How to Grow and Care for Rhododendron

Rhododendron is a genus of more than 1,000 deciduous or evergreen flowering shrub species, with more than 25,000 cultivars and hybrids. They come in a wide range of sizes, flower colors and shapes, and many are North American natives.

The bloom time is usually spring into summer, but some cultivars bloom in late winter or late summer.The growth rate and preferred conditions of rhododendron species vary widely. However, most prefer moist, well-draining soils, partial shade, and conservative fertilization. All rhododendrons are toxic to people and pets.

Learn how to grow stunning blooming rhododendron species with this care guide.

Rhododendron vs. Azaleas

Azaleas are flowering shrubs in the Rhododendron genus, so all azaleas are rhododendrons, but not all rhododendrons are azaleas. Azaleas typically have single-stem, tubular, early-spring blooming flowers, and rhododendrons have larger bell-shaped flowers that grow in clusters in early summer.

Common Name	Rhododendron
Botanical Name	Rhododendron spp.
Family	Ericaceae
Plant Type	Shrub
Mature Size	2-20 ft. tall, 3-15 ft. wide
Sun Exposure	Partial
Soil Type	Moist, well-drained
Soil pH	Acidic
Bloom Time	Spring, summer, fall
Flower Color	Red, pink, orange, yellow, purple, white
Hardiness Zones	4-9 (USDA)
Native Area	North America, Europe, Asia, Australia
Toxicity	Toxic to humans, toxic to pets

Rhododendron Plant Care

While rhododendron species vary in their care preferences, and many grow in a wide range of conditions, the typical main rhododendron requirements are:

• Plant in fertile, well-draining, acidic soil.
• Position in partial or dappled shaded.
• Water to maintain even moisture, but avoid soggy soils.
• Fertilize with a rhododendron formula annually only when soils are poor.

Light

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

Soil

Rhododendrons need moist, well-draining soils rich in organic matter. They're sensitive to soggy, heavy clay, or compacted soil with poor drainage, which can lead to root rot. These shrubs prefer acidic soils with a pH between 4.5 and 6.0.

Water

Rhododendrons have shallow roots and require even moisture to thrive, especially during the first year after planting. Water the plant slowly and deeply around twice a week during the first growing season, not leaving the roots in standing water.

Even if the plant doesn't show signs of drought stress, water established rhododenron every two to three weeks during dry spells. Adding a layer of mulch helps with water retention.

Temperature and Humidity

Temperature ranges vary depending on the rhododendron variety. Some tolerate heat and humidity, and others are cold-hardy. Most varieties don't like strong winds and are sensitive to temperature extremes.

Fertilizer

If you plant your rhododendron in rich soil with plenty of organic matter, there's usually no need to fertilize. In less fertile soil, use a formula for acid-loving rhododendron in late winter or early spring, following the label instructions.

Types of Rhododendrons

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

• Catawba rhododendron (R. catawbiense): Has dark, glossy, evergreen foliage and deep pink flowers. It can grow up to 20 feet tall and is native to the moist mountain woodlands of the Appalachian states. Suitable for USDA zones 4-8.
• Rosebay rhododendron (R. maximum): An evergreen shrub with white summer flowers. This Eastern North American native is especially hardy and grows 5 to 15 feet tall. Suitable for USDA zones 3-7.
• Pacific rhododendron (R. macrophyllum) : An evergreen shrub or small tree with pale pink to purple blooms. It grows 6 to 12 feet tall. Native to the Pacific Northwest, its the state flower of Washington.Suitable for USDA zones 6-9.
• Pinxterbloom azalea (R. periclymenoides): A bushy azalea native to the Eastern United States. Unlike most other varieties, it tolerates dry, nutrient-poor soils. It has lightly fragrant white to pink flowers in the spring and grows 3 to 8 feet tall.Suitable for USDA zones 4-9.

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.’): A heat- and cold-tolerant hybrid with bright lavender-purple blooms and dark evergreen leaves that turn mahogany in the winter. It grows 3 to 5 feet tall. Suitable for USDA zones 4-8.
• September Song Rhododendron (Rhododendron x ‘September Song’): An evergreen hybrid for regions with milder winters. It grows 4 to 5 feet tall and the spring flowers are pink, yellow, and orange. Suitable for USDA zones 6-8.
• Nova Zembla Rhododendron (Rhododendron x ‘Nova Zembla’): A hybrid evergreen shrub with bright red flowers that grows up to 5 feet tall. Suitable for USDA zones 4-8.
• Roseum Elegans Rhododendron (Rhododendron x ‘Roseum Elegans’) is a hybrid that grows 6 to 8 feet tall. The large pink-rose flowers resemble lilacs. 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, where its leaves turn red in the fall. The shrub, reaching 6 feet tall, has pink pendulous spring flowers. Suitable for USDA zones 4-8.

Pruning

Pruning rhododendrons is mainly done to remove dead, diseased, or broken branches. It can be done at any time.

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

Propagating Rhododendron

It only makes sense to propagate a straight rhododendron 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.

If you have a suitable rhododendron, take these steps to propagate it from cuttings in the early fall.

1. 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 the rest.
2. 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. 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 away from direct sunlight. Keep the soil evenly moist but not soggy. Large-leaf rhododendrons take three to four months to root.

Potting and Repotting

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, 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 plant a rhododendron variety that's suitable for your climate and it's shielded from chilly winter winds, there's no need for winter protection. Potted plants need winterizingbecause the roots are exposed and vulnerable to winter cold.

Extreme winter cold might damage buds. If the plant is small enough, loosely wrap it with burlap and add a layer of mulch to temporarily protect it. 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. The healthier the plant, the better equipped it is to fend off these pests and diseases.

Removing pest infestations varies by species, but sometimes it's possible to do so with a simple stream of water from a hose.

Alternatively, treat larger infestations with an insecticidal soap or horticultural oil, like neem oil. It's always a good idea to test the solution on a couple of leaves before treating all the foliage and spray early morning or at dusk to prevent the risk of leaf scorch.

How to Get Rhododendron to Bloom

Learn more about how to enjoy the best rhododendron blooms for as long as possible.

Bloom Months

Bloom months vary depending on the rhododendron species and cultivar, but most are known for being early spring to midsummer bloomers.

What do Rhododendron Flowers Look and Smell Like?

Rhododendron flower form and fragrance vary considerably depending on the species and cultivar. However, flowers are often bell-shaped, 1 to 3 inches wide and bloom in clusters at the end of branches. Colors include white, cream, orange, pink, red, and purple. Not all rhododendron flowers are fragrant, but the ones that are can be heady, spicy or sweet.

How to Encourage More Blooms

Some tips for encouraging prolific pruning include avoiding pruning too late in the season, providing adequate dappled light, offering consistent moisture, and amending soils high in nitrogen or with high pH levels.

Common Problems with Rhododendron

Rhododendron leaves turning yellow can indicate the pH levels are too high. This can happen when the plant is near a concrete sidewalk, driveway, or foundation, which leaches alkaline lime. Test the soil pH beforelowering 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.

From: thespruce