From a botanical standpoint, azaleas and rhododendrons are more alike than they are different. All azaleas are, in fact, a type of rhododendron. They both belong to the genus Rhododendron within the Ericaceae (heath) family. The genus is massive, containing over a thousand species, which botanists have divided into several subgenera, sections, and subsections. Historically, azaleas were classified in their own separate genus, Azalea, but were later reclassified based on a deeper understanding of their genetic and morphological characteristics. Today, the "azaleas" are primarily species and hybrids found within two subgenera of Rhododendron: Pentanthera (deciduous azaleas) and Tsutsusi (evergreen azaleas).
A key difference lies in their foliage habits. While this is a general rule with exceptions, it is a primary identifier for gardeners. Most of the plants commonly called "rhododendrons" are evergreen, meaning they retain their large, leathery, often elongated leaves (technically called lanceolate) throughout the winter. In contrast, many azaleas are deciduous, shedding all of their leaves in the autumn. There are, however, important evergreen azaleas (like the common Kurume and Southern Indian hybrids), which can cause confusion. These evergreen azaleas tend to have smaller, thinner, and sometimes glossier leaves than their large-leaved rhododendron cousins.
This is the most consistent botanical distinction. The flowers of both are showy and have a similar basic structure, but a close count of the stamens (the pollen-producing male parts) reveals the difference. Typically, rhododendron flowers have ten or more stamens per bloom. Azalea flowers, whether deciduous or evergreen, most commonly have only five stamens. This reliable characteristic is used by botanists to distinguish between the two groups within the larger genus.
While not a hard taxonomic rule, there are general trends in how the flowers are presented. Rhododendron flowers are often larger, bell-shaped or trumpet-shaped, and grow in large, rounded clusters called trusses at the end of the branches. Each truss is a single, impressive spherical bloom from a distance. Azalea flowers, on the other hand, are usually smaller, funnel-shaped, and appear either singly or in smaller, looser clusters all along the stem and at its tip. They tend to provide a more scattered coverage over the entire shrub.
There is a significant difference in their typical growth form and ultimate size. Rhododendrons are generally larger, woodier shrubs, often growing into sizable, dense, and rounded bushes that can reach heights of many feet, with some species becoming small trees. Azaleas are generally more compact and lower-growing shrubs. Deciduous azaleas can be quite upright and open in their growth habit, while evergreen azaleas are often densely branched and are frequently used for foundation plantings and hedges due to their smaller scale.
