Variability of Rhododendron Species
by Dr. Fred Coe, San Francisco, Calif.
A lecture delivered to the California Chapter, Oakland, California
The rhododendron genus with some 900 members probably has the largest number of showy species in the Heath family. Certainly the 'Cape heaths' with approximately 470 species have many showy members, but for flamboyant colors, size and relative ease of culture it's hard to beat rhododendrons.
The heath family including rhododendrons originated in the Tertiary period (30-40,000,000 years ago) before the ice ages and has continued to evolve up to the present day. The Himalayas and probably regions like New Guinea and Java are still evolving new species. This must be borne in mind when you mutter under your breath at the changes or merging in names of some favorite "species" you have been growing for some time. The species idea is one of convenience for people and means little to plants. The study of our native eastern azaleas has brought this clearly to view, for species readily hybridize in the wild and in a collection taken from the wild the collector may determine the size and form of the plant and flower according to what his idea of the particular species is. Although the collectors in the Himalayas say that little or no hybridizing occurs, this is hard to believe. Different insect pollinators at different altitudes and varying times of bloom probably keep species fairly pure, but no one has done extensive collecting of live specimens to make sure that plants do not hybridize where two or more species populations intermingle. This is highly technical work which requires microscopic study of chromosomes and, until such work is done, the question of hybrids occurring in this region will be debatable.
Some species populations show very marked variations in size, color, and even form of the flower (neriiflorum series), others have very uniform populations and are usually clear cut in appearance (ponticum series). Even in the latter case you've got to remember that some one person picked out the original plant or plants that is given the specific name. If you had to recollect in that same region you might have trouble telling whether you had the true original species.
All of the above is to show that not even the taxonomists who classify the plants are sure of their relationships. Two large groups of plants exist in the rhododendrons, those with scales (Lepidote) and those without (Elepidote). The taxonomists say that the most primitive members of the genus belong to the first group. There is great variation in this group, ranging from azalea-like flowers and deciduous plants of some of the triflorum series or very small flowers as in the lapponicums to enormous waxy, sweet-scented flowers in the maddenii series. A note of interest here should include that very nebulous group called javanicum rhododendrons. A thorough study has finally been made of this group and it is evident that it is far from being a uniform group and probably contains a number of species similar to those in the maddenii series in addition to the more typical javanicum. A letter written me a few days ago tells of the rediscovery of a species first collected in 1884 near Port Moresby, New Guinea. This species (R. toverenae) has the largest flowers in the genus with a tube approximately 12 cm in length (5") and 17.5 cm across (7"). The flower is white or pink and is borne on an epiphytic plant in high trees.
The wide variation in the plants in the lepidote series has brought up the question of whether they should all be considered rhododendrons in the true sense. Certainly they are difficult or almost impossible to hybridize with the elepidote group, but a few hybrids have been made. This means little, for crosses have been made between elepidote rhododendrons and other genera (kalmia and ledum).
The elepidote series show wide variation in size and form of plants and flowers. There are true trees in the R. arboreum and barbatum series, creeping shrubs (R. camtschaticum) and both evergreen and deciduous shrubs. Flowers again vary tremendously from those of the butterfly-like deciduous azaleas to large bowl or funnel-shaped flowers as in the campanulatum, auriculatum or other series.
Dr. Clement Bowers has arranged the series in groups of what are considered close relatives but even here this is not sure, for hybridizing may be difficult between these members. Chromosome number difference plays little part in the whole genus for only a few examples of polyploidy (more than the usual number of chromosomes) have been found.
To conclude this discussion of species with slides of some few species will show you the variation in the genus. The region around the San Francisco Bay is ideal for growing a high percentage of the species if you pick the ones that are suited to your conditions. To any one who is interested in a whole group that has hardly been touched for a hundred years, the 'javanicums' are a challenge. These are greenhouse shrubs, for the winter at least, but probably could be grown on a shaded patio in the summer. Hawaii is a wide-open field for these rhododendrons.