From a plant's perspective, survival is a cellular battle against ice. The primary threat of cold winter climates is the formation of ice crystals within the plant's cells. As water freezes, it expands, and these crystals act like microscopic daggers, piercing and shredding the delicate cell membranes and internal structures (organelles). Once these membranes are ruptured, the cell's contents leak out, and the cell dies. For a cactus, a plant that stores vast quantities of water in its specialized parenchyma cells, this is an extreme vulnerability. A fully hydrated cactus is, essentially, a bag of water primed for freezing destruction if no adaptations are in place.
The answer is not universal for all cacti; it depends entirely on their evolutionary heritage. Species native to the Sonoran or Chihuahuan deserts, like the iconic Saguaro (Carnegiea gigantea) or many Opuntia, are not equipped for hard freezes. Conversely, cacti from high-altitude or northern habitats, such as certain Opuntia (Prickly Pear) or Pediocactus species, have evolved specific physiological traits to survive freezing temperatures. Their survival is not a matter of luck but a suite of developed adaptations.
Cacti that can survive outdoors employ several sophisticated strategies. Firstly, they undergo a process called cold hardening. As temperatures drop and daylight shortens in autumn, these plants cease growth and begin to dehydrate themselves. They actively move water from inside their cells into the spaces between cells. Since ice forming between cells does less immediate damage than ice forming inside cells, this sacrifices some outer tissue to protect the vital core. Secondly, they increase the concentration of solutes (like sugars and certain proteins) within their remaining cell sap. This acts as a natural antifreeze, dramatically lowering the freezing point of the internal cellular fluid and preventing the formation of lethal ice crystals even as the temperature plummets below zero.
Survival is inextricably linked to dormancy. A cold-hardy cactus must enter its dormant phase completely. This state of suspended animation is triggered by environmental cues and is non-negotiable. A fundamental requirement for successful dormancy in freezing weather is extremely dry soil. Wet, cold soil is a death sentence. The roots, which are the most cold-sensitive part of the plant, will rot if sitting in frozen, saturated earth. Therefore, the most significant threat to an otherwise hardy cactus outdoors is not the air temperature itself, but excessive winter moisture around its roots, which prevents the plant from properly dehydrating and leads to fatal root rot.
