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Species In Our Midst
Rhododendron ponticum

by Elizabeth Carlhian

Rhododendron ponticum var. roseum
Picture by S & J Perkins
Rhododendron ponticum var. roseum in Concord, MA

Rhododendron ponticum

Rhododendron ponticum is an elepidote rhododendron (without scales). It is one of the more than 20 species in section Ponticum, subsection Pontica in the subgenus Hymenanthes. This subsection includes many of our most familiar horticultural species: R. hyperythrum, aureum, maximum, catawbiense, macrophyllum, brachycarpum, caucasicum, ungernii, smirnowii, yakushimanum and japonicum. Rhododendron ponticum has a wide native distribution, in Turkey in the Pontic Mountain ranges from sea level to 6000 feet in mixed deciduous and evergreen forests with beech, fir and spruce, hornbeam and alder with laurel and Vaccinium shrubs; in Lebanon from 2300-3500 feet in pine woods with sandy soil; in Spain near Cadiz and Algeciras; and in south Portugal on granite slopes and wooded valleys beside rivers. The specific name refers to the Pontus region, the old kingdom bordering the Black Sea, in modern northern Turkey.

Rhododendron ponticum forms a compact shrub to 15 feet in open areas, leggy in the shade. The bark is rough brown to dark brown. The leaves, elliptic to oblong, a good dark green, glabrous (without hairs) above and paler beneath, remain on the plant from two to four years. The leaves of R. ponticum do not curl up in cold weather. The inflorescence consists of 10 - 15 fairly compact campanulate flowers, 1- 2 inches long. The rachis (the 'stem' to which the individual flowers attach) and the pedicel (the stem of the whole inflorescence) are usually glabrous. Flower color ranges from lilac pink to pinkish purple in shades from pale to deep, and occasionally white. Some of the forms have much better color than others and Peter Cox speaks of some 'blue' forms rivaling R. augustinii. The calyx is small and the ovary and style glabrous. The whole plant is noticeably hairless, distinguishing it easily from its close relatives, like R. catawbiense.

Greer lists R. ponticum as hardy to -15oF. Cox, though, says it is not hardy in south Sweden and is frequently damaged in hard winters at his nursery in Glendoick, Scotland. Sweet says it is not hardy north of Philadelphia in the northeastern United States. There is a record of damage in a freeze in Seattle in 1955. New Englander gardens rarely include this species because of its tenderness, in spite of Greer's rating.

Several varieties of R. ponticum have been described: baeticum, the form found in Spain and Portugal; album, a form with white flowers; cheiranthifolium, with very narrow leaves, wavy at the margins, and a plant said to be dwarf and compact; lancifolium, also with narrow leaves and dwarf and compact; and variegatum with variegated foliage, white along the margins of the leaves. Also listed is 'Fastuosum Flore Pleno', a sterile double-flowered hybrid of catawbiense x ponticum; the flowers are double and lavender blue (Geber Francoisi, Ghent before 1846).

The earliest reference to R. ponticum is probably in the writing of the Greek historian Xenophon, 400 BC. He tells the story of the "Ten Thousand" Greek soldiers retreating through the mountains by the Black Sea, who ate the local honeycomb, which made them sick. The ancients believed this honey was made from the nectar the bees took from R. ponticum, although R. luteum and Nerium oleander are also likely toxic sources. In more recent times, Rhododendron ponticum was first described by Joseph Pitton Tournefort, when sent to the Levant by the king of France in 1700. He called the common mauve rhododendron 'CHAMAERHODODENDROS PONTICA MAXIMA', but collected neither plants nor seeds. Claes Alstroemer, a pupil of Linnaeus, found the same plant in 1750 in Spain between Cadiz and Gibraltar. Lineaus himself described and named it in 1762.

Rhododendron ponticum was brought to England from Gibraltar in 1763. By 1803 R. ponticum was grown and forced into flower as a pot plant for the London market. This species did very well in England. Its lush dark green leaves and strong vigorous growth, ability to thrive in different areas, and mauve colors meant that it was used often both as a species and to make new hybrids. The 19th century saw a great deal of hybridization as many new species, including the American ones, were introduced. As time went on, there was R. ponticum blood in many hybrids, but it became even better known as a good root stock on which to graft a desired plant. At the time, grafting was a more common way of increasing plants than growing from cuttings. The roots of R. ponticum are surprisingly hardy and do not seem to suffer from the cold. The undesirable characteristic created by the R. ponticum rootstock is its tendency to sucker, which has to be carefully controlled.

Two R. ponticum hybrids, 'Cunningham's White' (1850) and 'Chionoides' (1871), both good whites, are still with us after more than 100 years. There have been a great many purple hybrids, most notably 'Purple Splendor', 'Sappho', and 'Blue Peter', which have been used extensively in breeding. Very good photography of R. ponticum and many of its hybrids can be found in the book Rhododendron Portraits by van Gelderen and van Hoey Smith, pages 37 through 65.

Rhododendron ponticum has proved to be a major pest in the British Isles and other European countries. The species naturalizes aggressively and individual plants sucker, forming huge impenetrable mounds. In addition to being invasive, the plant is toxic to grazing animals. The UK now has a widespread, well publicized eradication program to stop its spread by grubbing out its roots.

I don't grow R. ponticum and I don't think I will ever try because of its lack of hardiness in New England, but the success of the forms of R. ponticum found at sea level in several countries where summer temperatures are very high, as in subtropical areas of lowland Japan, might suggest trying R. ponticum to create a more heat tolerant rhododendron, which we would welcome in the southern United States.

Elizabeth Carlhian, Concord, MA

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