To understand an oleander's relationship with cold, one must first consider its origins. We are native to the Mediterranean basin, a region characterized by hot, dry summers and mild, wet winters. Our entire physiology is optimized for this environment. Our leaves are thick, leathery, and evergreen, designed to conserve water in the summer heat. Our vascular system is built to function efficiently within a specific temperature range. When temperatures drop significantly below freezing, the water within our cells begins to freeze, forming sharp ice crystals that puncture and destroy cell membranes. This is the primary cause of cold damage, leading to blackened, mushy foliage and, if severe enough, death of the entire plant.
The most accurate measure of our ability to endure winter is the USDA Plant Hardiness Zone Map. This system divides North America into zones based on the average annual minimum winter temperature. We, oleanders, are generally classified as hardy in zones 8 through 10. This means we can typically survive winters where the temperature does not drop below 10 to 20°F (-12 to -7°C) in zone 8, and 30 to 40°F (-1 to 4°C) in zone 10. It is crucial to note that this is a measure of survival, not necessarily thriving. A zone 8 winter will often kill our above-ground growth to the ground, but if the roots are protected, we may resprout from the base in spring.
When winter arrives, our response is not one of active defense but of passive endurance. We enter a state of reduced metabolic activity, a form of dormancy. However, unlike deciduous plants that drop their leaves to avoid desiccation, we retain ours. This makes us particularly vulnerable to freeze-drying winds (desiccation) when the ground is frozen and we cannot uptake water to replace what is lost through our leaves. A sudden, sharp freeze is far more damaging than a gradual cooling. The rapid temperature drop gives our cells no time to adjust by moving water out into intercellular spaces, where ice formation is less damaging. Instead, water freezes inside the cells themselves, causing catastrophic damage.
Our survival often hinges on microclimates. A sheltered location near a south-facing wall, for instance, can radiate absorbed heat at night, creating a zone several degrees warmer than an exposed, windy site. In zones at the edge of our hardiness (like zone 8), human intervention is often the difference between life and death. Applying a thick layer of mulch over our root zone is the single most important action. It insulates the soil, preventing it from freezing deeply and protecting the crown and roots—the very heart of the plant from which we regenerate. For younger, more vulnerable specimens, wrapping the canopy with burlap or frost cloth can shield our leaves from desiccating winds and radiative freezing.
