Nerium oleander L. -- Arizona

Nerium oleander (oleander) is native to southern Europe, north Africa, the Middle East, and southwestern Asia (Kew, 2024). Oleander is a popular evergreen shrub known for its beautiful flowers and ability to thrive in various climates. It is native to the Mediterranean region but is now cultivated worldwide for its ornamental value (LiveToPlant, 2024). This is a very commonly used landscaping plant in the Phoenix urban area (USFS, 2024). GBIF (2024) lists Nerium oleander as introduced in 75 countries or islands.

GBIF (2024) maps Nerium oleander as being introduced in areas climate-matching with Arizona: southwestern USA, northern Mexico, South Africa, and southeast Australia. A weed alert from Arizona's Tonto National Forest (USFS, 2024) reports: this is a very commonly used landscaping plant in the Phoenix urban area. There are two sites where it has naturalized on the Tonto National Forest. Several clumps of it have attained great height, growing in Arnett and Telegraph Canyons, near Boyce Thompson Arboretum. Another large individual plant was found growing in Camp Creek, on the Cave Creek Ranger District, apparently naturalized from a nearby recreational residence. Oleander has not been considered to have invasive potential until fairly recently. A Red Alert was issued by the California Invasive Species Council for this plant in 2000. It had been found along the Sacramento floodplain near Redding, and riparian zones in southern California. This year, in Arizona, the Arizona Daily Star included oleander in a list of ornamental plants that were becoming invasive in Saguaro National Park.

GBIF (2024) lists Nerium oleander as introduced in 75 countries or islands, and cites evidence of impact in Fiji and South Africa. Henderson (1992) describes the naturalization of oleander in parts of South Africa, where it is spreading into many mountain river valleys of the Cederberg, the Gamka, the Baviaanskloof, and others. The plant is of concern due to its extreme toxicity and lack of ecological services. Oleander has been listed as a NEMBA category 1b invader in South Africa (Cronin et al, 2017).

GBIF (2024) lists Nerium oleander as introduced with evidence of impact in South Africa. GBIF (2024) maps occurrence of Nerium oleander in parts of eastern South Africa which are a climate match with Arizona. Henderson (1992) describes the naturalization of oleander in parts of South Africa, where it the plant is of concern due to its extreme toxicity and lack of ecological services. Arizona's Tonto National Forest has issed a weed alert for oleander (USFS, 2024) where projects have been initiated to remove infestations from Arnett and Telegraph Canyons, near Boyce Thompson Arboretum. Oleander is listed as an ornamental plant becoming invasive in Saguaro National Park.

Nerium oleander is the only species in the genus Nerium. GBIF shows the somewhat related genus Pachypodium to very rarely naturalize outside its native range (and these records may possibly be cultivated specimens?). Most species of Pachypodium are not tracked by GBIF. Cascabela thevetia is another related genus, native throughout Mexico and in Central America, and cultivated widely as an ornamental. GBIF shows this species as introduced in 27 countries, with evidence of impact in 6 countries, however these are not in climate matched regions.

GBIF (2024) maps Nerium oleander as occurring in many tropical or warm-temperate regions, including the southeast USA, southern Mexico, Central America, Brazil, Europe, India, southeast Asia, coastal Australia, and the North Island of New Zealand. These areas are not climate-matching with Arizona and cover a greater area than regions which are.

The HEAR (2005) Report rates oleander as not having a smothering growth habit and no evidence on forming dense thickets. However the plant can grow to be a large shrub.

The HEAR (2005) Report rates oleander as not creating a fire hazard in natural ecosystems. Henderson (1992) describes oleander as a rheophyte confined to the beds of swift running streams and rivers where it grows up to flood level, but not further than the reach of regular flash floods. This amphibious growth habit would not seem to be fire-prone.

The World Flora Online (WFO, 2024) describes oleander as producing abundant, poisonous, clear, sticky latex. The HEAR (2005) Report rates oleander as toxic to humans, toxic to animals, unpalatable to grazing animals. "One leaf of N. oleander may be sufficient to kill a sheep. The lethal dose of N. oleander leaves reported for several animal species is about 0.5
mg/kg." The US Forest Service (2024) cites information that all parts of the plant are extremely poisonous, containing 10 different cardiac glycosides. These compounds induce cardiac arrhythmia and eventual death. The lethal dose of green oleander leaves for cattle and horses is 0.005% of the animal’s body weight. Inhalation of smoke from a burning oleander also can cause poisoning.

The HEAR (2005) Report claims no evidence for oleander regarding formation of dense thickets.

The HEAR (2005) Report rates oleander as not reproducing vegetatively.

The HEAR (2005) Report rates oleander as not reproducing vegetatively. Suckers are produced at the base of the plant, but not a means of spread or natural fragmentation."

The HEAR (2005) Report rates oleander as "yes" for produces viable seed, however "All seedlings tracked died before completing one year of life, mostly because of desiccation during the first summer drought. It rarely produces fertile seed in the summer rainfall areas (of South Africa). Young oleander seedlings are extremely sensitive to drought and need a steady supply of soil moisture (Henderson, 1992). Experiments by Bufford et al, (2016) showed Nerium oleander had high germination in germination chambers.

The HEAR (2005) Report rates oleander as having prolific seed production (>1000/m2). "Fruits are many seeded follicles (181 seeds per fruit on average), so that individual plants often liberate thousands of seeds in spite of the low percentage of fruit set."

Experiments by Bufford et al, (2016) showed Nerium oleander exhibited the lowest percentage germination [of those species studied] in the field, but had high germination in germination chambers. Germination was significantly higher at the wet site and was higher in species [of those under study] with larger seeds. Hear (2025) reports no dormancy - seeds germinate rapidly after seed set. The hairy, water-dispersed seeds [of oleander] are released during the rainiest season in the year, and germination follows rapidly. All seedlings tracked died before completing 1 year of life, mostly because of desiccation during the first summer drought (HEAR, 2005). Young oleander seedlings are extremely sensitive to drought and need a steady supply of soil moisture (Henderson, 1992). These reports suggest high germination can occur under optimal conditions with abundant moisture, though high mortality can occur if moisture is not sustained.

The young plants (woody shrub) do not start to flower (in cultivation from seed) until the third year (Gardener, 2024). You need a little patience for breeding, because the small plants (from seed) do not bloom until they are three years old at the earliest - only now can you see what shape and color the flowers have so that you can select (HobbyGardeners, 2024).

The flowering season typically occurs during the summer months and continues into early fall (LiveToPlant, 2024). Oleander is cultivated in part, for its attractive flowers with a long blooming season.

The HEAR Report rates oleander as have propagules which are not bird dispered. The small feathery seeds are adapted to wind and water dispersal, and are unlikely to have any food value to animals (Henderson, 1992). All parts of the plant are extremely poisonous (USFS, 2024) thus unlikely to be consumed by animals.

The HEAR (2005) Report rates oleander as having propagules which can be water dispered. Henderson (1992) describes oleander as a "rheophyte" i.e. a plant species which is confined to the beds of swift running streams and rivers where it grows up to flood level, but not further than the reach of regualr flash floods. Many rheophytes can be easily cultivated outside of their rheophytic habitat. The small feathery seeds are adapted to wind and water dispersal (Henderson, 1992). Swift running water can be expected to disperse seed long-distance. The seed is described as having a crown of silky hairs which arises directly from the top of the seed. The fruit pod splits to release airborne seed (HEAR, 2024).

The HEAR (2005) Report rates oleander as not likely to disperse as a produce contaminant, a "no" to the category of propagules likely to be dispersed unintentionally (plants growing in heavily trafficked areas). No evidence that the seeds have any means of attachment.