The primary distinction between single and double bloom oleander varieties (Nerium oleander) lies in the fundamental morphology of their flowers, specifically the number and transformation of petals. A single bloom oleander presents the flower structure typical of the species in its wild form. It consists of five petals arranged in a radial, funnel-shaped corolla. This simple structure is not just aesthetically classic but also functionally efficient for the plant's reproductive process, providing a clear landing platform and guide for pollinators leading to the central reproductive organs—the stamens and pistil.
Double bloom varieties are a result of a horticultural mutation known as petalody, wherein the plant's reproductive organs, the stamens and sometimes even the pistil, are transformed into additional petal-like structures. Instead of producing pollen-bearing anthers, these transformed organs develop into narrow, ruffled, or twisted petaloids. This process creates the characteristic "double" appearance, with the flower head appearing much fuller, denser, and often more ornate due to the multiplied number of petaloid structures, which can far exceed the foundational five petals.
From a biological and evolutionary perspective, this morphological difference has significant implications. The single bloom oleander is a perfectly adapted reproductive unit. Its accessible nectar and prominent sexual organs facilitate effective cross-pollination by insects, leading to successful seed production. Conversely, the double bloom variant is often functionally sterile. The conversion of stamens into petaloids drastically reduces or completely eliminates pollen production. Furthermore, the dense mass of petals and petaloids can physically obstruct access to any remaining fertile parts, making pollination by insects exceedingly difficult. Consequently, the plant allocates a substantial amount of energy towards producing these showy but reproductively useless structures rather than into seed development.
This difference in reproductive strategy influences the plant's overall vegetative growth and cultivation. Because double-flowered varieties rarely set seed, they tend to channel the energy that would have been spent on fruit and seed production into additional vegetative growth. This can sometimes result in a marginally more vigorous shrub or a longer blooming period, as the plant is not diverting resources to reproduction. However, this sterility also means that double oleanders cannot be propagated true-to-type from seed; they must be cloned through vegetative methods such as cuttings or tissue culture to preserve their double-flowering characteristic.
