Rhododendron

Azaleas and rhododendrons are woody, evergreen or deciduous, rounded shrubs in the Ericaceae (blueberry) family. They are native to the United States and Asia and there are many cultivars and varieties, some native to North Carolina. They typically grow 6 to 10 feet tall and 5 to 8 feet wide. The genus name derives from the Greek words rhodo, which means rose, and dendron, meaning tree.

These plants prefer moist, well-drained, acidic soils in dappled shade and protection from afternoon sun which can scorch the leaves. They do not tolerate dense clay, so if you have clay soil, amend with organic material. The roots may rot if soil does not drain well. Plant the root ball high in the soil to help with drainage issues. Avoid overwatering and over-fertilizing. Pruning, if desired, may be done after flowering. 

Azaleas and rhododendrons are at home as understory shrubs planted in groups in woodland or naturalized areas, or as specimens in pollinator gardens where the flowers will attract hummingbirds and the specialized bee Andrena cornelli. Some varieties are grown as houseplants.

Per the American Rhododendron Society, "All azaleas are rhododendrons, but not all rhododendrons are azaleas." Here are some general characteristics to differentiate the two:

Quick ID Hints for Azaleas:

• Bloom in April
• Tubular or funnel-shaped flowers
• One flower per stem but has many stems that make the shrub look like it is covered in blooms
• 5 lobes per flower
• 5 stamens
• Multiple bloom colors including white, cream, pink, red, lavender, purple, orange, and yellow
• Foliage elliptical to obovate, thin, small, entire margins, thick pubescence, cluster at the tips of the branch, buds are smaller
• Most are deciduous but there are many evergreens

Quick ID Hints for Rhododendrons:

• Bloom later in the spring
• Bell-shaped flowers
• Blooms grow in clusters at the end of the branch
• 5 lobes per flower 
• 10 stamens 
• Blooms are typically white, pink, purple, red, and rarely yellow
• Leaves are green elliptic, entire margins, thick, leathery, glossy, undersides may be scaly or have small dots
• Terminal clusters of dried capsules release seeds

Insects, Diseases, and Other Plant Problems:  Insect problems include aphids, borers, lace bugs, leafhoppers, mealybugs, mites, nematodes, scale, thrips, and whitefly. Diseases include canker, crown rot, root rot, leaf spot, rust, and powdery mildew. This plant is frequently damaged by deer.

While these shrubs remain popular for landscape use, many cultivars are susceptible to Phytophthora root rot, which leads to leaf loss, reduced vigor, branch dieback, and wilting.  Implement good cultural practices first as described above. However, if you have a site with a history of this disease, consider planting root rot-resistant alternative species. See suggestions in the sidebar to the left or see this video created by Charlotte Glen as part of the Plants, Pests, and Pathogens series.

The Clemson Cooperative Extension Home and Garden Information Center has a factsheet on common diseases of rhododendron.  

VIDEO Created by Elizabeth Meyer for "Trees, Shrubs and Conifers" a plant identification course offered in partnership with Longwood Gardens.