JARS v39n4 - When Is a Rhododendron an Azalea?

When Is a Rhododendron an Azalea?
P.G. Valder
New South Wales, Australia
Reprinted from Australian Rhododendron Society Journal

"Look at that lovely blue azalea", they say, "I've never seen one that colour before." How on earth can one begin to explain that Rhododendron augustinii , or 'Blue Diamond' or whatever it might be, isn't an azalea. It certainly looks like one and bears little resemblance to most people's concept of a rhododendron. Anyway what is an azalea? After all, the deciduous and evergreen azaleas look as different from one another as they do from most rhododendrons. Unfortunately there isn't an easy answer, so you must either stop reading now or be prepared to put up with a bit of botanical history.

In the 18th century the Swedish botanist Linnaeus devised a system of classification of plants based on, among other things, the number of stamens. This system of classification, which turned out not to be a good one, was known, rather awkwardly, as Linnaeus's Sexual System. Following this system he divided the rhododendrons known to him between two genera. In his genus Rhododendron he placed the species with ten stamens ( R. ferrugineum , R. hirsutum , R. dauricum and R. maximum ). In the genus Azalea he placed those with five stamens ( A. indica , A. pontica , A. viscosa , A. lutea , A. lapponica , and A. procumbens ). Of these the first is an evergreen azalea, the next three are deciduous azaleas, the fifth is a scaly rhododendron that happens to have only five stamens, and the last is a prostrate shrub from Lapland, which is now not included in Rhododendron at all called Loiseleuria procumbens . This is ironic because Linnaeus based the genus on this plant, deriving the name Azalea from the Greek word "azaleos", meaning "dry", in allusion to its occurrence in dry places.

In spite of the fact that botanists have transferred Linnaeus's species of Azalea (other than A. procumbens ) to the genus Rhododendron, where they properly belong, the name has stuck. As a result this distinctive group of rhododendrons are kept apart by gardeners and often by nurserymen.

Unfortunately this is a little awkward since there are other equally distinctive groups. However what it amounts to is that all azaleas are rhododendrons but only some rhododendrons are azaleas.

Well then, I can hear you asking, how do you decide which is which? The number of stamens certainly isn't a reliable guide. In fact telling the difference isn't at all easy.

First of all you should look at the leaves, particularly the under-surfaces, with a microscope or magnifying glass and see whether or not scales are present. If they are present then it is not an azalea but one of the 600 or so scaly species, none of which fits the popular concept of a rhododendron either. R. augustinii and most other azalea-like rhododendrons have scales.

If scales are absent then it is either an azalea or an "ordinary" rhododendron. From this point on you can usually tell the difference by using commonsense. But if you really want to go all botanical then you'll need a microscope to look at the hairs these plants usually bear. The "ordinary" rhododendrons produce some hairs which branch, the azaleas never do.

Whereas most of the "ordinary" rhododendrons bear a considerable similarity to one another, this is not true of the azaleas, a group into which have been placed unrelated types. The deciduous azaleas seem very distinct from the evergreen azalea for instance. And there are some odd subtropical rhododendrons which seem more closely related to the azaleas than to other types of rhododendron. It will be some time before the botanists get the whole thing sorted out.

In the meantime you won't be far wrong if you recognise four main groups within the genius Rhododendron - ordinary, non-scaly rhododendrons (e.g. R. ponticum , 'Pink Pearl' etc.), tropical scaly rhododendrons (the vireyas, e.g. our own R. lochiae ), "ordinary" scaly rhododendrons (e.g. R. augustinii , R. nuttallii and the azaleas. And if you are a hybridist you will find that, while crosses between species within each of these groups are often successful, crosses between species from different groups rarely are. So it will probably be some time yet before we see a real azalea which is blue.

Within the azaleas themselves crosses between distinct types are rarely successful either. As a result breeders aiming to produce yellow evergreen azaleas have been making slow progress too. Even so there have been crosses between deciduous azaleas and "ordinary" rhododendrons which have produced useful garden plants. These are the so-called "azaleodendrons", of which 'Broughtonii Aureum' is a well-known example. These, however, seem to be sterile and thus have not been used in further breeding.

Doctor Valder is Senior Lecturer in Biology at the University of Sydney, Sydney, Australia.