Rhododendrons' Classification

Rhododendrons belong to the Ericaceae or erica family. Other familiar members of this family include the heaths and heathers (Erica and Calluna) mountain laurel (Kalmia), lily-of-the-valley shrub (Pieris), cranberry (Vaccinium) , Leucothe and Andromeda. Almost all of the ericaceous genera make good garden plants. Several other genera are often associated with rhododendrons but they do not belong in the same family: Camellia is in the Theaceae, Daphne in the Thymalaeaceae and Mag- nolia in the Magnoliaceae.

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With so many species discovered over several centuries it is perhaps not surprising that the genus Rhododendron has been split into more manageable groups and revised several times. Currently the genus is divided into eight subgenera:

  • Rhododendron
  • Hymenanthes
  • Pentanthera
  • Tsutsutsi
  • Azaleastrum
  • Candidastrum
  • Mumeazalea
  • Therorhodion

The first four of these subgenera contain by far the bulk of the species - the entire genus but for five species. Of the last four Candidastrum, Mumeazalea and Therorhodion each include only one species (R. albiflorum, R. semibarbatum and R. camtshaticum respectively) while Azaleastrum includes two species (R. ovatum and R. stamineum).

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Subgenera Rhododendron and Hymenanthes include the plants that gardeners recognize as the "true" rhododendrons. Some of these, the lepidote rhododendrons, have small scales on their leaves and make up the subgenera Rhododendron. Elepidote rhododendrons, those without leaf scales, form the subgenera Hymenanthes. Subgenus Pentanthera covers the deciduous azaleas and Tsutsutsi the evergreen azaleas.

Some of the subgenera are divided into sections, which are further divided into subsections. There are also groupings known as alliances and aggregates composed of very closely related species.

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Species names

Biological classification is based on the Linnean system of binomial (two names) nomenclature. Binomial names are also known as proper, scientific or Latin names. Latin (and to a lesser extent Greek) has always been the language of scholars and scientists and is very useful because it is an international language without borders.

The genus, the first name, is a grouping of closely related plants that share certain characteristics. The species, the second name, is a single plant type within the genus. So there can be many rhododendrons but only one Rhododendron yakushimanum.

To qualify as a species a plant must be genetically stable and capable of reproducing true to type from seed. Natural or artificial hybrids, mutants and selected forms are not regarded as new species because they cannot naturally replicate themselves. If they are capable of natural replication yet can still interbreed with the species they may be considered subspecies: naturally occurring, self-perpetuating variations. Subspecies (the name is often abbreviated to ssp or subsp) are usually geographical variations that occur after a population has been isolated for long periods. The genera and species are the last links in a long series of divisions and subdivisions. For most purposes simple identification by genus (Rhododendron) and species (forrestii) is adequate (remember that azaleas are part of the genus Rhododendron too).

Variety and cultivar names

Hybrids or unusual forms may occur naturally, and once a plant enters cultivation it is almost certain to be hybridized or developed in some way. Hybrids and cultivated forms fail the first test of a species -they cannot reproduce true to type from seed -so they must be classified in some other way.

Four terms are commonly used to describe these plants: variety (correctly varietas), cultivar, hybrid and clone.

All garden plants are commonly called varieties, but the botanical definition is more precise. A variety is a naturally occurring variation of a species, expressed as the abbreviation var, as in Rhododendron forrestii var tumescens. When cultivated it may also be known as a selected form. Forma is a term similar to variety but usually differs from the species in less botanically important details.

Cultivar is a contraction of cultivated variety and refers to plants, occurring either naturally or in cultivation, that are not capable of reproducing naturally and which must be perpetuated by vegetative propagation.

Hybrids are the result of crossing two plants of different botanical classification. That most commonly means two different species within the same genus, though hybrids between genera are also possible -with considerable limitations. When two hybrids are themselves crossed they no longer fully fit the description of a hybrid or a cultivar, so the term culton was instituted. Although a useful term, it is hardly ever used and such plants are usually referred to as cultivars. Clones are vegetative replicas of the original cultivar or hybrid. As most garden shrubs, they are not reproduced by repeatedly crossing the same parent strains, the term is used somewhat interchangeably with cultivar and hybrid.

What is an azalea?

Strictly speaking there is no such thing; all azaleas are rhododendrons. Initially the azaleas were classified separately from the rhododendrons but with time it became clear that the division was artificial.

The azaleas form two of the eight rhododendron subgenera. The deciduous azaleas make up the subgenera Pentanthera and the evergreen azaleas are classified under Tsutsutsi.

Obviously, deciduous azaleas drop all of their foliage in the fall. But that doesn't mean that evergreen azaleas retain their foliage. While they hold most of their leaves through fall, by the end of winter they can be almost bare, especially in cold climates. Because "evergreen" azaleas can shed much of their foliage, botanists prefer the term persistent-leaved to evergreen for these plants.

Evergreen azaleas have two distinct types of foliage, thus they are dimorphic. Look closely at the foliage of an evergreen azalea as it develops through the growing season and the two forms will be readily apparent. The new growth that develops in spring is light in texture and quite a bright green. These leaves last through summer but begin to drop in the fall and carry on dropping through winter. The second flush of new growth that develops in summer and early fall is of a heavier texture, is darker green and tends to persist through winter.


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