The Nerium oleander is a popular, flowering shrub known for its lush appearance and abundant blooms in warm climates. Although it is often mistaken for a deciduous species due to occasional leaf loss, the oleander is classified as an evergreen plant. It maintains green foliage throughout the year, making it a landscape staple widely cultivated across subtropical and Mediterranean regions.
The Botanical Classification of Oleander
The term “evergreen” describes a plant that retains functional green leaves throughout more than one growing season. Unlike deciduous plants, which shed all leaves annually, the oleander replaces its foliage gradually. In its native habitat, spanning from the Mediterranean to Southeast Asia, Nerium oleander continually holds its thick, leathery leaves. This growth habit allows the plant to maintain a dense, year-round screen or hedge. The persistent foliage is an adaptation to its warm, dry environment, allowing it to photosynthesize whenever conditions are favorable.
Environmental Factors Causing Leaf Drop
Although oleander is botanically evergreen, it drops large amounts of leaves when subjected to environmental stress, which often causes confusion. This leaf shedding is not a seasonal occurrence, but a survival mechanism triggered by adverse conditions.
The most common cause is exposure to freezing temperatures or frost, which damages leaf tissue and forces the plant to shed it to conserve resources. Established oleanders can tolerate brief dips near 20°F, but prolonged cold exposure causes significant defoliation.
Extreme or prolonged drought can also trigger leaf drop, despite the plant’s tolerance for dry conditions once established. The plant sheds foliage to reduce the surface area exposed to evaporation, conserving its internal water supply. Other stressors include sudden temperature fluctuations, waterlogging, or nutrient imbalances, all of which can cause a protective leaf-shedding response.
Key Identification and Toxicity Warning
Identification
The oleander is identified by its distinct foliage and striking flowers. The leaves are narrow, lance-shaped, and dark or grayish-green, typically growing in whorls of three or four around the stem. The flowers appear in clusters at the ends of the branches, blooming from late spring through fall. These funnel-shaped flowers are commonly found in shades of pink, white, or red.
Toxicity Warning
All parts of the Nerium oleander plant are toxic if ingested by humans, pets, or livestock. The plant contains potent cardiac glycosides, such as oleandrin and neriine, which severely affect the heart and nervous system. Symptoms of poisoning include vomiting, diarrhea, and an erratic pulse, and ingestion can be lethal. Since the plant remains toxic even when dried, gardeners should wear gloves when pruning and never use oleander wood for cooking fires.