The most fundamental difference lies in their approach to foliage. Evergreen azaleas, as the name implies, retain a set of leaves throughout the year. They are not entirely "ever-green" in the sense that they do undergo a gradual, cyclical leaf drop, but it is never a complete, simultaneous shedding. Older leaves from previous seasons are typically shed in the spring as new growth emerges, ensuring the plant is never bare. Their leaves are often smaller, thicker, leathery, and sometimes covered in small hairs (trichomes), adaptations that help reduce water loss.
In contrast, deciduous azaleas drop all of their leaves each autumn in a dramatic display of fall color before becoming completely bare for the winter. This is an adaptation to survive cold winters and drought conditions. Their leaves are generally larger, thinner, and softer than those of their evergreen cousins. The absence of a need for year-round water conservation allows for this less robust leaf structure.
The flowers of these two groups exhibit distinct morphological differences. Deciduous azalea blossoms are typically funnel-shaped or tubular with long, protruding stamens and pistils that extend significantly beyond the petals. The flowers are often larger and are frequently, though not always, fragrant. They are borne in loose, elegant clusters at the ends of the branches.
Evergreen azalea flowers are generally more salverform (a flat face with a narrow tube at the base) and have a different structure. Their stamens and pistils are shorter and are usually nestled within the corolla tube, not extending far beyond it. The blooms come in a wider range of forms due to extensive hybridization, including single, double, and hose-in-hose types. They produce such a prolific display of often smaller flowers that the foliage can be completely obscured during peak bloom.
Their growth patterns also differ. Evergreen azaleas tend to have a denser, more compact, and rounded growth habit. They are often wider than they are tall, forming a mound of twiggy branches and persistent foliage. This makes them ideal for formal hedges and mass plantings where a defined shape is desired.
Deciduous azaleas possess a more open, upright, and spreading habit. Their branching structure is more visible and often considered elegant or architectural, especially in the winter months when the leaves are gone. They grow as multi-stemmed shrubs and generally reach a larger mature size than most evergreen varieties, fitting better in naturalized woodland garden settings.
Botanically, these groups belong to different subgenera within the genus Rhododendron. The vast majority of evergreen azaleas are classified in the subgenus Tsutsusi and are primarily native to Japan. Modern evergreen hybrids are complex crosses derived from these East Asian species.
Deciduous azaleas are classified in the subgenus Pentanthera and are native to North America and parts of Europe and Asia. Species like the Florida Flame Azalea (Rhododendron austrinum) and the Swamp Azalea (Rhododendron viscosum) are examples of natives that have contributed to hybrid lineages. This difference in origin explains many of their adaptations to climate and soil conditions.
