Nerium oleander, the oleander, is a shrub or small tree native to the Mediterranean region east to southern Asia, from Morocco and Portugal east to India and Myanmar; the Flora of China also cites it as native in Yunnan in China.Nerium oleander in the Flora of China It is the only species currently classified in the genus Nerium, belonging to subfamily Apocynoideae of the dogbane family Apocynaceae. It is also cultivated widely in temperate and subtropical areas as an introduced ornamental and landscaping plant.
It is tolerant to both drought and inundation, but not to prolonged frost. It is a highly poisonous plant but its bitterness renders it unpalatable to humans and most animals, so poisoning cases are rare and the general risk for human mortality is low. Ingestion of larger amounts may cause nausea, vomiting, excess salivation, abdominal pain, bloody diarrhoea and irregular heart rhythm. Prolonged contact with sap may cause skin irritation, eye inflammation and dermatitis.
Description
Oleander grows to tall, with erect stems that splay outward as they mature; first-year stems have a glaucous bloom, while mature stems have a greyish bark. It is most commonly grown in its natural shrub form, but can be trained into a small tree with a single trunk. The leaves are in pairs or whorls of three, thick and leathery, dark-green, narrow lanceolate, long and broad, and with an entire margin filled with minute reticulate venation web typical of eudicots. The leaves are light green and very glossy when young, maturing to a dull dark green.
The flowers grow in clusters at the end of each branch; they are diameter, pink, with a deeply 5-lobed fringed corolla round the central corolla tube in natural wild plants. White to dark red, larger (to 5 cm diameter), and double-flowered, cultivars are also common in cultivation.The "yellow oleander" is Cascabela thevetia They are often, but not always, sweet-scented.In the past, scented plants were sometimes treated as the distinct species N. odorum, but the character is not constant and it is no longer regarded as a separate taxon. The fruit is a long narrow pair of follicles long, which splits open at maturity to release numerous downy seeds.
Taxonomy Nerium oleander is the only species currently classified in the genus Nerium. It belongs to (and gives its name to) the small tribe Nerieae of subfamily Apocynoideae of the dogbane family Apocynaceae. The genera most closely related thus include the equally ornamental (and equally toxic) Adenium G.Don and Strophanthus DC., both of which also contain (like oleander) potent cardiac glycosides that have led to their use as arrow poisons in Africa. The three remaining genera Alafia Thouars, Farquharia Stapf and Isonema R.Br. are less well known in cultivation.
Synonymy The plant has been described under a wide variety of names that are today considered synonymous:
Oleander Medik. Nerion Tourn. ex St.-Lag. Nerion oleandrum St.-Lag. Nerium carneum Dum.Cours. Nerium flavescens Spin Nerium floridum Salisb. Nerium grandiflorum Desf. Nerium indicum Mill. Nerium japonicum Gentil Nerium kotschyi Boiss. Nerium latifolium Mill. Nerium lauriforme Lam. Nerium luteum Nois. ex Steud. Nerium madonii M.Vincent Nerium mascatense A.DC. Nerium odoratissimum Wender. Nerium odoratum Lam. Nerium odorum Aiton Nerium splendens Paxton Nerium thyrsiflorum Paxton Nerium verecundum Salisb. Oleander indica (Mill.) Medik. Oleander vulgaris Medik.
Etymology The taxonomic name Nerium oleander was first assigned by Linnaeus in 1753. The genus name Nerium is the Latinised form of the Ancient Greek name for the plant nẽrion (νήριον), which is in turn derived from the Greek for water, nẽros (νηρός), because of the natural habitat of the oleander along rivers and streams.
The origins of the species name are disputed. The word oleander appears as far back as the first century AD, when the Greek physician Pedanius Dioscorides cited it as one of the terms used by the Romans for the plant. Merriam-Webster believes the word is a Medieval Latin corruption of Late Latin names for the plant: arodandrum or lorandrum, or more plausibly rhododendron (another Ancient Greek name for the plant), with the addition of olea because of the superficial resemblance to the olive tree (Olea europaea).Cf. oleaster Another theory posited is that oleander is the Latinised form of a Greek compound noun: οllyo (ὀλλύω) 'I kill', and the Greek noun for man, aner, genitive andros (ἀνήρ, ἀνδρός). ascribed to oleander's toxicity to humans.
The etymological association of oleander with the bay laurel has continued into the modern day: in France the plant is known as "laurier rose", while the Spanish term, "Adelfa", is the descendant of the original Ancient Greek name for both the bay laurel and the oleander, daphne, which subsequently passed into Arabic usage and thence to Spain.
The ancient city of Volubilis in Morocco may have taken its name from the Berber name alili or oualilt for the flower.
It is also sometimes known as rose laurel, be-still tree, or rosebay.
Distribution and habitat
Nerium oleander is either native or naturalised to a broad area from northwest Africa through the Mediterranean region and warmer areas of the Black Sea region, Arabian Peninsula, southern Asia, and as far east as Yunnan in southern parts of China.Pankhurst, R. (editor). Nerium oleander L. Flora Europaea. Royal Botanic Garden Edinburgh. Retrieved on 2009-07-27.Bingtao Li, Antony J. M. Leeuwenberg, and D. J. Middleton. "Nerium oleander L.", Flora of China. Harvard University. Retrieved on 2009-07-27. INCHEM (2005). Nerium oleander L. (PIM 366). International Programme on Chemical Safety: INCHEM. Retrieved on 2009-07-27Huxley, A.; Griffiths, M.; Levy, M. (eds.) (1992). The New RHS Dictionary of Gardening. Macmillan. . It can be found in tropical, subtropical, and temperate regions. The plant typically occurs in dry locations with a lot of sun near stream beds where it can alternatively tolerate long seasons of drought and inundation from winter rains.
Ecology Some invertebrates are known to be unaffected by oleander toxins, and feed on the plants. Caterpillars of the polka-dot wasp moth (Syntomeida epilais) feed specifically on oleanders and survive by eating only the pulp surrounding the leaf-veins, avoiding the fibres. Larvae of the common crow butterfly (Euploea core) and oleander hawk-moth (Daphnis nerii) also feed on oleanders, and they retain or modify toxins, making them unpalatable to potential predators such as birds, but not to other invertebrates such as spiders and wasps.
The flowers require insect visits to set seed, and seem to be pollinated through a deception mechanism. The showy corolla acts as a potent advertisement to attract pollinators from a distance, but the flowers are nectarless and offer no reward to their visitors. They therefore receive very few visits, as typical of many rewardless flower species.Shmida, A., Ivri, Y., and Cohen, D. The enigma of the oleander. Eretz VeTeva, January–February 1995 (in Hebrew). Fears of honey contamination with toxic oleander nectar are therefore unsubstantiated.
Leaf scorch
A bacterial disease known as (Xylella fastidiosa subspecies sandyi) has become a serious threat to the shrub since it was first noticed in Palm Springs, California, in 1992. The disease has since devastated hundreds of thousands of shrubs mainly in Southern California, but also on a smaller scale in Arizona, Nevada and Texas. The culprit is a bacterium which is spread via insects (the glassy-winged sharpshooter primarily) which feed on the tissue of oleanders and spread the bacteria. This inhibits the circulation of water in the tissue of the plant, causing individual branches to die until the entire plant is consumed.
Symptoms of leaf scorch infection may be slow to manifest themselves, but it becomes evident when parts of otherwise healthy oleanders begin to yellow and wither, as if scorched by heat or fire. Die-back may cease during winter dormancy, but the disease flares up in summer heat while the shrub is actively growing, which allows the bacteria to spread through the xylem of the plant. As such it can be difficult to identify at first because gardeners may mistake the symptoms for those of drought stress or nutrient deficiency.
Pruning out affected parts can slow the progression of the disease but not eliminate it. This malaise can continue for several years until the plant completely dies; there is no known cure. The best method for preventing further spread of the disease is to prune infected oleanders to the ground immediately after the infection is noticed.
The responsible pathogen was identified as the subspecies sandyi by Purcell et al., 1999.
Cultivation
Beyond the traditional Mediterranean and subtropical range of oleander, the plant can also be cultivated in mild oceanic climates with the appropriate precautions. It is grown without protection in warmer areas in Switzerland, southern and western Germany and southern England and can reach great sizes in London and to a lesser extent in Paris due to the urban heat island effect. This is also the case with North American cities in the Pacific Northwest like Portland, Seattle, and Vancouver. Plants may suffer damage or die back in such marginal climates during severe winter cold but will rebound from the roots.
On the East Coast of the United States, it grows as far north as Virginia Beach, while in California and Texas miles of oleander shrubs are planted on median strips. There are estimated to be 25 million oleanders planted along highways and roadsides throughout the state of California. Because of its durability, oleander was planted prolifically on Galveston Island in Texas after the disastrous Hurricane of 1900. They are so prolific that Galveston is known as the 'Oleander City'; an annual oleander festival is hosted every spring. Moody Gardens in Galveston hosts the propagation program for the International Oleander Society, which promotes the cultivation of oleanders. New cultivars are hybridised and grown on the Moody Gardens grounds, encompassing every named cultivar.
History
Nerium oleander has a history of cultivation going back millennia, especially amongst the great ancient civilisations of the Mediterranean Basin. Some scholars believe it to be the rhodon (rose), also called the "Rose of Jericho", mentioned in apocryphal writings (Sirach 24:14) dating between 450 and 180 BC.
The ancient Greeks had several names for the plant, including rhododaphne, nerion, rhododendron and rhodon. Pliny the Elder said that the Romans had no Latin word for the plant, but used the Greek terms instead. Pedanius Dioscorides states in his 1st century AD pharmacopeia De Materia Medica that the Romans used the Greek rhododendron but also the Latin Oleander and Laurorosa. The Egyptians apparently called it scinphe, the North Africans rhodedaphane, and the Lucanians (a southern Italic people) icmane.
Both Pliny and Dioscorides stated that oleander was an effective antidote to venomous snake bites if mixed with rue and drunk. However, both rue and oleander are poisonous themselves, and consuming them after a venomous snake bite can accelerate the rate of mortality and increase fatalities.
A 2014 article in the medical journal Perspectives in Biology and Medicine posited that oleander was the substance used to induce hallucinations in the Pythia, the female priestess of Apollo, also known as the Oracle of Delphi in Ancient Greece. According to this theory, the symptoms of the Pythia's trances (enthusiasmos) correspond to either inhaling the smoke of or chewing small amounts of oleander leaves, often called by the generic term laurel in Ancient Greece, which led to confusion with the bay laurel that ancient authors cite.