Oleander, known scientifically as Nerium oleander, is a versatile and resilient evergreen shrub belonging to the Apocynaceae family. From a botanical perspective, it is a plant supremely adapted to survive in challenging conditions, featuring long, leathery, lance-shaped leaves arranged in pairs or whorls of three. These leaves possess a thick cuticle and sunken stomata, two key xerophytic adaptations that minimize water loss, making the plant exceptionally drought-tolerant. All parts of the plant contain toxic cardiac glycosides, a potent defense mechanism against herbivores. Its showy, fragrant flowers are terminal cymes, meaning the flower clusters form at the end of the branches, attracting pollinators like bees and butterflies.
The most common way to categorize oleander varieties is by the structure of their flowers, which directly relates to their reproductive strategy. This is a key botanical distinction.
Single-Flowered Varieties: These cultivars feature flowers with five petals radiating from a central corolla tube. This is the wild, or species, form. The simple structure provides easy access for pollinators to the nectar and pollen, making them highly attractive to bees and other beneficial insects. Examples include the classic white 'Album' or the vibrant red 'Rubrum'.
Double-Flowered Varieties: These cultivars have been selected for extra petals, often appearing rose-like. Botanically, these extra petals are stamens that have been transformed into petal-like structures (a phenomenon known as petalody). While this creates a lush, dramatic visual effect, the altered floral anatomy often makes the nectar and pollen less accessible, reducing the flower's value to pollinators. 'Mrs. Roeding' is a popular double pink variety.
Oleanders also exhibit variation in their growth form, which is crucial for garden planning from a structural perspective.
Standard Large Shrubs: Most oleander varieties grow as large, sprawling, multi-stemmed shrubs, typically reaching 8 to 12 feet in height and width. They are excellent for use as tall informal hedges, privacy screens, or as standalone specimen plants. Their vigorous growth habit allows them to fill a space quickly. The common pink or red oleanders often fall into this category.
Dwarf Cultivars: These varieties have been bred for a compact, densely-branched growth habit, maturing at around 3 to 5 feet tall. This makes them suitable for smaller gardens, container planting on patios, or low borders. Their reduced size does not diminish their flower power, as they still produce prolific blooms. A prominent example is the Petite series, which includes 'Petite Pink' and 'Petite Salmon'.
Beyond form and habit, specific cultivars offer unique botanical characteristics. 'Hardy Red' is often noted for its slightly increased cold tolerance, potentially surviving brief dips to lower temperatures than other varieties due to perhaps a higher solute concentration in its cells. The 'Variegata' types, such as 'Sister Agnes' with white-edged leaves, have foliage with less chlorophyll in the white sectors. This can sometimes make them slightly less vigorous than their solid-green-leaved counterparts, as their photosynthetic capacity is reduced. The pigments in the flowers themselves, ranging from pure white through yellows, pinks, salmons, and deep reds, are due to various carotenoids and anthocyanins present in the petals.
