Nerium oleander
(oleander)

Nerium oleander is a shrub native to the Mediterranean region which is widely used as an ornamental in the tropics, subtropics and warm temperate areas worldwide. Although reported only as cultivated, persistent or as an occasional escape in some countries, it has naturalized in others mainly through water dispersion. Although very few details are available about its invasiveness, N. oleander is reported as capable of producing many seedlings and being very competitive. Another major concern about the species is that all plant parts are very poisonous, especially to small children, pets and livestock. Limiting the invasiveness of N. oleander is its preference for intermittent waterways, low pollination and fruit production, adaptation to water dispersal and the requirement of seedlings for moist shaded environments to survive. N. oleander is reported as invasive in South Africa, Hawaii (USA), Australia, New Zealand and Niue. It is also listed as invasive in two US National Parks (Death Valley National Park in California and Lake Mead National Park in Nevada) and in Florida but without further details. It is listed as potentially invasive in Cuba, being categorized as a naturalized species with a tendency to proliferate in the country and with the capacity to produce a high number of seeds.

Nerium oleander is native to the Mediterranean region, including northern Africa, southern Europe and South East Asia (HEAR, 2017). It is extensively cultivated worldwide throughout the tropics and subtropics (PIER, 2018). This species occurs in Africa, Asia, North America, Central America, the Caribbean, South America, Europe and Oceania (See Distribution Table for details; Acevedo-Rodríguez and Strong, 2012; PIER, 2018; Euro+Med, 2020; Flora do Brasil, 2020; Missouri Botanical Garden, 2020; PROTA, 2020; Smithsonian Museum of Natural History, 2020; USDA-ARS, 2020).

Nerium oleander is one of the most used ornamental shrubs in the tropics and subtropics (PIER, 2018). It has a high risk of introduction due to its popularity as an ornamental and high availability through local nurseries and the internet. Although widely distributed, there are still some areas without reports of its occurrence. The development of cultivars more resistant to colder temperatures could expand the species range into northern latitudes (Dave's Garden, 2020).

Nerium oleander is a shrub found in tropical, subtropical and warm temperate climates (HEAR, 2017). It is reported in forests and shrublands along riverbanks and creeks, in intermittent watercourses among rocks, gravelly places, damp ravines and slopes and in the vicinity of springs in arid areas (Herrera, 1991; HEAR, 2017; PIER, 2018; PROTA, 2020). In South Africa and in the Mediterranean region, N. oleander is usually found naturally only along watercourses, with a preference for gravelly and rocky streambeds (Henderson, 1992). It can be found from sea level to elevations of almost 2000 m (Herrera, 1991). N. oleander is also cultivated as an ornamental and a landscape shrub in urban areas and is a persistent relic in old towns and graveyards (HEAR, 2017; PIER, 2018).

Genetics

Nerium oleander is a popular ornamental shrub with many cultivars developed with a wide array of sizes and different flower colours (HEAR, 2017). The chromosome numbers reported for the species are n = 11, 2n = 22 (IPCN Chromosome Reports, 1977). Germplasm collections are available for the species (Kew Royal Botanic Gardens, 2020; USDA-ARS, 2020).

Reproductive Biology

Nerium oleander is propagated by seed and when cultivated, vegetatively through cuttings (HEAR, 2017; PIER, 2018).

A description of the reproductive characteristics of N. oleander is given by Herrera (1991):
The flowers are self-compatible, but selfing is prevented by the spatial separation of the pollen and stigma development. Plants produce low pollen quantities and no nectar. Pollination is low, with some bees [Apidae] and flies [Diptera] visiting the flowers deceived by the massive fragrant flower production. Fruits reach maturity about 40 to 45 days after pollination. Unvisited flowers do not set fruit. Fruit abortion ranges from 8 to 54%. Despite a low fruit set, the seed output per plant is high as the mean number of seeds per fruit ranges from 40 to 300. The seeds have a tuft of long hairs which can aid in wind dispersal, but a relatively large percentage of the seeds land close to the parent plant. The main means of dispersal is though water during the rainy season. The seeds can float for long periods of time and even germinate while floating; germination is close to 100%. Seedlings can also float, and remain submerged for up to 5 months and only become established if the water depth is less than 10 mm. Seedling survival is low; most die in open sites and need damp, shaded places. About 46% of the seedlings die during spring and summer due to desiccation. Heavy flooding is also a cause of mortality of established seedlings.

Physiology and Phenology

Nerium oleander has a high fire tolerance (HEAR, 2017). According to Pagen (1988) flowering time throughout the range of this species varies with five zones distinguished: southern France and northern Italy (June-September); Portugal, Spain, southern Italy, Malta, Albania, Greece, Turkey and Cyprus (April, May-September); Morocco, Algeria, Tunisia, Niger and Libya (March-October); Syria, Lebanon, Israel, Jordan and Iraq (April-November); Oman, United Arab Emirates, Iran, Afghanistan, Pakistan, India, Nepal and China (almost year round); fruits can be found at any time of the year in all regions. Seed dispersal occurs during the peak of the rainy season (Herrera, 1991).

Longevity

Nerium oleander is a perennial shrub (PROTA, 2020).

Environmental Requirements

A shrub native to intermittent streambeds of the Mediterranean region, N. oleander is now distributed in tropical and subtropical areas worldwide (Missouri Botanical Garden, 2018; PROTA, 2020). It prefers full sun, will tolerate reflective heat and partial shade but not full shade (HEAR, 2017). Across its natural range, N. oleander appears to show an affinity for areas with a steady supply of soil moisture. It prefers well drained medium to heavy soils with  pH 5.5-7.8 (HEAR, 2017; PROTA, 2020); although it tolerates poor soils it will do best in fertile soils (Missouri Botanical Garden, 2018). It is salt and drought tolerant (HEAR, 2017; PIER, 2018). In areas with colder temperatures, N. oleander will tolerate some frost down to -6°C; in these areas, the species should be planted in containers to be brought indoors and maintained at temperatures above 10°C (Missouri Botanical Garden, 2018).

Various pests and diseases have been reported for N. oleander including: insects such as Aphis nerii, Aspidiotus nerii, Daphnis nerii, Caenocoris nerii, Oncopeltus fasciatus and Syntomeida epilais (Miller and Dingle, 1982; Bristow, 1991; Herrera, 1991; Popenoe et al., 2019; PROTA, 2020); viruses such as  Cucumber Mosaic Virus (CMV), Strawberry Latent Ringspot Virus (SLRV) and Tomato Spotted Wilt Virus (TSWV) (Betti and Canova, 1989; Bellardi et al., 1996); fungi Fusarium brachygibbosum, Pseudocercospora neriella and Sphaeropsis tumefaciens (Kavak, 2007; Mirhosseini et al., 2014; HEAR, 2017); and  bacteria Pseudomonas savastanoi and Xylella fastidiosa (Temsah et al., 2010; HEAR, 2017).

Environmental

• Agroforestry
• Amenity
• Boundary, barrier or support
• Erosion control or dune stabilization
• Landscape improvement
• Soil conservation
• Windbreak

General

• Botanical garden/zoo

Materials

• Dyestuffs
• Pesticide
• Poisonous to mammals
• Rubber/latex

Medicinal, pharmaceutical

• Traditional/folklore

Ornamental

• Christmas tree
• Cut flower
• garden plant
• Potted plant
• Propagation material
• Seed trade

Nerium oleander is sometimes confused with the olive tree (Olea europaea), due to the leaves being similar in shape. They are easily distinguished by the fact that N. oleander is a multi-trunked shrub and O. europaea a tree. The fruits of N. oleander are pods while those of O. europaea are drupes. The flowers are also very different: N. oleander has bigger flowers in a wide array of colours while the flowers of O. europaea are smaller and a cream-white colour (Flora of China Editorial Committee, 2020).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Eradication

The South African government recommends the removal and destruction of N. oleander plants, including prohibiting its trade (Invasive Species South Africa, 2020). In California where this species has naturalized and is sold as an ornamental, the recommendation is that it be considered for removal from the trade following talks with the horticulture industry and also monitored for further spread into wildlands (Brusati et al., 2014).

Physical/Mechanical Control

Nerium oleander can be physically controlled by cutting the stumps (Brisbane City Council, 2020).

Chemical Control

Basal bark treatments are recommended for the control of N. oleander (Brisbane City Council, 2020).

More information is needed about the effects of N. oleander on habitats and native species where reported as invasive. There is also a lack of information about the naturalization of this species in areas where reported as introduced and/or cultivated.

11/06/2020 Original text by:

Jeanine Vélez-Gavilán, University of Puerto Rico at Mayagüez, Puerto Rico