The plants that make up what we call the genus Rhododendron have developed and diversified throughout the whole northern hemisphere. Resulting from this, there are many hundreds of separate and distinct, but generally related, species. Thus, the genus has presented botanists, especially those who strive to correlate, name and group the numerous taxa, with many taxonomic problems. Just who is related to whom?
Over the years, I have followed arguments for and against the "series" system devised by the Englishman Sir Baley Balfour to handle the influx of many new rhododendron taxa (a word used to describe previously unstudied wild material) that resulted from the prodigious efforts of plant explorers during the first third of this century. The system has been called artificial and denounced as obsolete and an anachronistic, but even those who state this admit that it is useful.
Sleumer, Cullen and Chamberlain and others have developed a different system which they claim is more "natural" and which takes into account more recent information that was not available to Balfour. This system is gradually replacing the series concept and may eventually be accepted by all as the only system for rhododendron nomenclature. The series idea dies hard, though, and many of us automatically still think in its terms. So the taxonomists plow onward and study and think in scientific and technical ways. But what about the majority of us who delight in collecting and growing mainly for a horticultural interest? Perhaps we need to let the professional taxonomists study morphological, phytochemical and cytological characteristics for grouping these plants and confine ourselves to horticultural features. Cullen and Chamberlain admit in their monograph on the revision of the taxonomy of the genus that they completely ignore the horticultural traits of the various species.
For nearly 30 years I have grown and admired diverse rhododendron species. My interest in these plants has been catholic and I admire species in all the Balforian series or the subgenera, sections and sub-sections of the Sleumer-Cullen and Chamberlain scheme. Of particular interest to me have been plants that we generally refer to as elepidote and broad-leaved rhododendrons. I have now (with a little bit of tongue-in-cheek bravado) divided these into seven horticultural subsections with no regard to any cytological or morphological affinities, but group the many species into horticultural subsections based on their ease of growth, cultural requirements, insect and disease resistance, temperature response and readiness to flower. While all of us remark on plant appearance and foliage, we really want them to bloom and to flower at an early age! Alas, some of them disappoint us and these sit in our gardens for years with scarcely any formations of flower buds.
My seven horticultural subsections are: 1. Paucibloomia 2. Reluctantia 3. Susceptabilia 4. Exasperatia 5. Humdrumia 6. Agreeabilia 7. Salubria.
I think the names are self-explanatory. As one might expect, the species that are easiest to grow and flower are in the subsections Agreeabilia and Salubria. Many, but not all, of my favorite rhododendron species are in these subsections. The Paucibloomia and Reluctantia subsections, as the names imply, are very slow to flower, but contain some very interesting things. I have, however, seen them bloom in other gardens. I have grown a number of them for 25 to 30 years and have never seen them bloom in my garden! The Susceptabilia subsection contains plants that are prone to succumb to root problems, extreme temperatures, poor drainage, insect damage, etc. Some of them are very nice - if you can keep them alive. They require a lot of attention with more or less constant vigilance for problems to appear. But at least most of these species can do fairly well, if care is taken. The most difficult are the species in the Exasperatia subsection. These can be a real trial. Some of them will fail despite best efforts. They seem to fade away gradually or suddenly no matter what you do! To grow a R. pronum or a R. proteoides to perfection and mature flowering is a real accomplishment. Alas, I must confess that I have never succeeded with these two, but others have and I salute them.
Here is an alphabetical listing of most of the species in the subsection Hymenanthes that I am growing or have grown for at least three years in my garden in Camas, Wash., listed under seven horticultural subsections:
Paucibloomia: R. aeruginosum, R. citriniflorum, R. clementinae, R. erubescens, R. fulgens, R. fulvastrum, R. globigerum, R. longesquamatum, R. parmulatum, R. praestans, R. przewalskii, R. thomsonii, R. wasonii.
Reluctantia: R. adenogynum, R. balfourianum, R. beanianum, R. calophytum, R. caloxanthum, R. cerasinum, R. didymum*, R. eudoxum, R. lanigerum, R. martinianum, R. recurvoides, R. repens*, R. roxieanum, R. schizopeplum, R. sphaeroblastum, R. wiltonii.
Susceptabilia: R. arboreum, R. arizelum, R. barbatum, R. basilicum, R. crinigerum, R. diaprepes, R. elliottii, R. eximium, R. gymnocarpum, R. habrotrichum, R. lacteum, R. macabeanum, R. montroseanum, R. pachytrichum, R. sherriffii, R. sinogrande, R. spilotum.
Exasperatia: R. bathyphyllum, R. chaetomallum, R. eclecteum, R. erosum, R. lanatum, R. mallotum, R. pronum, R. proteoides, R. rude, R. rufum, R. souliei.
Humdrumia: R. catawbiense, R. fauriei, R. maculiferum, R. macrophyllum, R. maximum, R. monosematum, R. ponticum, R. ungernii, R wallichii.
Agreeabilia: R. alutaceum, R. anwheiense, R. apodectum, R. bainbridgeanum, R. brachyanthum, R. campanulatum, R. chrysanthum*, R. croceum*, R. dichroanthum, R. dryophyllum, R. elegantulum, R. galactinum, R. hemidartum, R. hodgsonii, R. houlstonii, R. irroratum, R. makinoi, R. metternichii, R. neriiflorum, R. orbiculare, R. planetum, R. praevernum, R. sanguineum, R. uvariifolium, R. vellereum, R. vernicosum, R. wightii.
Salubria: R. adenophorum, R. argyrophyllum, R. bureavii, R. campylocarpum, R. caucasicum, R. decorum, R. degronianum, R. discolor, R. fictolacteum, R. floribundum, R. fortunei, R. haematodes, R. hemsleyanum, R. hyperythrum, R. morii, R. oreodoxa, R. pingianum, R. pseudochrysanthum, R. rex, R. ririei, R. smirnowii, R. strigillosum, R. sutchuenense, R. wardii, R. yakushimanum.
These are all based on my own experience and observations. Others may have different conclusions. Of what use is this? Perhaps none, but owing to its being based on long experience, perhaps this system could be used by individuals starting a species collection. Certainly those of middle age or more would do well to stay away from the Paucibloomia and Reluctantia subsections, unless they can be satisfied with foliage and few flowers. The Susceptabilia and Exasperatia subsections present some goals for which to aim. Successfully growing and flowering these species is a real accomplishment!
The Salubria subsection contains six out of my list of 10 favorite rhododendron species: R. yakushimanum, R. haematodes, R. pseudochrysanthum, R. wardii, R. fortunei and R. strigillosum.
I select these because of their ready flowering with beautiful blooms, their good foliage and their ease of growing. For anyone starting a species collection, they certainly have much to recommend them.
* Editor's Note:
R. didymum is a synonym for R. sanguineum ssp. didymum.
R. repens is a synonym for R. forrestii ssp. forrestii.
R. chrysanthum is a synonym for R. temenium ssp. chrysanthum
R. croceum is a synonym for R. wardii var. wardii
Dave Goheen is author of several Journal articles, including "Rhododendron Collecting on Mr. Kinabalu" (v37:n1), "Some Thoughts on Rhododendron Species" (v42:n4), "The Centennial Azalea" (v39:n3) and "Welcome to the International Species Symposium 1985" (v39:n1).