By: Sandy Feather ©2006
Penn State Extension
Q. During the past few weeks, I have enjoyed the brilliant blooms of my azaleas and my neighbors' azaleas and rhododendrons. Are there major differences between these plants? The rhododendrons are larger, but the flowers appear to have a similar shape. Is it true they can survive with very little watering?
A. Rhododendrons and azaleas both are classified in the genus Rhododendron, so you are correct that they are very similar.
Rhododendrons are not always larger than azaleas; some grow only 18 to 24 inches tall. There is tremendous variation in this genus, so it is difficult to set hard and fast rules for telling the difference.
Delaware Valley White azalea
Rhododendron flowers usually have 10 or more stamens, while azaleas usually have five. Stamens are the pollen-bearing male reproductive organs found inside the throat of a flower. (The ovule-bearing female reproductive organs are called pistils.)
Rhododendroms usually are evergreen, while most azaleas are deciduous (lose their leaves in fall), including all of the azalea species native to Western Pennsylvania -- R. arborescens, R. periclymenoides and R. viscosum. That said, evergreen azaleas are more commonly grown as ornamentals in Western Pennsylvania landscapes.
Rhododendron leaves may be scaly or have small dots on the underside of the leaves. A few rhododendrons have dense, soft hairs on the undersides. Azalea leaves, both deciduous and evergreen, are often covered top and bottom with fine hairs.
Most rhododendron flowers are borne in terminal clusters known as trusses; azalea flowers are often borne singly or in small terminal clusters.
Large purple rhododendron
Rhododendron species are members of the Heath family, or Ericaceae. In general, members of this family prefer a well-drained, yet evenly moist, acidic soil that has good organic matter content.
Our native species, both deciduous and evergreen, usually grow as understory plants in the high shade of mature trees, often along streams. They are not considered extremely drought resistant. They actually require an even, adequate source of moisture. They tend to be shallow-rooted plants and perform best when protected from strong winds, especially in the winter.