In Britain, the rhododendron is the showiest of a triumvirate of invasive plants whose introductions are frequently blamed on the enthusiasms of Victorian estate owners, and their gardeners, for botanical novelty.
The name Rhododendron, literally 'rose tree', was adopted by Carolus Linnaeus from the classical Greek for oleander (Nerium oleander; Apocynaceae), because of the superficial similarities between the leaves of these two shrubs. The genus has approximately 850 species and is distributed through the northern Hemisphere, south-east Asia and Australasia. Its greatest diversity is in the Himalayas.
Rhododendron ponticum, native to the eastern and western Mediterranean, was associated with Turkey by Linnaeus, hence the specific name. Rhododendron was discovered by Joseph Tournefort, whose classification scheme Linnaeus had learnt as a student, on his pioneering expedition to the Levant (1700-1702). Tournefort's travels in the region inspired those of the Oxford-based academic John Sibthorp over 80 years later.
Rhododendron ponticum has two subspecies. Subspecies ponticum is distributed from Bulgaria to Turkey and east to Georgia, whilst ssp. baeticum is native to Spain and northern Portugal. Rhododendron is thought to have been introduced to Britain in 1763 from the Iberian Peninsula, although subsequent introductions occurred from the Black Sea region.
In antiquity, the region around the Black Sea was known for producing an intoxicating, so-called 'mad', honey. The fourth-century BCE Greek philosopher and soldier Xenophon reported soldiers in a Greek army invading Asia Minor had been poisoned by honey stolen from hives. In 69 BCE, Mithridates deliberately used 'mad honey' to subdue Roman soldiers before his army attacked them. The honey was apparently produced by bees visiting rhododendron bushes. 'Mad honey' is still produced in regions of the Caucuses (and parts of Nepal) as a recreational drug. The effects are produced by a class of neurotoxic diterpenes called grayanotoxins which are found in several members of the Ericaceae. These compunds have their effects by interfering with the sodium channels of neurons.
In the late-nineteenth century, rhododendron was commonplace in Victorian shrubberies, along with many of its North American and Himalayan cousins. Today, the rhododendron ranks as one of the most invasive plants in the British landscape; it 'leapt' the garden wall, established itself in the wild and has significant environmental effects. Genetic analyses confirm that almost all naturalised Rhododendron ponticum in Britain is ssp. baeticum, although there has been some interbreeding with two species introduced from North America, Rhododendron catawbiense and Rhododendron maximum.
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