From a botanical perspective, petunias (Petunia × atkinsiana) are classified as tender perennials. This means that while their genetic programming allows them to live for multiple years, they are inherently sensitive to cold and cannot survive freezing temperatures. Our survival mechanism is not geared towards dormancy like that of a hardy shrub or bulb. Instead, we are optimized for rapid growth and prolific flowering during the warm season, investing our energy in reproduction rather than building robust, frost-resistant structures. Our native habitat in South America informs this physiology; we thrive in full sun and warm conditions, making the icy grasp of frost an existential threat.
Frost is lethal to us because of its effect on the water within our cells. The vast majority of our tissue is composed of water. When the temperature drops to 32°F (0°C) or below, the water inside and between our cells begins to freeze. As it freezes, it expands, forming sharp, jagged ice crystals. These crystals act like tiny knives, physically piercing and shredding the delicate cell membranes that are vital for controlling what enters and exits the cell. Once these membranes are ruptured, the cell's contents leak out, and the cell dies. This damage is not superficial; it is systemic. From our leaves and stems to our roots, this cellular destruction leads to the plant becoming mushy, blackened, and ultimately unable to function or recover.
It is important to understand that cold damage occurs on a spectrum and does not begin precisely at the freezing point. We petunias begin to suffer physiological stress well before 32°F. Temperatures dipping into the mid-40s°F (around 7°C) can significantly slow our growth and flowering. As temperatures approach the freezing mark, the damage becomes more severe. A very light frost might only kill the most exposed and tender tips of our growth, while a hard freeze, where temperatures drop significantly below 32°F for an extended period, will be fatal to the entire plant, including the root system. There is little to no variation among common garden petunias; none possess meaningful frost hardiness.
While we cannot alter our fundamental sensitivity to cold, certain actions by gardeners can create a microclimate that offers temporary protection from a light frost. Covering us with breathable fabric like frost cloth or even a bedsheet before nightfall can trap geothermal heat radiating from the soil, raising the immediate temperature around us by a few critical degrees. It is crucial that the covering extends to the ground and does not touch the foliage, as contact can transfer cold. For petunias in containers, moving us to a sheltered location like a garage or porch for the night provides the best protection. These are, however, short-term strategies for unexpected early or late-season cold snaps, not a solution for enduring winter.
