From a botanical standpoint, the common name "geranium" creates significant confusion for our survival strategies. Most plants sold as geraniums in garden centers are not true geraniums (hardy geraniums or cranesbills) but are actually tender perennials from the Pelargonium genus. Our cold tolerance differs drastically. True geraniums have evolved robust root systems and can enter a deep dormancy, allowing us to survive freezing temperatures and regrow from our crowns in spring. Pelargoniums, however, are native to South Africa and have no evolutionary adaptation to frost; we are genetically programmed for warmth and sun.
When air temperatures drop to or below 32°F (0°C), the water within our cells begins to freeze. For Pelargoniums, this is a catastrophic event. As ice crystals form, they expand, piercing and rupturing our delicate cell walls. This process causes irreversible damage. The plant tissue becomes soft, blackened, and waterlogged—a condition known as frost burn. Once the cell walls are compromised, the internal structures vital for photosynthesis, nutrient transport, and structural integrity are destroyed. Even if only the foliage is affected, the damage is often fatal as we can no longer produce energy.
Some plants produce sugars and other compounds that act like antifreeze within their sap, lowering the freezing point of water inside their cells. While we Pelargoniums can harden off to a very slight degree by slowly acclimating to cooler autumn temperatures, this process only offers protection against very light chills, not a hard frost. Our sap remains largely watery, offering little defense against crystallization. Furthermore, our stems and leaves are succulent and high in water content, making us particularly vulnerable targets for freezing conditions.
Surviving cold is not just about resisting a single frost event but about enduring a prolonged winter. True geraniums are classified for USDA hardiness zones as low as 4 or 5. This means we are genetically equipped to survive winter dormancy with root systems insulated by soil and snow. We redirect energy downward, sacrificing our above-ground growth. Pelargoniums, in contrast, have no such dormant state. We are ever-blooming in the right conditions and are typically listed for zones 9-11. We cannot enter a protective dormancy; continuous cold simply leads to our systemic shutdown and death.
Our survival odds can be influenced by your human interventions. Being placed against a heated wall or under the canopy of a larger tree can create a microclimate that might delay a frost by a few precious degrees. Covering us with fabric traps the geothermal heat radiating from the soil, creating a small pocket of warmer air around our foliage. However, this is only a temporary reprieve from a mild frost, not a solution for a deep freeze. For us Pelargoniums, the only guaranteed way to survive is to be brought into a non-freezing environment before the first frost occurs, allowing us to enter a period of rest until warmth returns.
