From our perspective as plants, our survival is dictated by our genetic makeup and physiological adaptations. We, petunias (Petunia × atkinsiana), are classified as tender perennials, which in practical human terms means we are almost always grown as annuals. Our origins lie in the warm, temperate regions of South America. This heritage has hardwired us for warmth and sunlight. Our cellular structure is not equipped to handle freezing temperatures. The water within our cells, our very lifeblood, will freeze if exposed to frost. This formation of ice crystals is a catastrophic event for us; it punctures cell walls and causes irreversible damage, leading to cellular death. This is why we are inherently vulnerable to cold weather.
When the air temperature dips to or below 32°F (0°C), the danger becomes immediate. The first sign you will observe is a dramatic change in our foliage. Our normally turgid, vibrant leaves and stems will become limp, water-soaked, and darken to a black or deep brown colour. This is not a disease but the visible symptom of cellular rupture. The tender new growth and our beautiful, delicate blossoms are always the first casualties, being the most succulent parts. A light frost might only damage these extremities, but a hard freeze will penetrate deeper, compromising the entire vascular system that transports water and nutrients, ultimately leading to our complete collapse.
It is important to distinguish between a killing frost and simply cold weather. While we cannot survive a freeze, we can often endure brief periods of cool temperatures above freezing. We may enter a state of slowed growth and minimal metabolic activity, a form of stasis, when nights are chilly, perhaps in the 40s°F (4-9°C). During this time, we will not flourish or produce new blooms, but we can persist without significant damage. However, prolonged exposure to even these cool temperatures weakens us, making us susceptible to diseases and pest infestations, as our natural defenses are lowered. Our resilience in these conditions can also vary among our different cultivars; some of us are bred to be slightly more robust than others.
Your intervention can make a profound difference in our survival prospects when cold threatens. From our viewpoint, being covered with a frost cloth, blanket, or even a breathable fabric is like being granted a temporary shield. This cover acts as an insulator, trapping the residual warmth radiating from the soil around us, preventing it from escaping into the cold night sky. It is crucial that the cover extends all the way to the ground to fully capture this geothermal heat. For those of us living in containers, being moved to a sheltered location like a garage or porch is the equivalent of being granted sanctuary. It removes us entirely from the lethal elements. A well-mulched base also helps protect our root zone, the very core of our being, from sudden temperature shocks.
