By RSBG Nursery Manager & Assistant Curator, Atsuko Gibson
The most frequently asked question here at the garden is,
“What is the difference between a rhododendron and an azaleas?”
There is a simple answer and a long answer.
The simple answer is, all azaleas are rhododendrons, but not all rhododendrons are azaleas. All azaleas belong in the genus Rhododendron. Azalea is a horticultural term, not a scientific or botanical name. For example, the botanical name for the western azalea is Rhododendron occidentale.
Here is the long answer.
The number of stamens might give you a clue about how to tell them apart, but there are many exceptions in this large genus of around 1200 species. Typically, azaleas have five stamens, whereas rhododendrons have ten or more stamens. The majority of deciduous azaleas belong to subgenus Pentanthera, whereas all evergreen azaleas as well as a dozen or so deciduous azaleas (mainly the three-leaved azaleas from Japan) belong in subgenus Tsustusi. R. canadense and R. vaseyi are the outlier species in subgenus Pentanthera, with 7-10 stamens.
All azaleas are elepidotes, which means they lack scales. All evergreen azaleas have appressed hairs, which are hairs that are pressed closely against the leaf surface. All evergreen azaleas occur in east Asia, such as E. China, Taiwan, Korea, and Japan.
Azaleas can be evergreen or deciduous. There are also deciduous rhododendrons such as R. mucronulatum and R. dauricum, but one can always tell they are not azaleas due to the presence of scales on the leaf surface.
You might be wondering, what about Rhododendron albiflorum? It is deciduous, lacks scales, but has 10 stamens. R. albiflorum is the sole member of subgenus Candidastrum and is considered a rhododendron, not an azalea. I assume botanists decided that having the blooms along the stem is not a typical characteristic of azaleas!
There you have it. The ultimate answers for the differences between rhododendrons and azaleas. When I googled this question, I was surprised that there was so much misinformation out there. It is a good reminder that you can’t trust everything on the internet!
R. mucronulatum foliage w
Japanese three leaved aza
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