QBARS - v10n4 The Different Types of Rhododendrons and Their Names

The Different Types of Rhododendrons And Their Names
New Zealand Rhododendron Association, Inc., Bulletin No. 16


Each distinct type of wild rhododendron is called a species (a word which is the same in both singular and plural; to use the word "specie" for the singular is quite incorrect). The well known Rhododendron arboreum is a good example. In practice, if we are obviously writing about rhododendrons, the word Rhododendron used with the name of the species is usually shortened to "R." Thus we write R. (occasionally "Rh.") arboreum . There are about 1,000 species of rhododendrons described in the book "The Species of Rhododendron." A convenient book in which to read brief descriptions of the species is the "Rhododendron Handbook" published each five years by the Royal Horticultural Society (R. H. S. for short). The next publication is due for 1957.


Most of the species, if grown from seed, show differences between different seedlings. For instance, R. arboreum seedlings usually have red flowers, but some plants have white flowers. The white flowered plants are called a variety of the regular type, and the full name is then written: " R. arboreum variety album " (the word "album" being Latin for "white"). Generally the word "variety" is abbreviated or omitted so that the name would be written: " R. arboreum var. album " or " R. arboreum album ." It should be noted that the name of the species " arboreum " ("a tree") and " album " ("white") are both written in Latin, because that is the international language.

Names of Cultivated Forms of Rhododendrons

To the naming of the wild species and varieties of plants, as described above has recently been added a new international code for naming of cultivated plants. This new code is not concerned with the wild species and varieties as mentioned above; but only with the naming of new types such as sports and hybrids, which have arisen under cultivation. An account of the new scheme is given in the R. H. S. Journal, May 1956, page 157.


The new name "cultivar" has been coined (apparently a telescoped form of "cultivated variety") to describe a variety which has arisen under cultivation or which is common in cultivation but occurs so rarely in the wild state that it does not merit being made a botanical variety. An example of a cultivar is: R. arboreum 'Mrs. Henry Shilson'. From 1953 onwards it is recommended that the cultivar or "fancy" name should not be in Latin, and that it should be written with inverted commas.

The term "cultivar" is to be used only for one original plant and for plants propagated vegetatively (i.e. by cuttings, division, layers, budding or grafting) from it. A cultivar maybe a selected form of a hybrid such as R. X Loderi 'King George'.

Hybrid Cultivars

It will be seen that a hybrid name may cover a very wide range of good and poor types, because thousands of seedlings may be raised as a result of crossing two species by several different breeders. For instance, R. X Loderi was raised by crossing R. auklandii X R. fortunei , and all plants raised by anyone from crossing these species must be called R. X Loderi. Any particularly good plants of R. X Loderi must be given a "cultivar" name so that all plants layered or grafted from it can be identified, e.g. R. X. Loderi 'King George' or R. X Loderi 'Irene Stead'" or R. (Scarlet King) 'Orchard'. It cannot be too strongly emphasized to rhododendron growers that they should be very clear and very careful on this point. The first breeder to make across such as X Loderi may have gone to much trouble to select particularly good types of the two parent species to make the cross. He may have raised some hundreds of the hybrid seedlings from which he selected a few of the best plants to give them cultivar names such as 'Venus', 'King George' and the like. Any other breeder may repeat the same using poor specimens of R. auklandii and R. fortunei as parents, yet he is not only entitled, but compelled to call all of the seedlings R. X Loderi. This sort of thing is being done very extensively. There is no harm in the practice if buyers are told that they are buying such seedlings. It may be that some of the plants will prove to be better than those originally raised under the name; the whole thing depends on the skill and integrity of the man or organization raising the seedlings.

One practice which appears quite wrong is frequently employed by some English Rhododendron nurseries. A certain hybrid such as R. X 'Elizabeth' may include a plant to which the R. H. S. has awarded its First Class Certificate (F.C.C.) or Award of Merit (A.M.). Some nurseries raise large batches of seedlings by repeating the same cross, and advertise these plants as R. X 'Elizabeth' F.C.C. This leads gardeners to think they are buying plants propagated vegetatively from the one which received the F.C.C. Award.

Buyers should therefore always try to buy hybrids which have a cultivar name. That is to say, do not buy simply R. X Loderi, but R. X Loderi 'Venus' or 'King George' or 'Irene Stead.'  Strictly speaking, if you buy any Rhododendron which has the letters "A.M." or "F.C.C." after it in the catalogue or list, you should get only that cultivar, whether it has a cultivar name or not. For instance, the hybrid R. X 'Naomi' has the forms: R. X Naomi A.M.; R. X Naomi 'Pink Beauty'; R. X Naomi 'Stella Marris' F.C.C.; R. X Naomi `Exbury Form'.

Where species of rhododendron are concerned, most growers are satisfied if they buy seedlings of these species raised from a good type of parent plant. It must, however, be recognized that the seedlings of species include some which are better than others. Using R. arboreum as an example again, there is one particularly good bush at Ilam which for convenience may be called R. arboreum 'Ilam form.' Only plants propagated vegetatively from this one are entitled to be described under the name of this cultivar.

Note that botanical varieties are not cultivars. For instance, R. arboreum var. album or R. nuttallii var. stellata are botanical varieties which can fairly be raised from seed and sold as those varieties. If someone finds an extra good seedling of R. arboreum album , he is quite at liberty to call it R. arboreum album 'Purity' or any such cultivar name.

New members of the Association are often puzzled as to whether any given rhododendron is a species or a hybrid. The Rhododendron Handbook will provide the answer. If the name of the rhododendron in question is not found in the list of species or in the list of synonyms, it can safely be assumed that the plant is a hybrid. The handbook has lists of hybrids and the name in question is likely to be found there. Some of the old hybrids like R. 'Fragrantissimum' are often but mistakenly thought of as species. It would be much simpler if the sign for a hybrid (an "X") were placed before the second part of the name (e.g. "R. X Fragrantissimum"). In practice this is rarely done, though strictly speaking it should be done.

The Main Groups of Rhododendrons

There used to be two separate genera-Azalea and Rhododendron-but these have both been merged into one and called Rhododendron. This was done because there was not a clean cut distinction between Azalea and Rhododendron.

Series and Sub-series

Since about 1,000 species of Rhododendron are known, it is convenient to sub-divide this large group into smaller groups for reference. One series of rhododendrons is a group of maybe twenty or thirty species. For convenience we refer to the series by the name of one of the best known species it contains. For instance, one group is known as the Arboreum series, after R. arboreum which belongs to that group.

Sub-Series. The Series Arboreum contains about twenty species, which fall into two natural groups, one lot appearing closely related to R. arboreum , the others having more resemblance to R. argyrophyllum . These two smaller groups are called respectively the "Sub-series Arboreum," and the "Sub-series Argyrophyllum." For instance, R. delavayi belongs to the series Arboreum and sub-series Arboreum. R. ririei belongs to the series Arboreum, subseries Argyrophyllum. It all sounds very complicated, but once grasped it is simple. Knowing that any rhododendron belongs to a certain series and subseries gives a definite clue as to its main characteristics. It shows to which branch of the family the plant belongs.

Some of the main series of rhododendrons are:

The Azalea series, which contains six subseries. This group includes not only the parent plants from which the common evergreen azaleas have been bred, but also the rhododendron species which drop their leaves in winter.

The Grande series, a group of large leaved species of which R. grande is the best known. R. sinogrande and R. sidereum are among the species in this series.

The Maddenii series. This is a very large group of which R. maddenii is best known.

Lepidote and Elepidote Rhododendrons

The Maddenii series and several other series of rhododendrons are described as lepidote or scaly-leafed as compared with the elepidote or non-scaly groups. This scaliness is an easily observed character and a most important one in rhododendrons. The lepidote and non-lepidote rhododendrons are very distinct types of plant and this feature (scaliness or not) should be about the first thing to look for when trying to identify a rhododendron. The scales, to be seen easily, need to be examined with a magnifying glass, searching the underside of the leaves. The scales are like small round dots scattered over the lower side of the leaf and also over the young stems. Once you have seen them you should have no doubt about it.

The lepidote rhododendrons generally are not the popular type of variety, but include large numbers of smaller leaved sorts, mostly with white or pale-colored flowers, but including the best "blue" shades, and mauve, lavender and yellow. They mostly have smallish flowers, though some, like R. maddenii , R. nuttallii, R. dalhousiae , R. rhabdotum and others have large flowers and large leaves. R. cinnabarinum is one lepidote species which has red flowers. R. lochiae , the one Australian species of Rhododendron, is a red flowered lepidote species, and probably the tropical brightly colored Javanese rhododendrons are also lepidotes.

It is generally considered that lepidote rhododendrons cannot be crossed with non-lepidote species. Once or twice hybrids have been raised between these two groups ( R. griersonianum X R. dalhousiae ) but in general it is useless to try.

NOTE: The rule in writing the botanical name of a species is that the species name begins with a small letter, not a capital. Thus we should write " R. griersonianum ," not " R. Griersonianum ." It used to be the rule that the name of the species could begin with a capital letter, if it was named after a person or place. Many species of rhododendron are called after people and for that reason it has in the past been customary to use the capital letter. Strictly speaking, the small letter should now be used.