The lowest temperature a succulent can survive depends on its specific species and natural adaptations. Succulents are a diverse group of plants defined by their ability to store water in their fleshy leaves, stems, or roots. This stored water makes them vulnerable to cold temperatures, as the moisture inside the cells expands when it freezes, rupturing the cell walls and destroying the tissue. This cellular damage is the primary mechanism of frost injury, resulting in the characteristic mushy, translucent appearance of a frozen succulent.
Defining the Cold Hardiness Scale for Succulents
Succulents fall into three broad categories based on their tolerance for freezing temperatures. Tender succulents are the most sensitive, thriving in warm climates and suffering damage when temperatures drop below 40°F (4°C). These plants, which include most Echeveria, Crassula, and Kalanchoe species, cannot survive a hard frost and will be killed when the temperature reaches the freezing point of 32°F (0°C).
Semi-hardy succulents possess a slightly greater tolerance, often surviving brief dips below freezing, typically down to about 25°F or 20°F (-4°C to -7°C). Survival at these lower temperatures is conditional on the plant being completely dry and the cold snap being short-lived. Certain Agave varieties and some Pachyveria hybrids fall into this middle category.
Hardy succulents are the most resilient, having evolved mechanisms to survive prolonged and deep freezes, sometimes tolerating temperatures as low as 0°F to -20°F (-18°C to -29°C). These species include Sempervivum (Hens and Chicks), Sedum (Stonecrop) species, and certain Opuntia (Prickly Pear) cacti. These plants typically survive by entering a deep dormancy and reducing the water content in their cells to prevent fatal ice crystallization.
Variables That Affect Cold Survival
The temperature thresholds for succulent survival are not absolute. The most immediate threat is the duration of the cold event, where a brief overnight frost is far less damaging than a prolonged freeze lasting several days. Sustained freezing temperatures penetrate the soil, freezing the roots and leading to a complete plant kill, even for otherwise cold-tolerant species.
A plant’s ability to withstand cold is enhanced by acclimation. This physiological process is triggered by gradual exposure to cool, non-freezing temperatures in the fall, which prompts the plant to thicken its protective cuticle layer and change the chemical composition of its cellular fluids. Succulents that are abruptly moved from warmth to cold are more susceptible to damage than those that have slowly adapted.
The moisture content in the soil and the plant tissue is a major determining factor. Wet soil freezes more quickly and holds the cold longer, transferring it directly to the roots and increasing the risk of cold-related rot. While the wind chill index does not directly affect plants, high wind velocity can exacerbate damage by increasing evaporative cooling and moisture loss from the leaf surfaces, leading to desiccation damage.
Practical Steps for Winter Protection
Tender succulents should be protected when nighttime temperatures consistently drop below 40°F (4°C). This temperature serves as a practical trigger point, signaling that the plant is no longer safe outdoors and should be moved to a protected location, such as a garage, basement, or bright indoor space. This relocation prevents the dangerous cellular rupture that occurs at the freezing point.
For semi-hardy varieties that remain outdoors, temporary protection can be provided using breathable materials like a lightweight frost cloth. This fabric prevents frost crystals from forming directly on the foliage and can provide several degrees of localized temperature protection, but it must be applied before nightfall. Old cotton sheets can also be effective, but plastic sheeting should be avoided as it traps moisture and can cause more harm.
Water management is the most proactive step to encourage cold tolerance. Watering should be ceased in late fall and throughout the winter months to keep the soil dry and induce a state of dormancy. Utilizing a sheltered microclimate is also beneficial, such as placing potted plants against a south-facing wall or under a roof overhang, where the structure can radiate warmth and provide a barrier against frost and precipitation.
Recognizing and Treating Cold Damage
Visual cues indicate the extent of damage sustained by the plant following a freezing event. Signs of damage include leaves or stems that appear translucent, water-soaked, or mushy, as well as the appearance of black or dark brown spots on the foliage. The plant structure may also collapse entirely, which indicates that the internal cellular structure has been compromised and destroyed.
The most important step in post-freeze care is to exercise patience and avoid rushing to prune or remove the damaged parts. The full extent of the injury may take several days to become clearly defined, and the damaged tissue can sometimes offer a protective layer to the underlying healthy parts. Once the mushy areas have dried out and turned papery or crispy, they can be carefully pruned away using a sterilized tool.
Damaged succulents should not be watered or fertilized immediately after a cold snap. Adding water to the soil will only increase the risk of fungal infection and rot in the damaged tissues. Instead, keep the plant in a cool, dry area until new, healthy growth appears, signaling that the plant has recovered.