While often confused for one another, rhododendrons and azaleas share a fascinating botanical relationship. Both belong to the genus Rhododendron within the Ericaceae (Heath) family. Therefore, all azaleas are technically rhododendrons, but not all rhododendrons are azaleas. The differences between them are primarily based on horticultural and morphological distinctions rather than a strict scientific separation into different genera.
Botanically, the genus Rhododendron is divided into several subgenera, sections, and subsections. Most true rhododendrons fall into the subgenus Hymenanthes, which includes the large-leaved elepidote (non-scaly) species. Conversely, most azaleas are classified within the subgenera Pentanthera (deciduous) and Tsutsusi (evergreen). A key identifying feature is the foliage. True rhododendrons typically have large, leathery, evergreen leaves that are often oval-shaped and can be quite substantial in size. Azalea leaves are generally smaller, thinner, and more delicate. While some azaleas are evergreen, many are deciduous, losing their leaves in the fall, a trait uncommon in the larger rhododendron group.
The flowers provide the most noticeable differences. Rhododendron flowers are typically bell- or funnel-shaped and are known for their large, impressive trusses, which are spherical clusters of blossoms. Each flower usually has ten or more stamens. Azalea flowers, on the other hand, are most often funnel-shaped but are frequently borne in smaller, more scattered clusters rather than dense trusses. The most reliable floral distinction is the number of stamens: azalea blossoms typically possess five stamens per flower, whereas rhododendron flowers have ten or more.
The overall growth form of these plants also differs. Rhododendrons generally grow as large, robust, woody shrubs or even small trees, developing a thick, tree-like trunk and branches over time. Their structure is often denser and more substantial. Azaleas tend to be smaller, more delicate shrubs with a finer branching pattern. They are often less woody and have a more open, airy growth habit compared to the solid, imposing structure of many rhododendron species and hybrids.
A more technical botanical characteristic involves the presence of scales on the leaf underside. Many plants in the Rhododendron genus have small, rust-colored dots or scales called lepidotes on the underside of their leaves. However, the subgenus Hymenanthes (which contains the large-leaf rhododendrons) is elepidote, meaning it lacks these scales. Azaleas, belonging to other subgenera, are generally lepidote and will often have these tiny scales on their leaves, a trait easily observed with a magnifying glass.
