| Rhododendron ponticum | |
|---|---|
 | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Eudicots |
| Clade: | Asterids |
| Order: | Ericales |
| Family: | Ericaceae |
| Genus: | Rhododendron |
| Species: | R. ponticum |
| Binomial name | |
| Rhododendron ponticum | |
| Synonyms[1] | |
List
| |
Rhododendron ponticum, calledcommon rhododendron orpontic rhododendron, is a species offlowering plant in theRhododendron genus of the heath familyEricaceae. It is native to theIberian Peninsula in southwestEurope and theCaucasus region in northernWest Asia.
R. ponticum is a dense,suckeringshrub or smalltree growing to 5 m (16 ft) tall, rarely 8 m (26 ft). Theleaves areevergreen, 6 to 18 cm (2.4 to 7.1 in) long and 2 to 5 cm (0.79 to 1.97 in) wide. Theflowers are 3.5 to 5 cm (1.4 to 2.0 in) in diameter, violet-purple, often with small greenish-yellow spots or streaks. Thefruit is a drycapsule 1.5 to 2.5 cm (0.59 to 0.98 in) long, containing numerous smallseeds.
It has twosubspecies:
| Image | Name | Distribution |
|---|---|---|
 | R. p. baeticum (Boiss. & Reut.)Hand.-Mazz. | Found in central and southernPortugal and southernSpain (in theProvince of Cádiz).[2] |
 | R. p. ponticum | Found around the southernBlack Sea Basin (easternBulgaria, northernTurkey,Georgia,Northern Caucasus) and centralLebanon.[3] |
And avariegated variety:
The species has twodisjunct populations, one in the southwesternIberian Peninsula (central and southernPortugal and southwesternSpain) and the other near the southernBlack Sea Basin (easternBulgaria, northernTurkey,Georgia, andNorthern Caucasus). It has also been introduced toMadeira,Myanmar,Belgium, theBritish Isles,Netherlands andFrance.[1][3]
The range in the Iberian Peninsula is limited to mountain ranges, theCaramulo mountains, theMonchique range and theAljibe range. A remnant of the originallaurissilva forests that covered the peninsula 66 million years ago.[5]
Though the common rhododendron was present inGreat Britain prior to themost recent ice age, it did not recolonise afterwards and the modern ecology of the island developed without it. Its presence today in Great Britain is due to humans introducing it, and it easily naturalises and becomes a pest in some situations, often covering whole hillsides (especially inSnowdonia and western Scotland). In the British Isles, it colonises moorlands, uplands, shady woodlands (alongside escaped laurels and the native holly) and in areas of acid soils.[6]
Fossil evidence shows it had a much wider range across most of southern and western Europe before theLate Glacial Maximum, or until about 20,000 years ago.[6]
It was noted by the botanistJoseph Pitton de Tournefort during his travels in the Near East in 1700–1702, and so received its name fromLinnaeus to identify the ancient kingdom on the south shores of theBlack Sea,Pontus, in which it grew. At the other end of its range, in southern Spain, Linnaeus' friend and correspondentClas Alströmer found it growing witholeander.[citation needed] It was introduced to Britain as an ornamentalshrub in 1763, and later planted as cover for game birds. It is now considered to be aninvasive species.[7]
Rhododendron ponticum subsp.baeticum is one of the most extensively cultivated rhododendrons in western Europe. It is used as anornamental plant in its own right, and more frequently as arootstock onto which other more attractive rhododendrons aregrafted. The plants were first grown in Britain in the 1760s, supplied byConrad 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 severehypotension andbradycardia in humans if consumed in sufficient quantities, and is known asmad honey due to toxic diterpenes (grayanotoxins).[8] This poisonous honey plays a significant part in the 2023 filmA Haunting in Venice.
Sap from a freshly cut branch can be used to treat toothaches.[9]
Suckering of the root, together with its abundant seed production, has led to it becoming aninvasive species over much of western Europe and in parts ofNew Zealand. Rhododendron control is a key element in nature conservation in those areas.[10]Conservation organisations in Britain now believeR. ponticum has become "a severe problem" in the nativeAtlanticoakwoods of thewest highlands ofScotland and inWales andIreland, and onheathlands in southernEngland, crowding out the native flora.[11] Clearance strategies have been developed, including the flailing and cutting down of plants with follow-upherbicide spraying. Injection of herbicide into individual plants ha s been found to be more precise and effective.[12]
InIreland, particularly in theKillarney National Park it is extremely invasive and the exact date that Rhododendrons were introduced is unknown. It has no natural predators in Ireland and is avoided by grazing animals as the stems and leaves contain toxic levels of grayanotoxins. It is suggested that the shrub was planted in Derrycunihy wood in the 19th century or before.[13]
A study[14] in the journalFunctional Ecology also showed that invasive rhododendron nectar was toxic to European honeybees (Apis mellifera), killing individuals within hours of consumption. It also paralyzed bees of the speciesAndrena 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 (Bombus terrestris) was not affected by the rhododendron nectar. It is important not to see Rhododendron as a problem species for honey bees as they actually avoid the flowers owing to their ability to detect the toxin nectar. The toxicity is caused bygrayanotoxin 1 which is one of several highly hydroxylated diterpenoid defence chemicals produced in the leaves ofRhododendron to protect againstherbivores – e.g. theThripsHeliothrips haemorrhoidalis.[15] Some species of honey bee (Apis mellifera sub sppcaucasica) tolerate the toxin and make so-called "mad honey".
Recent efforts to manage the spread ofRhododendron ponticum in the United Kingdom has led to some controversy, particularly within the grounds ofTaymouth Castle in highland Perthshire. There remains debate over appropriate identification of the plant, particularly where it is interspersed amongst clumps of Cherry Laurel (Prunus laurocerasus) in areas of mixed woodland. Tensions surrounding the management of these cohabiting species were brought to a head in early December 2022 when prominent arboriculturalists faced strong opposition from machine operators within the estate over correct identification and subsequent management of the plant, culminating in the destruction of a large thicket of laurel. While there remains debate over the associated similarities and differences between the plants, Cherry laurel may indeed be viewed and treated in a like manner to rhododendron within this habitat niche.[16]
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