Rhododendrons of the Caucasus
Species and Hybrids
Translated by David W. Goheen
First of all, a small overview of this region. Since, indeed, two rhododendron species (Rhododendron luteum and R. ponticum) occur along the eastern shore of the Black Sea, we will bring into our consideration the district of the Colchis as well as the Caucasus, proper. It is that territory between the Black Sea in the west and the high mountainous approaches of the Great and Lesser Caucasus in the east. Here, the climate is uniformly moist and warm and mostly frost-free. The yearly precipitation is approximately 2,500 mm (98 in.). Here, in low situations, grows the Colchis oak forest, composed of Quercus hartwissiana, Carpinus betulus and Fagus orientalis. One encounters at a height of about 900 m (3,000 ft.), a species rich forest vegetation with an interesting shrubby stratum. Here occur, in addition to R. luteum and R. ponticum, also other evergreen leafed trees, such as Buxus sempervirens, Prunus laurocerasus and Ilex quinquefolium. The yet further east mountainous slopes of the Caucasus are especially favored with precipitation; yearly the average is over 3,000 mm (118 in.). Conifers join the woods at heights of about 1,800 m (6,000 ft.), consisting of Abies nordmanniana and Picea orientalis. At this high border zone, the occurrence of R. caucasicum begins, often in pure stands mixed with birches (Betula carpatica and Betula raddeana) and Sorbus aucuparia ssp. caucasica.
The Great Caucasus which, in Mt. Elbruz, reaches a height of 5,633 m (18,481 ft.) is strongly glaciated. Yet further east, the precipitation amount becomes distinctly lower (east Caucasus is about 1,000 mm, 39 in.) and the contrasts in temperature become larger. Only small outposts of Rhododendron luteum can be found here on the mountain slopes. This, therein, is again evidence that the genus Rhododendron is bound to and favors an optimum precipitation level.
Classification Key for the Caucasus Species
- Deciduous, leaves ciliate; corolla dark yellow -- R. luteum
+ Evergreen, leaves glabrous or with indumentum; corolla white or pink to purple -- 2
- Leaves glabrous -- R. ponticum
+ Leaves with an adpressed to lanate indumentum at maturity -- 3
- Dwarf shrub, 0.3 to 1.5 mm; leaves with a compacted indumentum beneath; corolla usually creamy white, rarely yellowish -- R. caucasicum
+ Shrub 1 to 6m; leaves with a thick compacted to lanate indumentum beneath; corolla white-flushed pink or pink -- 4
- Leaves 11.5 to 21 cm long; calyx 5 to 9 mm long; ovary and petioles stipitate-glandular -- R. ungernii
+ Leaves 7 to 11.5cm long; calyx 2 to 3 mm; ovary and petioles eglandular -- R. smirnowii
R. ponticum L
A strongly growing species that, in the Caucasus, is mostly up to 6 m, rarely to 10 m in height. The leaves are bare on both sides. The flower trusses consist mostly of 6 to 15 florets, in exceptional cases, up to 30 florets. The flower color is pale rose to purple-violet, rarely white.
The species is especially abundant in the rainy forests and mountain slopes east and south of the Black Sea. It ascends from sea level (0 m) up to 2,200 m (7,200 ft.) in the Caucasus. In the deciduous woods, shrubby beds of this species are formed. Especially in the sub-alpine high beds, it grows in impenetrable thickets. The great variability of the species is a prominent feature. Several garden forms are possibly clones from wild habitats.
In addition to the Caucasus, this species also grows in the southwesterly mountains of the Iberian Peninsula in Spain and Portugal (Gibraltar, Sierra de Monchique, Sierra de Algarne and Sierra Morena). This geographical form was earlier described as Rhododendron baeticum BOISS and REUT. Additional occurrences are found in Turkey (mountains to the south of the Black Sea), as well as an isolated occurrence in the coastal mountains of Bulgaria on to Argas, north of Istanbul. This form was earlier separated as variety scorpilii DONIN. In the mountains of Lebanon and Antilebanon, one finds this species yet again. It was formerly distinguished as var. brachycarpum BOISS. Fossil finds in the Southern Alps and at Innsbruck have been regarded by several paleobotanists as evidence of a connected area between Western Europe and Asia Minor. Others describe these of a true, yet very closely related, species, R. sordellii TRALAU.
Rhododendron ponticum was introduced into garden culture very early (England from Gibraltar in 1763). In agreeable habitats, as in Great Britain and Ireland, the species naturalizes easily. Therefore, the garden value is frequently discussed and is questioned. The species inclines strongly toward hybridization and many old plants in European park plantings are hybrids with R. catawbiense and R. maximum. Around 1820, the first crossing 'Morelianum' between R. catawbiense and R. ponticum originated. Still today, 'Multimaculatum' (Tondeleyo) is found abundantly in gardens, grown in 1860 by Waterer in England. In Thompson's Nursery in about 1875, a hybrid between R. periclymenoides and R. ponticum was obtained. This was an azaleodendron with the name of 'Odoratum' with a bright lilac-blue flower color. In the following years, many breeders made crosses with R. ponticum and today one can hardly keep up with the number of varieties that resulted. Mentioned here are only a few garden forms. Possibly, the question is whether these are selections or clones from wild habitats.
'Aucubifolium': Leaves with small yellow flecks.
'Cheiranthifolium': Compact growth; leaves lanceolate with irregular borders.
'Imbricatum':* Compact, thickest growth to about 1 m; leaf form clearly irregular, elliptic apex and base, round-arched, 3 to 6 cm long at the sprout ends; flowers small, violet; not so winter-hardy as the standard form.
'Lancifolium':* to about 1.5 m high; leaves narrowly lanceolate with smooth edges; flowers smaller than the standard form, corolla margin purple with the middle brighter; to this also belong 'Graciella'* and 'Linearifolia'*.
'Variegatum': Growth weaker than the type; leaves often with an indented margin, yellowish white; not flowering abundantly.
| R. yakushimanum x R. smirnowii.
Photo by Photo courtesy of Erhard Moser
R. smirnowii TRAUTVETTER
The 1 to 2m, rarely in the wild, to 4 m shrub is, especially at the time of new growth, of an attractive appearance because of its silvery tomentum. The leaves are also covered on the undersides with white to brownish indumentum. The rose-purple to bright carmine-purple decorative flower trusses contain from 10 to 14 individual florets with wavy margins.
This species occurs in the Caucasus in birch forests at heights of from 1,000 to 1,600 m (3,280 to 5,249 ft.), often joined with R. ponticum and R. ungernii. In northern Turkey, it is found up to 2,300 m (7,546 ft.). The area in the Caucasus is relatively tightly constrained. Wherefore, the species seldom becomes investigated and graded. There should be considered a closed-district in order to prevent the dying out of natural stands. R. smirnowii is a very beautiful wild species, with a good garden value and excellent winter-hardiness, that became introduced into garden culture in 1880. Therefore, it has been used frequently until now for crossing in order to obtain hybrids for winter-cold regions. Unfortunately, most of these hybrids do not increase well through cuttings. These root with difficulty and therefore should become propagated better through plant cultivation (layering).
R. ungernii TRAUTVETTER
In its native habitat, this often grows to a 6 m high shrub. Also with this species, the underside of the leaf has a thick, white - later brownish - indumentum. The silvery tomentum is very decorative. The blooming trusses are complexes of mostly 15 to 20 individual florets. The corolla color is a soft rose with a lighter throat.
This species also occurs from 600 m (1,969 ft.) in a narrowly limited space in birch forests. Rarer in the coniferous woods and in sub-alpine regions up to 2,500 m (7,218 ft.), where it can form thick, compact bushes. It is most usually found between 600 to 1,900 m (1,969 to 6,234 ft.). Although already introduced into garden culture by 1886, it is even now rarely found in gardens. One reason is surely the partial lack of winter-hardiness. There are, however, truly hardy types, In suitable climates, it is a charming species, especially owing to its initial white felt and later brownish indumentum on the leaf undersides.
| R. caucasicum in the Caucasus mountains.
Photo courtesy of Erhard Moser
| R. caucasicum
Photo courtesy of Erhard Moser
R. caucasicum PALLAS
The dwarfish, up to 1.5m high species of the Caucasus. The leaf undersides have a thin, rusty indumentum. The white to creamy-yellow, occasionally rose tinted flowers, often display a greenish or reddish pattern. The truss has 7 to 10 individual florets. It is the only alpine species of the Caucasus and often forms crooked, woody vegetation at heights of from 1,600 to 3,000 m (5,250 to 9,845 ft.). This often next to impenetrable zone many times occupies a spread of 500 m. Especially luxuriant stocks are found on the north slopes of the Greater and Lesser Caucasus at suitably moist places with an undergrowth of Oxalis acetosella and Vaccinium myrtillus. It is a very beautiful and, for small gardens, a very suitable species. Unfortunately, the culture many times in deep soil is difficult. Even large plants can sicken from phytophthora and can suddenly die. Often, the species in garden culture, blooms a second time in the autumn. Since it was introduced in 1803, R. caucasicum has become frequently used in hybridization. R. caucasicum hybrids have a compact, often flat form and bloom often very abundantly and earlier than other hybrids. They also can be reproduced from cuttings very easily. Many of these hybrids have been cultivated for a long time and are to be found still today in our gardens. Already by 1835, the crossing with R. arboreum by Waterer and other breeders in England had taken place. Today, 'Nobleanum' is one of the earliest blooming types. Perhaps at about the same time, came about 'Jacksonii'. The question is whether it was the same combination or if it was a cross between 'Nobleanum' and R. caucasicum. Perhaps the best known hybrid is 'Cunningham's White'. This was developed about 1830 in Edinburgh, Scotland from a combination of R. caucasicum with R. ponticum album. It is a robust, healthy and winter hardy type that in Europe has often become utilized as an understock for plant cultivation.
| R. luteum
Photo courtesy of Erhard Moser
R. luteum SWEET (R. flavum G. DON, Azalea pontica L.)
A mostly to 2 m, rarely to 4 m, shrub. In diameter, the bush may attain 6 m. The soft leaves are hairy and ciliate. The flower truss is composed of from 7 to 12 single florets. The flower color is mostly a strong yellow.
It occurs in the Caucasus in western Transcausica from the Black Sea coast up to the sub-alpine stage (2,200 m, 7,218 ft.). It is especially found in thin birch woods. Between 1,400 to 2,000 m (4,593 to 6,562 ft.) on the mountain ridges, small stands are formed. As described before for R. ponticum, relict populations occur in Europe in the Ukraine, White Russia into Poland and isolated stands are found along the southeastern edges of the Alps in Krani and Carinthia. Surely in earlier times, the stock of this species also formed a connected territorial range. In European garden culture, it is indeed the most frequently grown and the most frequently used for hybridization azalea. The species has good winter hardiness and adapts itself easily in agreeable habitats. In Middle Europe, it can easily be naturalized, which is most desirable. Of note, is its strong fragrance at blooming time as well as the beautiful leaf color in the fall. Introduced into England by 1792, its garden worth was known even earlier. By 1820, Mortier in Belgium began hybridization work by crossing, especially with American species such as R. calendulaceum, R. periclymenoides and R. viscosum as well as the Chinese R. molle. He originated in the following years the Ghent hybrids, that, even today, are very widespread. They grow and bloom freely and show good winter hardiness. Their colors reach from bright yellow to dark carmine-red, simply and abundantly blooming.
| R. x kesselringii, a natural hybrid.
Photo courtesy of Erhard Moser
R. ponticum, R. smirnowii and R. ungernii grow together in similar habitats and R. caucasicum also shows some overlapping of area boundaries. This leads, occasionally, to the formation of hybrids with all possible intermediates. Thus, difficulty in recognizing each species for sure can take place and has, especially with R. ungernii, led to many complexities. Regarding the evaluation of winter hardiness, it is conceivable there is some genetic influx from R. smirnowii. However, it should be emphasized that, in normal cases, R. smirnowii and R. ungernii in their natural habitats can be clearly differentiated.
Under the natural hybrids, one should mention R. x kesselringii E. WOLF. The description came about in 1910 through Wolf from plants that were raised from seeds from the natural habitat. With certainty, it is surely the same hybrid of R. ponticum x R. smirnowii that can be found also in the wild. Cultivated plants of these show good winter hardiness and bloom abundantly in clear rose, almost without markings. The leaf undersides are thinly felted, especially in the juvenile stage. Almost as sensational, however, was the 1967 description of the hybrid between R. ponticum and R. caucasicum: R. x sochadzeae CHAR AND DAVLIAN from Megreline in Transcaucasica. In our gardens, this hybrid has long been known as 'Cunningham's White'. The hybrid in its natural habitat varies from rose to cream colored, partly inclined more to R. ponticum and partly inclined more to R. caucasicum. Also, hybrid swarms between all four of the evergreen species are possible (Cox, 1979). Often, it is decidedly difficult to clarify the exact origin of rhododendron hybrids, if they occur together in the wild with other species.
Chamberlain, D. F. (1982): Revision of Rhododendron 2. Notes from the RBG, Edinburgh.
Cox, P. A. (1979, 1990): The Larger Species of Rhododendron, London.
Schmidt, P., Alexandrova, M. S. (1981): The Wild Growing Rhododendron Species of the USSR. Contributions to Wood Science, Berlin (in German).
Moser, E. (1991): Rhododendrons of the Caucasus, a retrospective (in German).
Moser, E. (1991): Rhododendrons - Wild Species and Hybrids, Radebeul.
Erhard Moser is the author of the recently published Rhododendron which was reviewed by David Goheen in the Summer 1992 issue of the Journal.
Editor's Note: * designates an unregistered hybrid. The name does not conflict with that of a registered clone.