Rhododendrons and azaleas belong to the genus Rhododendron of the Ericaceae family.  The Ericaceae family includes the heaths, heathers, cranberries, blueberries, mountain laurels, Japanese andromeda (pieris) and other ornamental plants.

The genus Rhododendron has about a thousand different species.  These species are currently organized into subgenera, sections and subsections, with the species in each group having certain points of similarity to each other.  A family tree for rhododendron species can be viewed here.  Links to photo images and plant descriptions are provided.

All azaleas are rhododendrons, but not all rhododendrons are azaleas. Evergreen azaleas belonging to subgenus Tsutsusi, and deciduous azaleas are classified in the subgenus Pentanthera.  Rhododendrons with scales, which are mostly the small-leaved rhododendrons, belong to subgenus Rhododendron, while rhododendrons without scales, which are mostly the large-leaved rhododendrons, belong to the subgenus Hymenanthes. When botanists use the word "rhododendron" they are including azaleas. On the ARS website we utilize the definitions common to the commercial trade and refer to both "rhododendrons" and "azaleas".

When botanists categorize rhododendrons and azaleas as to whether they have "scales" on their leaves and stems or not, they are referring to small structures that are about the diameter of a human hair and are easily visible with a hand lens. Botanists use the term "lepidote" to describe rhododendrons with scales and use the term "elepidote" to describe rhododendrons without scales. Most large-leaved rhododendrons and all azaleas are elepidotes. About one third of all rhododendron species are in a group of tropical rhododendrons called "vireyas". Vireyas and most small-leaved rhododendrons are lepidotes.

Rhododendron species are found growing in the wild in many parts of the world.  A large number of cultivated rhododendrons are derived from species coming from Asia, especially the Himalayan foothills, in western China, northern India, Myanmar, Sikkim and Nepal.  Other rhododendrons are native to Japan, Korea, Europe and some are native to eastern and western North America.

About 300 rhododendron species grow in warm climate locations in southeastern Asia, principally in Borneo, New Guinea, Sulawesi, Sumatra and the Philippines which are referred to as "vireya rhododendrons". For more information about vireya species and hybrid cultivars please visit Vireya.net.