An evergreen shrub or small tree, attaining a height of 25 ft in the wild; branchlets soon glabrous. Leaves oblanceolate or elliptic-oblanceolate, 4 to 8 in. long, 1 to 21⁄2 in. wide (but sometimes larger in the Caucasian forests), dark green above, paler beneath, glabrous; petiole 1⁄2 to 11⁄8 in. long. Inflorescence a terminal corymb with up to fifteen flowers, opening in late May or June; rachis 1 in. or slightly more long, glabrous or slightly glandular (tomentose in subsp. baeticum, and occasionally in the typical subspecies); pedicels 3⁄4 to 13⁄4 in. long, lengthening in fruit, glabrous or slightly glandular. Calyx very small, with five blunt teeth about 1⁄10 in. long. Corolla funnel-shaped, about 2 in. wide, light purple or rosy purple, the upper lobe usually marked with greenish or brownish spots. Stamens ten, filaments hairy at the base. Ovary and style glabrous. Bot. Mag., t. 650. (s. and ss. Ponticum)
R. ponticum has its main and most continuous distribution in the region of the Black Sea. East of the sea, it is an important ingredient of the rich forests of western Transcaucasia (Colchis), growing under beech, Picea orientalis or Abies nordmanniana, also ascending above the tree-line, where it occurs in dwarfed form at well over 6,000 ft altitude. It is, or was, also found at sea-level in the neighbourhood of Batum, forming thickets up to 20 ft high in association with the cherry laurel (Prunus laurocerasus). South of the Black Sea it extends through the Pontic ranges of N. Anatolia from Lazistan and N. Kurdistan in the east to Bithynia in the west. Only in this century was it established that R. ponticum is also a native of the Balkans, where it occupies a small area in the Strandja mountains of N.W. European Turkey and bordering parts of S.E. Bulgaria. Here it occurs with other elements of the Colchic forests, e.g., the cherry laurel, Daphne pontica and Vaccinium arctostaphylos. Well to the southeast of its main area, R. ponticum occurs in the mountains of the Lebanon, where it is scattered.
subsp. baeticum (Boiss. & Reuter) Hand.-Mazz. R. baeticum Boiss & Reuter; R. ponticum var. myrtifolium (Lodd.) G. Don, in part, nom. confus., not R. myrtifolium Lodd. – It is a remarkable and surprising fact of plant geography that R. ponticum should also occur as a genuine native in parts of Portugal and southern Spain. But the fossil evidence shows that it is an ancient inhabitant of western Eurasia, which existed in Russia early in the Tertiary Period and even in comparatively recent geological times occurred in Central Europe and as far north as Ireland. Its existing stands, both in the east and the west, are the remnants of a once much wider area, which has become fragmented by climatic change. The precise distribution of R. ponticum in the Iberian peninsula at the present time is not known in detail. But in Portugal it is certainly to be found in Alta Beira province at the north-western end of the Serra do Caramulo; in the Serra de Monchique above Foia (Algarve) and near Odemira, also in S. Portugal (Alentejo). In Spain R. ponticum is confined to S. Andalucia, where it occurs (or did in the last century) in several localities in an area about 35 miles long from south to north and 6 or 7 miles wide, stretching from the hills near Tarifa on the Atlantic coast through the mountains behind Algeciras and then as far north as the Sierra de Galina. In Portugal, and probably in Spain also, it is near the limits of its climatic tolerance, and confined to streamsides and other wet or shaded places.
Whether R. ponticum, in what might be termed its Atlantic area, is really distinct enough from the eastern race to merit botanical distinction is a moot question. The most reliable difference is that the rachis of the inflorescence in specimens from Spain and Portugal appears to be invariably tomentose, especially near the apex and at the insertions of the pedicels, whereas in Caucasian and Anatolian specimens it is usually glabrous, but by no means invariably so. There is no constant difference in foliage nor, so far as can be ascertained, in any other character discernible in an herbarium specimen, though no doubt experimental cultivation would reveal many minor points of distinction.
In Alton’s Hortus Kewensis the date of introduction of R. ponticum is given as 1763. It may be that plants or seed were acquired by Kew in that year, but the first commercial introduction, according to Loudon, was by Conrad Loddiges, whose career as a nurseryman started in 1770. In Weston’s compendium of the catalogues of the London nurseries, called Flora A.nglicana, R. ponticum makes its appearance in the supplement (1780); in the main work (1775) there is no mention of it. Whether the early introductions were of typical R. ponticum from the Black Sea region, or of subsp. baeticum from Spain or Portugal, is not known for certain. We have it on the authority of the German botanist Pallas, writing in 1784, that the plants in English gardens originated from Gibraltar, which may be true at least of the Kew plants, for there is a specimen in Bishop Goodenough’s herbarium collected, probably at Kew, in the 1780s, which has the tomentose rachis characteristic of subsp. baeticum. But there may well have been commercial importations of typical R. ponticum too.
No rhododendron has obtained so secure a footing in Britain as this. On light acid soils it is the commonest of exotic evergreens and, in the right place, as by lakesides, one of the most beautiful. Although no longer much planted as an ornamental, it is useful as a windbreak and draught-stopper and, where there is room, as a cheap, fast-growing informal hedge. It is commonly used as stock on which the garden hybrids are grafted, and to this practice, no doubt, the presence of great drifts of this shrub in many gardens is due. For when plants are left unwatched the stock frequently sends up sucker-growths, and it then becomes only a matter of time before the finer-bred and less assertive scion is overwhelmed.
R. ponticum is an invasive plant, establishing itself on acid soils where the natural vegetation has been disturbed by fire or by the planting of conifers. For this reason it is disliked by both conservationists and foresters. Its presence far from human habitation is, however, not always the result of self-seeding, for it was at one time much planted as game covert and many thriving woodland colonies are probably the result of this misguided practice. Once established, it is difficult to eradicate, cutting it to the ground being quite futile unless the regrowth is treated with a suitable brushwood killer.
It is questionable whether the cultivated and naturalised plants are entitled to the name R. ponticum, for there can be no doubt that the species has become contaminated by hybridisation. As early as 1806 it was stated that hybrids had sprung up between R. ponticum and R. maximum, and it seems so likely as to be almost certain that crossing between R. ponticum and the Hardy Hybrids has taken place continuously during the last 150 years. Some plants sent out by nurseries as R. ponticum are obvious hybrids and none the worse for that. Naturalised plants on the Surrey-Sussex border, examined recently, resembled R. ponticum in foliage and colour but showed signs of hybridity in the larger calyx and very glandular pedicels deriving from R. maximum, and in the presence of hairs on the ovary, which suggest the influence of R. catawbiense, another parent of the Hardy Hybrids. But another colony, a mile or so away, and probably seeded from a different property, agreed perfectly with R. ponticum subsp. baeticum.
The cultivated rhododendron named R. obtusum by Watson in 1825 is of uncertain identity. If it came from Armenia, as he stated, it must have been a form of R. ponticum or a hybrid between it and R. smirnowii. It has been treated as a variety of R. ponticum, but the Swiss botanist de Candolle, who was acquainted with the living plant, considered it to be near to R. catawbiense and possibly a hybrid. It was a low bushy plant with thick glabrous elliptic leaves obtuse at the apex, slightly cordate at the base, and rosy flowers (Watson, Dendr. Brit., t. 162; de Candolle, Prodromus, Vol. 7, p. 722).
The following were treated as variants of R. ponticum in previous editions, but there is a possibility that some are really hybrids:
f. album (Sweet) Zab. – White-flowered forms of R. ponticum are rare, the ‘white ponticum’ sometimes seen among seedling plants being an obvious hybrid with R. maximum or, more likely, with some Hardy Hybrid with R. maximum in its parentage. But what appears to have been a white form of the true R. ponticum was in cultivation at Kew. See also the note by Cox and Hutchison in R.C.Y.B. 1963, p. 66.
cv. ‘Cheiranthifolium’. – Flowers pale purple; the plant dwarf and compact; leaves 2 to 3 in. long, 3⁄8 to 1⁄2 in. wide, very wavy at the margins. It is very unlike R. ponticum in every respect, and almost certainly a hybrid.
cv. ‘Lancifolium’. – A small edition of the type; leaves 2 to 4 in. long, 1⁄2 to 3⁄4 in. wide. Flowers in small trusses, almost white in the centre, suffused with purple towards the margin. The plant is dwarf and compact, rarely more than 6 to 8 ft high. It is distinguished from ‘Cheiranthifolium’ by the leaves being flat, not wavy.
cv. ‘Variegatum’. – Leaves smaller and narrower than in the type, edged with creamy white. Another variegated form was ‘Aucubifolium’, with yellow-spotted leaves.
An interesting variant of R. ponticum still in cultivation has bronzy purple leaves which hold their colouring well in the winter. It is possibly the same as ‘Foliis Purpureis’, for which the nurseryman William Paul received a First Class Certificate in 1895.