JARS v53n3 - What Are Those Azaleas?

What Are Those Azaleas?
Donald H. Voss
Vienna, Virginia

This tale concerns names used for two commonly grown azaleas. Most azalea fanciers would identify a cultivated evergreen azalea with narrow strap-like petals as Rhododendron linearifolium , some would respond R. macrosepalum var. linearifolium , and a very few might say R. stenopetalum . For a certain cultivated Japanese evergreen azalea with white flowers, most would probably respond R. mucronatum , while others might say R. ripense var. mucronatum or just cultivar R. 'Mucronatum'.

Each of the azaleas mentioned is a cultivated "garden-form" plant, and each has been associated with a species found in the wild. When first named, these plants were accorded species status: the strap-petaled azalea as R. linearifolium Sieboid & Zuccarini and the white-flowered plant as Azalea mucronata Blume [= R. mucronatum (Blume) G. Don]. Ties to other taxa were not recognized for many decades.

The nomenclatural changes shown in Tables 1 & 2 reflect taxonomic decisions on classification of the plants. Such decisions involve comparative studies of the plants' morphology, habit, provenance, etc. - possibly seasoned with a dollop of subjective preference. Some of the nomenclatural questions encountered with respect to these plants include:

• What if, in a given genus, the same epithet is applied more than once at a given rank, say that of species? (A botanical name usually comprises the genus name plus a specific epithet, followed by the name[s] of the author[s])

• What if a name is published but is not apprehended by the botanical community (does a tree's falling in the forest make a sound if no one is there to hear it)?

• What specific epithet is chosen if two species are merged?

•  What if a plant is determined to be a garden form unknown in the wild?

• What if a plant bearing a specific epithet is determined to be an interspecific hybrid?

Answers and guidance relating to these questions are found in two codes governing the naming of plants. The following sections summarize several of the rules and then discuss their application in the nomenclatural history of the azaleas in question.

Nomenclatural Rules and Their Application

In his text on taxonomy, Lawrence (1951) stated that there can be no valid distinction between taxonomy of indigenous plants and cultivated plants. In practice, this view has not been accepted universally. The research objectives of many botanists do not engage them in detailed consideration of cultivated plants and in attention to the literature in which names for many such plants are published.

Before a separate code for the nomenclature of cultivated plants was instituted in 1953, Latin-form botanical names were the only ones suitable for accurate communication of the identity of a plant. Subsequently, cultivar names became available. A given plant may have many vernacular (common or colloquial) names, differing from place to place - or even in the same locality. The correct botanical or cultivar name for a given plant, in contrast, is unique and has universal application.

Rules . - Two codes govern the formation of plant names, the International Code of Botanical Nomenclature 1994 (Botanical Code; Greuter etal., 1994) and the International Code of Nomenclature for Cultivated Plants (Cultivated Plant Code; Trehane etal., 1995). Many considerations enter into the choice of new botanical and cultivar epithets. Among them are:

• The rule of priority in the Botanical Code, which states that "the nomenclature of a taxonomic group is based upon priority of publication...Each taxonomic group...can bear only one correct name, the earliest that is in accordance with the Rules."

• In applying the rule of priority, the Botanical Code specifies (with certain exceptions): "For any taxon below the rank of genus, the correct name is the combination of the final epithet of the earliest legitimate name of the taxon in the same rank, with the correct name of the genus ..."

• The Botanical Code provides: "Plants brought from the wild into cultivation retain the names that are applied to the same taxa growing in nature...nothing precludes the use for cultivated plants of names published in accordance with the requirements of the present [Botanical] Code."

• The Botanical Code also provides that certain groups of plants may appropriately receive cultivar epithets under the Cultivated Plant Code.

• The Cultivated Plant Code notes that the Botanical Code governs Latin-form names for both cultivated and wild plants. However, "Distinguishable groups of cultivated plants, whose origin or selection is primarily due to the intentional actions of mankind [emphasis added], are to be given [cultivar] epithets formed according to the Rules and provisions of this [Cultivated Plant] Code." The "intentional actions" include deliberate hybridization or accidental hybridization in cultivation, selection from cultivated stock, or selection from wild-plant variants maintained solely by continued propagation.

• Different rules may now apply, according to the origin of plant material -

- Species based on wild material

- Hybrids found in the wild

- Distinguishable groups of cultivated plants - due primarily to intentional actions of mankind.

The Botanical Code provides that a plant of hybrid origin may be - but need not be - given a Latin-form hybrid name (either a hybrid epithet or a hybrid formula). The hybrid name applies, however, to all plants derived from representatives of the specified parent taxa. For hybrid plants within the purview of the Cultivated Plant Code, a cultivar epithet is appropriate. In this way, individual hybrid clones may receive distinctive names. Note that the multiplication sign (x), which precedes a Latin-form botanical hybrid specific epithet or genus name, is not to be used before cultivar epithets for hybrids.

The following statement in the preamble of the Botanical Code also bears on the name changes under discussion: The only proper reasons for changing a name are either a more profound knowledge of the facts resulting from adequate taxonomic study or the necessity of giving up a nomenclature that is contrary to the rules.

Subjectivity . - Determining the correct name for a plant often calls for resolution of conflicting interpretations arising in the application of nomenclatural rules. Differences also arise with respect to taxonomic judgments affecting the classification (and, consequently, nomenclature) of plants.

Botanists must consider not only the origin and nature of plant material being named and the publication dates of the names, but also whether media in which the names are published meet code requirements for effective publication. This can be a contentious issue.

With the flood of exotic plant introductions into Europe in the 1800s, many new plant names appeared first in nursery catalogs. According to Mabberley, these (as well as less ephemeral horticultural publications) have been considered by some botanists as "obscure, a word botanists tend to trot out when anything but a mainstream botanical journal is being considered...widely circulated books, often best sellers, have been disregarded, one suspects partly through botanical snobbery" (Mabberley, 1990).

Priority is determined by the date of publication. Subsequent to publication, however, other botanists' recognition of a name and its claim to priority necessarily depends on their being aware of the publication. And the subjectively delimited scope of their bibliographical research constrains awareness. Mabberley attributes failure of later botanists to adopt Hogg's Azalea stenopetala (and other names) to their failure to search horticultural publications for new plant names.

Interpretations of the priority rule are sometimes Byzantine. For example, one botanist may believe that distribution to a few colleagues of printed pages containing an encyclopedia article prior to issuance of the bound volumes establishes the date of publication, but others may differ.

Disagreement as to the legitimacy of a published name may affect application of the priority rule. A case of interest to horticulturists is that of the Japanese deciduous azalea, Rhododendron japonicum (A. Gray) Suringar (Brummitt, 1987). In sorting out a nomenclatural problem in the genus, one botanist denied the legitimacy, hence priority, of Suringar's combination on the basis that Suringar had only "proposed" it. Thus, it was asserted, the name R. japonicum (Blume) Schneider is the legitimate name for the broad-leaved rhododendron familiar in horticulture as R. metternichii (an illegitimate name).

An interested group (including deciduous azalea expert and former U.S. National Arboretum director Henry Skinner) petitioned for rejection of R. japonicum for the broad-leaved rhododendron. (Rejection of names is a procedure under the Botanical Code; it sometimes can take years.) The petition cited the long-standing use of the name for the azalea and the disruption and cost that the name change would impose on the horticultural community. The responsible committee sidestepped the issue of rejection by rereading Suringar and deciding that his words constituted more than a proposal - and thus that his combination is legitimate. Rhododendron japonicum survives today in the infraspecific epithet for the Japanese deciduous azalea, R. molle subsp. japonicum (A. Gray) K. Kron.

Circumscription of taxa at different ranks (for example, species and infraspecific taxa) may depend not only on botanists' views on the relative importance of various plant characteristics but also on personal proclivities that separate "splitters" from "lumpers." One individual may find that color or color-pattern differences call for the naming of new varieties or forms, while another may see these differences as merely expressions within the bounds of a species. The annals of botanical nomenclature, as well as those of ornamental horticulture, are littered with names applied by splitters and later abandoned by lumpers.

Where botanists deem minor variants within a taxon, say a species, to be sufficiently important, they provide infraspecific epithets to recognize variant groups. The ranks of varietas and forma are most often encountered in this context. For example, Wilson (1921) cited the following formae under R. linearifolium var. macrosepalum Makino:
f. dianthiflorum Wilson
f. rhodoroides Makino
f. decandrum Wilson
f. hanaguruma Makino

Beginning in 1959, cultivars have been given epithets in a modern language ("fancy names"). Earlier Latin-form epithets applied to taxa comprising cultivated plants may be converted into cultivar epithets without change. For example, Rhododendron linearifolium var. macrosepalum f. rhodoroides Makino is designated R. 'Rhodoroides' in the Rhododendron Handbook - 1998 (Argent, etal., 1997).

The use of cultivar names for distinguishable variants is recognized under both codes. Disagreements persist among botanists and between botanists and horticulturists with respect to the naming of these variants. Some believe that in certain circumstances Latin-form epithets are a desirable alternative - or, indeed, supplement - to the use of cultivar epithets. The situation may be further complicated by options for registering cultivar names with an international registration authority, by the availability of trademarking (see Voss, 1997) and, in some cases, by statutory provisions for registration.

Paths to Cultivar Status

In the cases examined here, the paths started with garden forms never documented in the wild. Each was designated a species. Later, taxonomic observations led to classification decisions linking the garden plants to wild species or hypothesizing hybridity. Nomenclatural changes ensued (Tables 1 & 2). Not included in the discussion are some familiar synonyms (such as "Indicum album" and "LedifoIium album" for R. 'Mucronatum') and names crafted for minor variants of horticultural interest.

'Linearifolium'. - Although Poiret had published the name R. linearifolium in 1804, the application of his name is obscure (see sidebar). Chamberlain etal. (1996) indicated that the Poiret taxon is "unplaced" as to subgenus. Although this implies that the identity of Poiret's plant is not known, his prior publication of the name in genus Rhododendron at the rank of species precludes - according to the priority rule in botanical nomenclature - use of Siebold & Zuccarini's linearifolium as the specific epithet for the strap-petaled azalea. J. D. Hooker (apparently unaware of Poiret's R. linearifolium ) preferred to keep azaleas in a separate genus, Azalea , and he published Azalea linearifolia (with R. linearifolium Siebold & Zuccarini cited as a synonym) in Curtis's Botanical Magazine (Hooker, 1869). The accompanying illustration showed the familiar plant with narrow leaves and narrow strap-like petals. Although illegitimate as an epithet at the rank of species (because of the priority of Poiret's epithet), linearifolium has provided the familiar varietal and cultivar epithets for the strap-petaled azalea.

In 1908, Makino, a Japanese botanist, determined that R. linearifolium Siebold & Zuccarini is closely related to and should be considered a variety of R. macrosepalum , the large-sepal azalea. (A common plant in central Japan, R. macrosepalum had been named by the Russian botanist Maximovich in 1870.) Several years later, probably from consideration of priority of publication, Makino switched to linearifolium as the specific epithet. In 1918, Komatsu reverted to Makino's 1908 R. macrosepalum var. linearifolium , a change consistent with the illegitimate status of the Siebold & Zuccarini name.

Wilson (1921) pointed out that this strap-petaled plant is a rare monstrous garden form from Japan, unknown in the wild state. Some association with the wild plant named by Maximovich continues to the present - with important modifications. The first of these was noted in the Rhododendron Handbook - 1980 (Leslie, 1980). The Latin-form varietal epithet was dropped, reappearing as 'Linearifolium', a cultivar epithet. Leslie's comment that " R. linearifolium is apparently unknown in the wild and is now regarded as a cultivar" may imply that more adequate taxonomic study revealed the cultivar to be a distinguishable group of cultivated plants resulting primarily from the intentional actions of mankind.

Chamberlain & Rae (1990) did not include the strap-petaled azalea in their list of species. They speculated, however, that " R. macrosepalum may be the wild species from which R. linearifolium is derived," accepting the relationship posited by Makino.

Mabberley (1990) unearthed the name Azalea stenopetala , published in the Gardener's Yearbook (Hogg, 1865), leading to a further change on the basis of priority. The specific epithet stenopetala is certainly appropriate for R. 'Linearifolium': from the Greek, stenos = narrow, petalon = leaf, petal (Brown, 1956). Considering this plant to be conspecific with R. macrosepalum Maximovich (1870), Mabberley replaced R. macrosepalum with R. stenopetalum (Hogg) Mabberley. The Gardener's Yearbook contained many new names gathered in part from nursery catalogs of the era. In his article, Mabberley states that the names he cites from this source were accompanied by "reasonable descriptive matter."

Chamberlain etal. (1996) accepted R. stenopetalum (Hogg) Mabberley but retained the cultivar epithet 'Linearifolium'. The Rhododendron Handbook - 1998 lists R. stenopetalum (Hogg) Mabberley (incl. R. macrosepalum Maximovich). It states that Siebold & Zuccarini's R. linearifolium is equivalent to the type of R. stenopetalum and is an aberrant plant found only in cultivation. It follows Mabberley in not providing separate status for 'Linearifolium'.

Many Rhododendron taxonomists now regard the strap-petaled azalea as 'Linearifolium', a cultivar. The combination R. stenopetalum (based on Hogg's 1865 publication of A. stenopetala ) has been accepted as the correct name for the species, displacing the later-published R. macrosepalum .

'Mucronatum'. - It is interesting to note some similarities and differences between the cases of R. 'Linearifolium' and R. 'Mucronatum'. The latter - a familiar white-flowered plant long considered a garden form - was named as a species by Blume in 1826. Nearly a century later, Wilson (1921) observed the plant known as R. mucronatum (Blume) G. Don in Japanese gardens and studied herbarium specimens at Kew and Arnold Arboretum. He concluded that it is a rare, albino form (unknown in the wild) of the lilac-purple wild azalea named R. ripense by Makino in 1908. Placing the two forms in one species, Wilson retained the earlier specific epithet mucronatum and published the new combination R. mucronatum var. ripense for the wild plant.

The Rhododendron Handbook - 1980 regarded the white-flowered form as R. 'Mucronatum', a hybrid cultivar of uncertain parentage. Yet Chamberlain and others (1990, 1996) continued to view the white-flowered form as R. mucronatum var. mucronatum (with the notation "only known in cultivation") and the lilac-purple form as R. mucronatum var. ripense .

Although in their revision of R. sub-genus Tsutsusi , Chamberlain & Rae (1990) considered both the white and lilac-purple varieties as members of the same species - R. mucronatum - their decision embraced an element of uncertainty: "Var. mucronatum is the widely cultivated white form of the species. It may [emphasis added] occur in the wild as the albino form of var. ripense ."

The Rhododendron Handbook - 1998 (Argent etal., 1997) refers to the white-flowered plant as a presumed artificial hybrid derived from R. ripense and R. stenopetalum . It reverses past treatment, specifying Makino's R. ripense as the inclusive species and following the 1980 Handbook in designating the white-flowered form as a cultivar: R. ripense 'Mucronatum'.

Table 1 . A Nomenclatural History of Rhododendron stenopetalum
Petals strap-like Corolla broad funnel-shaped
Poiret (1804) R. linearifolium (unplaced to subgenus)
(later use of linearifolium refers to the plant described by Siebold and Zuccarini, not that of Poiret)
Siebold & Zuccarini (1846) R. linearifolium (illegitimate name)
Hogg (1865) A. stenopetala
Hooker, J.D. (1869) A. linearifolia
Maximovich (1870) R. macrosepalum
Makino (1908) R. macrosepalum var. linearifolium
Makino (1913) R. linearifolium var. linearifolium R. linearifolium var. macrosepalum
Komatsu (1918) R. macrosepalum var. linearifolium
Leslie R. macrosepalum 'Linearifolium'
(notes that "R. linearifolium is apparently unknown in the wild and is now regarded as a cultivar")
R. macrosepalum
Chamberlain etal. (1996) (No entry; but suggests this form derived from R. macrosepalum) R. macrosepalum
(syn.: R. linearifolium Siebold & Zucc. var. macrosepalum (Maxim.) Makino)
Mabberley (1990) R. stenopetalum (Hogg) Mabberley
(syn. in part: R. macrosepalum 'Linearifolium'; R. linearifolium Siebold & Zucc, not Poiret)
R. stenopetalum (Hogg) Mabberley
Chamberlain etal. (1996) R. stenopetalum (Hogg) Mabberley 'Linearifolium' R. stenopetalum (Hogg) Mabberley
Argent etal. (1997) R. stenopetalum (Hogg) Mabberley
(incl. R. macrosepalum Maxim.)
(syn. in part: R. linearifolium Siebold & Zucc; equivalent to type of R. stenopetalum )
R. stenopetalum (Hogg) Mabberley
(incl. R. macrosepalum Maxim.)
(syn. in part: R. macrosepalum Maxim. excluding 'Linearifolium')

Table 2. A Nomenclatural History of Rhododendron mucronatum (Or Is It R. ripense ?)
Corolla white Corolla pale lilac-purple
Blume (1826) A. mucronata
Don, G. (1834) R. mucronatum (Blume) G. Don
Makino (1908) R. ripense
Wilson, E.H. (1921) R. mucronatum G. Don [var. mucronatum ] R. mucronatum var. ripense (Makino) E.H. Wilson
Leslie, A.C. (1980) R. 'Mucronatum'  (hybrid of uncertain parentage)
("only known in cultivation... white form of the species.
It may occur in the wild as the albino form of var. ripense ")
R. ripense
("widely cultivated, with many distinct
garden forms...[including var. mucronatum ]")
Chamberlain & Rae (1990) R. mucronatum var. mucronatum R. mucronatum var. ripense (Makino) E.H. Wilson
Chamberlain etal. (1996) R. mucronatum var. mucronatum R. mucronatum var. ripense (Makino) E.H. Wilson
Argent et al. (1997) R. ripense 'Mucronatum' (Blume) G. Don
(as syn. of: A. mucronata Blume;
a presumed artificial hybrid between R. ripense and R. stenopetalum )
R. ripense Makino

The Bottom Line

It is evident that, with respect to both pairs of plants (Tables 1 & 2), botanists' views on the status of the garden forms have differed over the years. Explanations for the resulting nomenclatural changes reflect both taxonomic decisions and the application of nomenclatural rules. In addition, individuals' interpretations and preferences may have been factors.

The Rhododendron cultivar epithets 'Linearifolium' and 'Mucronatum' began their nomenclatural "lives" as Latin botanical epithets designating species. Both were long known as garden forms derived from wild plants. As late as Chamberlain etal. (1996), distinctions were preserved in their respective species for the garden forms - R. stenopetalum 'Linearifolium' and R. mucronatum var. mucronatum .

The Rhododendron Handbook - 1998 approaches each case differently. On the basis of priority, the aberrant plant that began as R. linearifolium Siebold & Zuccarini is placed (together with Maximovich's R. macrosepalum ) in R. stenopetalum - and, indeed, provides the type for the species. This treatment does not, however, explicitly mention the strap-petaled plant as cultivar 'Linearifolium'.

The plant that started its nomenclatural journey as A. mucronata [= R. mucronatum ] appears in the Rhododendron Handbook - 1998 as the cultivar R. ripense 'Mucronatum'. The Handbook authors note that 'Mucronatum' is a "presumed artificial hybrid between R. ripense and R. stenopetalum ."

These Rhododendron Handbook - 1998 treatments are puzzling. For R. 'Linearifolium', though we are told that it is an aberrant cultivated plant, no epithet (botanical or cultivar) indicating a separate status is mentioned. The cultivar R. 'Mucronatum', in contrast, is placed in R. ripense though the plant is presumed to be of hybrid origin (Argent etal., 1997).

If 'Mucronatum' is a hybrid, then it must be something other than the species R. ripense ; but the Rhododendron Handbook - 1998 authors apparently are not crossing this bridge until hybridity is more certainly established. If it is not a hybrid but rather a variant form from the species comprising mainly the lilac-purple variety, the rule of priority calls for use of the earliest name, R. mucronatum , as was done in Chamberlain & Rae's revision (1990).

"What Are Those Azaleas?" The title of this article calls for answers, but honesty suggests equivocation. Meanwhile, the world of horticulture needs names for these plants. Until further refinement in classification is forthcoming, one solution is to call the strap-petaled azalea R. stenopetalum 'Linearifolium', recognizing the illegitimacy of Siebold & Zuccarini's species name R. linearifolium (the name was published earlier for a different plant by Poiret) and the priority of Hogg's specific epithet over Maximovich's macrosepalum .

The white azalea (long known as R. mucronatum ) may be designated simply R. 'Mucronatum' (Mucronatum Group). Closely related variants of the latter plant can receive similar treatment, for example: R. 'Liliiflora' (Mucronatum Group). Pending definitive determination of the origin of R. 'Mucronatum', perhaps by the techniques of molecular biology, the latter course of action has the merit of not assigning a presumed hybrid to a single species. We must in any case recognize that the affinities of these cultivars are problematical.

We do know that both of the garden-form plants discussed above belong to genus Rhododendron . Thus, they can be correctly designated as cultivars R. 'Linearifolium' and R. 'Mucronatum', recognizing that their existence today is attributable primarily to a century and a half of propagation - intentional actions of mankind in cultivation. Single quotation marks and italics aside, this brings us full circle (though stripped of taxonomic controversy) to the descriptive names used by Siebold and Zuccarini (1846) and G. Don(1834)!

Argent, George, etal. 1997. The Rhododendron Handbook - 1998: Rhododendron species in cultivation . London: Royal Horticultural Society.
Brown, Roland Wilbur. 1956. Composition of Scientific Words (rev. ed.). Washington, DC: Smithsonian Institution Press.
Brummitt, R.K. 1987. Report of the committee for Spermatophyta: 31 (Proposal 660). Taxon 36(1):72-73.
Chamberlain, D.F. & Rae, S.J. 1990. A revision of Rhododendron : IV subgenus Tsutsusi. Edinb. J. Bot. 47(2):113-114.
Chamberlain, David, etal. 1996. The Genus Rhododendron: its classification & synonymy . Edinburgh: Royal Botanic Garden Edinburgh.
Cox, Peter A. & Cox, Kenneth N.E. 1997. The Encyclopedia of Rhododendron Species . Perth, Scotland: Glendoick Publishing.
Galle, Fred C. 1987. Azaleas (rev. & enl. ed.). Portland, OR: Timber Press.
Greuter, W., etal. 1994. International Code of Botanical Nomenclature . Koenigstein, Germany: Koeltz Scientific Books.
Hooker, J. D. 1869. Curtis's Botanical Magazine . Third ser., Vol. 25, t. 5769.
Lawrence, George H.M. 1951. Taxonomy of Vascular Plants . New York: Macmillan Co.
Leslie, Alan. 1980. The Rhododendron Handbook - 1980: Rhododendron Species in Cultivation . London: Royal Horticultural Society.
Mabberley, D. J. 1990. The significance of the three independent 'Kew' editions of Johnson's Gardener's Dictionary." Feddes Repertorium 101(5-6):263-276.
Trehane, P., etal. 1995. International Code of Nomenclature for Cultivated Plants - 1995 . Wimborne, UK: Quarterjack Publishing.
Voss, Donald H. 1997. Cultivar epithets - again! The Azalean 19(2):28-31.
Wilson, Ernest Henry. 1921. The azaleas of the old world, (in Wilson & Rehder. A Monograph of Azaleas . Publications of the Arnold Arboretum, No. 9). Cambridge, MA: [Harvard] University Press.

The author expresses appreciation to Dr. Robert J. Griesbach and Dr. Joseph H. Kirkbride for instructive comments on drafts of this article. Errors and infelicities remain the sole responsibility of the author.