Hybridizing Myths of Evergreen Azaleas
K. Wada, Yokohama, Japan
Breeders of evergreen azaleas in the past seem to have confined their breeding efforts to hybridizing trials between species and/or hybrids of the Azalea Series, Obtusum Subseries. Those species and/or hybrids in most cases hybridize with each other very freely and give plenty of desired cross pollinated seeds. But not many breeders seem to have tried hybridizing between species or hybrids of the Azalea Obtusum Subseries and species or hybrids other than these. At least, not many of such results have been recorded.
Rhododendron 'Brocade' is recorded as a hybrid between R. williamsianum and 'Vervaeniana'. We take this latter name to mean Indica azalea Vervaeniana. Most rhododendron-minded people, more or less, doubt this parentage, because one of the parents was an evergreen azalea. But nobody could tell what else the other parent might be instead of 'Vervaeniana'. Frankly, I have not yet seen the flowers of 'Brocade' and as far as my observations of the superficial botanical characteristics of the plant go, I have found nothing to confirm the described parentage. However, I can not assume such a great rhododendron man as Mr. Lionel de Rothschild could leave such a parentage description without being sure of it. If he had thought it doubtful, he should have put a question mark after the parentage description. I presume it is possible he was by himself when he did the hand-pollinating for that cross. 'Brocade' seems not to show any characteristics of an evergreen azalea in any part of the plant. But if Mr. Lionel de Rothschild had got many sister seedlings, some of them must have looked more like an azalea.
More recently, Willem Hardyzer in Boskoop, Holland, raised a series of Kurume azalea hybrids with lepidote R. racemosum. Some of them were named and propagated, and are now becoming popular plants in our gardens. These hybrids are nearly indistinguishable from the regular Kurume azalea hybrids except the leaves are devoid of the hairs characteristic to the Obtusum Subseries of Azalea and the inflorescences more or less racemosed.
When R. dauricum (a lepidote rhododendron) was crossed with an evergreen azalea, plants were obtained that look nearly the same as R. dauricum without the characteristic hairs of the Azalea Obtusum Subseries and with the legitimate purple flowers of R. dauricum and the same type of calyx. Still they had some different characteristics.
Recently, a Japanese enthusiast has succeeded in raising several seedlings from an evergreen azalea crossed with R. yakushimanum and these seedlings do not look identical to each other. Some look more like a rhododendron proper but others look more like an evergreen azalea. None of them have the indumentum beneath the leaves. The one so far flowered has leaves with very pronounced azalea hairs both above and beneath the leaves. It has produced flowers pinkish in the opening and fading to white, as generally seen in R. yakushimanum, with 7 to 8 flowers to each truss. The evergreen azalea used has brilliant red flowers and can not have a gene of white in its ancestors. The said azalea is called 'Shojo-no-Mai' and is supposed to be a hybrid of some kind of Satsuki azalea and a form of the azalea R. scabrum. This particular azalea seems to hybridize rather freely with rhododendrons proper and is believed to be a tetraploid.
Another enthusiast has successfully flowered his hybrid between Satsuki azalea 'Benigasa' and R. degronianum. Satsuki azalea 'Benigasa' has red flowers and is reported to be a tetraploid. The seedling has given the pink flowers of R. degronianum, forming a truss, but looks more like an azalea rather than a rhododendron when not in bloom. A diploid evergreen azalea may be used as well for hybridizing with rhododendrons proper, but probably with more difficulty. What factor or factors may make cross-pollinating between evergreen azaleas and rhododendrons possible is not known to me.
These azaleodendrons show great tolerance to Phytophthora cinnamomi rot, as does the Rothschild's 'Brocade'.
Kurume azalea cultivars hybridize with R. keiskei as they do with R. racemosum. However, evergreen azalea species or hybrids have proved very much reluctant to cross with the deciduous azalea species belonging to the Azalea Luteum Subseries. So attempts to bring yellow color into the evergreen azaleas of the Obusum Subseries should best be done by mating R. wardii, dichroanthum, burmanicum, (good yellow Rock form), chrysodoron, etc.
Most people don't seem to know that evergreen azaleas hybridize rather freely with a number of the tri-foliated deciduous azaleas included in the Azalea Schlippenbachii Subseries. This group comprises more than one dozen different species with colors of lilac, wisteria purple, russetish purple, deep purple, salmon, orange, orange-red, etc. according to species and form. They also freely hybridize with the azalea R. tashiroi, an evergreen azalea separated by botanists either from the Obtusum Subseries or from the tri-foliated deciduous azalea group. As R. tashiroi has lilac tinted white flowers, and is most drought and heat tolerant, it is very valuable for expanding the color range and increasing the toughness of the hybrid's constitution.
Azalea R. poukhanense, belongs to the Azalea Obtusum Subseries but is deciduous to withstand the most rigorous winter temperatures. Some forms of Kaempferi azalea of the most northerly distribution are made nearly deciduous from the same winter temperatures. Nobody would deny the tri-foliated deciduous azaleas must have some physical body structure that can stand lower winter temperatures than evergreen azaleas. If the more winter hardly constitution of the trifoliate azalea group can be bred into the present Southern Indica hybrid azaleas, some of the progeny may be hardier, have the original large flowers and may grow satisfactorily in colder climates. Thus a new strain of larger flowers combined with more winter hardiness may be established. I suggest that the azalea R. amagianum, with large brick to orange-red flowers of the tri-foliated group might be used for this purpose. For your information, the R. amagianum plants gave plenty of seedlings with evergreen foliage when crossed with large-flowering evergreen azalea R. sublanceolatum and some of the seedlings may be as hardy as R. amagianum with flowers as large as R. sublanceolatum. R. sublanceolatum and its relatives are native to the most southerly part of Japan and were an important parent of the present Southern Indica hybrids.
Among the Azalea Schlippenbachii Subseries, botanists included many deciduous azaleas which apparently have no relationship to each other. The tri-foliated deciduous azaleas make one group closely related to each other. But the azaleas R. schlippenbachii, albrechtii and quinquefolium are of an entirely different origin. I presume they are derived from the Stamineum Subseries of Rhododendron. In Japan we have a deciduous azalea called R. nipponicum which looks very similar to R. schlippenbachii of the Azalea Series superficially but its origin may be of the Vireya Section. In any case, except for the tri-foliated deciduous azalea group I have heard no report that any other of the Schlippenbachii Subseries has crossed with species of the Obtusum Subseries.
The present article is written at random, but I hope it may foretell that new large-flowering hardier evergreen azaleas could be derived by bringing the blood of the tri-foliated deciduous azalea group into the present tender large flowering evergreen types or even the blood of hardy rhododendrons proper.