From the perspective of a succulent plant, the primary danger of cold temperatures is not the cold itself, but the formation of ice crystals within its cells. Succulents are characterized by their thick, fleshy leaves and stems, which are specialized for storing large quantities of water. This very adaptation that allows them to thrive in arid, hot conditions becomes their greatest vulnerability in the cold. When the temperature drops below freezing (32°F or 0°C), the water inside the plant's cells can begin to freeze. As water freezes, it expands. This expansion causes the cell walls to rupture, leading to irreversible damage. Once the temperature rises again, the damaged cells leak their contents, resulting in the characteristic symptoms of soft, mushy, and translucent leaves—a state often described as frost damage. This damage is fatal to the affected parts of the plant, as the cells can no longer function.
It is a misconception to assign a single universal temperature threshold for all succulents. Different species have evolved varying degrees of cold hardiness based on their native habitats. For a plant like a Sempervivum (Hens and Chicks), which originates from mountainous regions in Europe, temperatures well below freezing, even as low as -20°F (-29°C), are tolerable. These hardy varieties have developed natural antifreeze proteins and other cellular mechanisms that allow water to freeze in the spaces *between* cells without rupturing the cell walls themselves. In contrast, a tender succulent like an Echeveria or a Aeonium, native to milder climates in Mexico and Africa, has no such defenses. For these plants, any exposure to temperatures at or just below 32°F (0°C) can be damaging, and prolonged exposure is often fatal. Therefore, the question of "too cold" must first be answered by identifying the specific plant species.
The temperature on a weather forecast is only part of the story for an outdoor succulent. Several environmental factors significantly influence how a plant experiences the cold. Moisture is a critical element. A plant with dry soil is far more cold-tolerant than one with wet soil. Wet soil freezes more solidly and can damage the roots, while also creating a colder microclimate around the plant. Furthermore, the duration of the cold is as important as the intensity. A brief, light frost of 30°F (-1°C) that lasts only an hour may cause minor damage to tender leaf tips, while a hard freeze of 28°F (-2°C) that lasts for several hours will cause extensive, often fatal, damage. Other factors include the plant's acclimatization; a succulent gradually exposed to cooler autumn temperatures will harden off and become more resilient than one subjected to a sudden cold snap. Wind chill can also strip away any insulating warmth from the plant's surface, making the effective temperature much colder.
When temperatures approach a plant's known tolerance limit, protective actions can be taken. From the plant's viewpoint, being moved under a patio roof or covered with a frost cloth provides a crucial buffer. The cover traps geothermal heat radiating from the ground and heat released by the plant's own metabolic processes, creating a slightly warmer microclimate. However, these measures have their limits, typically offering only a few degrees of protection. For a tender Echeveria, a frost cloth might save it from a light frost at 30°F (-1°C), but it will not provide enough protection during a hard freeze at 25°F (-4°C). The ultimate protective measure, from the plant's perspective, is to be brought indoors or into a greenhouse when prolonged cold weather is forecast. This is the only guaranteed way to prevent cellular damage from freezing for species that lack inherent hardiness.
