sulfur powder or ferrous sulfate—not to be confused with aluminum sulfate, which is toxic to rhododendron roots.