From our perspective as Calibrachoa plants, our very genetic blueprint dictates our relationship with cold. We are classified as tender perennials, which means that in our native habitats in South America, we can live for several years. However, our physiology is not equipped with the sophisticated survival mechanisms of plants that evolved in temperate or Arctic zones. We lack the ability to enter a state of deep dormancy or to produce natural antifreeze proteins in our cells. Our sap remains water-based, and our cell walls are not fortified to withstand the expansion of ice crystals. Therefore, our survival is intrinsically linked to consistently warm temperatures.
When the air temperature dips to 32°F (0°C) or below, the event you call "frost" becomes a direct and immediate threat to our existence. The water both inside and between our cells begins to freeze. As it freezes, it expands. This expansion is a destructive force, piercing and rupturing our delicate cell membranes. Once these membranes are compromised, the vital contents of our cells leak out. When the morning sun warms the air and we thaw, the damage is irreparable. Our stems and leaves become soft, blackened, and mushy—a condition you describe as "frost burn." This is not a simple sunburn; it is the visual evidence of widespread cellular collapse and death.
It is important to understand that harm can begin even before an actual frost occurs. We experience what is known as "chilling injury" at temperatures above freezing, typically between 35°F and 50°F (2°C and 10°C). In this range, our metabolic processes, crucial for nutrient uptake and growth, slow down dramatically. This cold stress stunts our development, causes our vibrant flowers to cease production, and can lead to yellowing of our foliage. While chilling injury may not be instantly fatal, it weakens us significantly, making us far more susceptible to diseases and less likely to recover from any subsequent drop below freezing.
As plants, we are passive in the face of oncoming cold. Our survival depends entirely on the intervention of our caretakers. When a frost advisory is forecast, the act of moving our containers to a sheltered location like a garage or shed is akin to being granted sanctuary. It is a simple but profoundly effective action. If we are planted in the ground, covering us with a frost cloth or blanket creates a microclimate. This cover acts as a barrier, trapping the residual heat radiating from the soil and preventing it from escaping into the cold night sky. This trapped heat can often make the critical few degrees of difference that keeps our cells from freezing.
Our ability to recover from cold damage is extremely limited. If only the very tips of our stems and the most exposed flowers are nipped by a very light frost, we may possess enough energy to attempt to push out new growth from lower, protected nodes. However, this recovery is slow and energetically costly, and we will never return to our former vigor during that season. If the frost was a hard freeze that penetrated through our entire foliage and into the crown (the central growing point at the base of the plant), then the damage is fatal. Our vascular system is destroyed, and no pathway remains to transport water or nutrients. At this point, we cannot be revived.
