R. ponticum is a dense, suckering shrub or small tree growing to 5 m (16 ft) tall, rarely 8 m (26 ft). The leaves are evergreen, 6 to 18 cm (2.4 to 7.1 in) long and 2 to 5 cm (0.79 to 1.97 in) wide. The flowers are 3.5 to 5 cm (1.4 to 2.0 in) in diameter, violet-purple, often with small greenish-yellow spots or streaks. The fruit is a dry capsule 1.5 to 2.5 cm (0.59 to 0.98 in) long, containing numerous small seeds.
The two subspecies are:
- R. p. ponticum, found from Bulgaria east to Georgia
- R. p. baeticum (Boiss. & Reut.) Hand.-Mazz. found in Spain and Portugal
Distribution and habitat
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In Asia it occurs in Turkey, Lebanon, Georgia, the Krasnodar area of southern Russia, the Himalayas, Afghanistan, Tajikistan, Northern Pakistan, and parts of Indian-administered Kashmir into the northern Republic of India (Himachal Pradesh, Uttarakhand). It is the state flower of Azad Jammu and Kashmir.
Though it had been present in Great Britain before the last Ice Age, it did not recolonise afterwards and the ecology of the island grew up without it. Its presence today is due to humans introducing it, and it easily naturalises and becomes a pest in some situations, often covering whole hillsides (especially in Snowdonia and the western British Isles). In the British Isles, it colonises moorlands, uplands, shady woodlands (alongside escaped laurels and the native holly) and in areas of acid soils, often in shaded areas.
It was noted by the botanist Joseph Pitton de Tournefort during his travels in the Near East in 1700–02, and so received its name from Linnaeus to identify the ancient kingdom on the south shores of the Black Sea, Pontus, in which it grew. At the other end of its range, in southern Spain, Linnaeus' friend and correspondent Clas Alströmer found it growing with oleander. It was introduced to Britain as an ornamental shrub in 1763, and later planted as cover for game birds. It is now considered to be an invasive species.
Cultivation and uses
Rhododendron ponticum subsp. baeticum is one of the most extensively cultivated rhododendrons in western Europe. It is used as an ornamental plant in its own right, and more frequently as a rootstock onto which other more attractive rhododendrons are grafted. The plants were first grown in Britain in the 1760s, supplied by Conrad Loddiges, and became widely distributed through the commercial nursery trade in the late 18th and early 19th centuries. The roots readily send up suckers from below the graft, often allowing it to overtake the intended grafted rhododendron.
Honey produced with pollen from the flowers of this plant can be quite poisonous, causing severe hypotension and bradycardia in humans if consumed in sufficient quantities, due to toxic diterpenes (grayanotoxins).
Suckering of the root, together with its abundant seed production, has led to it becoming an invasive species over much of western Europe and in parts of New Zealand. Rhododendron control is a key element in nature conservation in those areas. Conservation organisations in Britain now believe R. ponticum has become "a severe problem" in the native Atlantic oakwoods of the west highlands of Scotland and in Wales, and on heathlands in southern England, crowding out the native flora. Clearance strategies have been developed, including the flailing and cutting down of plants with follow-up herbicide spraying. Injection of herbicide into individual plants has been found to be more precise and effective.
A recent study in the journal Functional Ecology also showed that invasive Rhododendron nectar was toxic to native honeybees (Apis mellifera), killing individuals within hours of consumption. It also paralyzed bees of the species Andrena carantonica, a solitary mining bee. Bees became paralysed and exhibited excessive grooming or other distress behaviours after feeding on Rhododendron nectar, and ate less food than bees fed a control nectar. In contrast the buff-tailed bumblebee was not affected by the rhododendron nectar.
- Alice M. Coats, Garden Shrubs and Their Histories (1964) 1992, s.v. "Rhododendron"; http://www.countrysideinfo.co.uk/rhododen.htm#Introduction%20to%20Britain .
- Hayes, Andrew Wallace (2007). Principles and methods of toxicology. CRC Press. p. 998. ISBN 978-0-8493-3778-9.
- "New flora and fauna for old". The Economist. 2000-12-21. Retrieved 2008-12-14.[dead link]
- "Rhododendron: A killer of the Countryside". Offwell Woodland & Wildlife Trust. 2004. Retrieved 30 May 2010.
- "BREAKTHROUGH IN BATTLE AGAINST PROBLEM PONTICUM". Forestry Commission. 30 July 2004. Retrieved 30 May 2010.
- Tiedeken, Erin Jo; Egan, Paul A.; Stevenson, Philip C.; Wright, Geraldine A.; Brown, Mark J. F.; Power, Eileen F.; Farrell, Iain; Matthews, Sharon M.; Stout, Jane C.; Manson, Jessamyn (November 2015). "Nectar chemistry modulates the impact of an invasive plant on native pollinators". Functional Ecology: n/a–n/a. doi:10.1111/1365-2435.12588.
- Flora Europaea: Rhododendron ponticum
- Rhododendron Ponticum is the emblem and symbol of Bulgaria's most exotic National Park – The Strandja mountains
- Germplasm Resources Information Network: Rhododendron ponticum
- Centre for Conservation Strategy: Rhododendron ponticum in Britain
- Danish Rhododendron Society: Rhododendron ponticum in Europe
- Milne, R. I., & Abbott, R. J. (2000). Origin and evolution of invasive naturalized material of Rhododendron ponticum L. in the British Isles. Molecular Ecology 9: 541–556 Abstract.
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