Azaleas and rhododendrons are perennial shrubs commonly found in evergreen gardens. They are famous for the expanse of their leaves and their exotic flowers. Both azalea and rhododendrons have been classified as ‘Rhododendron’ according to the conventional taxonomical method. As a result, there is some confusion between these two shrubs.
The word ‘rhododendrons’ is generally used to refer to either of these. Apart from the similarity in their names, they also share many similar requirements such as the pH of the soil and the moisture level. This rule of thumb is used by botanists to differentiate azaleas—all azaleas can be classified as rhododendrons but not vice-versa.
Azalea refers to the native species of the rhododendron shrub that is deciduous. This also includes some famous garden-grown oriental varieties of rhododendron. Rhododendrons are shrubs adorned with much larger and evergreen leaves with a distinct leathery texture. Apart from the nomenclature, some identifying features can be used to differentiate azaleas from rhododendrons.
According to the new classification updates of the Balfourian System, besides being classified as a rhododendron, azalea has been grouped under the subgenus pentanthera (to indicate its deciduous nature) and subgenus tsutsusi (to indicate its evergreen foliage).
Azaleas have typical features in the form of small hair-like structures. These hair are found growing parallel to the leaf’s surface. This feature can be clearly seen on the midrib of the leaf, i.e. along its underside. The evergreen variety of azaleas has a visible hair growth pattern. However, rhododendrons don’t have hair-like growth but they do have small, scaly dots, found underneath the leaf.
Some rhododendrons may have oblong spots that may be confused with sticky, thick hair-like projections but an azalea leaf will never be dotted or have scales. The commonest varieties of azaleas are deciduous, i.e. they shed their leaves in the winter season. Except R. dauricum and R. mucronulatum, all rhododendrons are evergreen.
The flower size and flowering patterns of these shrubs are very different. Azaleas have slightly smaller and slender, funnel-shaped flowers. The flowers of true rhododendrons are bell-shaped and much bigger. Azaleas tend to have just one flower per stem, known as terminal blooming. Most garden rhododendrons have their flowers in small batches, in a clustered formation.
Azaleas are more seasonal in nature, in terms of flowering, i.e. their flowers have a particular time for blooming. Garden azaleas tend to have the best foliage during the onset of the spring season. Except for a few varieties, rhododendrons tend to maintain some degree of flowering throughout the year with some periods of dense flower sprouting.
Some azalea plants are known to be toxic to herding animals. No rhododendrons are known to be toxic.
Most azaleas will have just one stamen on each of the flower lobes. Since they have five lobes in total, these shrubs characteristically have five stamens. Rhododendrons, however, tend to have two or more stamens per lobe. Thus, their total stamen count is 10 or more.