Arnold Arboretum, Boston
The genus Rhododendron includes both Rhododendrons and Azaleas. In this genus, both Rhododendron and Azalea are used as common names. The distinctions that follow are made by gardeners. The Rhododendron plants are usually evergreen and those labeled Azalea plants are deciduous, although there are a few evergreen Azaleas like those in the florist or nursery trade. To the typical gardener, azaleas are conspicuous because they don't have trusses.
Rhododendrons have ten or more stamens, while Azaleas have five or six.
Rhododendrons have large, paddle-shaped leaves and large, bell- or funnel-shaped flowers borne in terminal trusses. Azaleas have small, elliptical leaves and trumpet- or tubular-shaped flowers at the ends of the shoots. Still some rhododendrons, called lepidoes have small leaves and small flowers and some rhododendrons are naturally miniature have miniscule leaves and flowers. It is true that all azaleas have small leaves compared to the large leaved rhododendrons, the elepidotes.