To understand our winter survival, you must first understand our origins. We are native to the arid, desert regions of South Africa and Namibia. Our entire evolutionary purpose is to survive in an environment defined by intense sun, scorching heat, and, most critically, very little and highly unpredictable rainfall. Our unique stone-like appearance is a masterclass in camouflage to avoid being eaten. Our succulent nature is not for storing moisture in a single body like many cacti; instead, we consist of two fused leaves that hold all the water we need to survive months of drought. Our growth cycle is perfectly attuned to seasons, but not cold, snowy ones. We are actively growing in the fall and are completely dormant during the hottest, driest part of the summer.
Our physiology has one major, non-negotiable weakness: we are extremely susceptible to cold and, above all, frost. Our cells are filled with water. When the temperature drops below freezing (32°F or 0°C), the water inside our leaves freezes, expands, and ruptures the cell walls. This damage is irreversible and fatal. It turns our firm, healthy leaves into a mushy, rotten mess. While a brief, light chill just above freezing might not immediately kill us, sustained temperatures at or near freezing will. We simply lack the genetic programming to produce the natural antifreeze compounds that allow hardy perennials to survive freezing conditions.
When you ask about surviving winter outdoors, the ambient air temperature is only one part of the danger. The greater, and often overlooked, threat is soil moisture combined with cold. Our dormancy period, which for us occurs during the peak summer heat, is a defense mechanism against water loss. If you subject us to cold, wet soil, you are creating a worst-case scenario. Our roots, which are designed to dry out completely, will rot and succumb to fungal infections in cold, soggy conditions. This combination of "cold and wet" is a death sentence far more certain than a brief, dry cold snap. Furthermore, winter precipitation in the form of rain or snow directly onto our bodies will pool in the crevice between our leaves, inevitably leading to rot.
Given our absolute intolerance for freezing temperatures and wet conditions, we cannot survive winter outdoors in the vast majority of US Hardiness Zones. The USDA plant hardiness zone map is based on average annual extreme minimum temperatures. We would only theoretically be safe in the ground year-round in the very warmest parts of zone 10a (30-35°F) and all of zone 10b and 11 (above 35°F), and even this is risky if an unusual cold snap occurs. This essentially limits our permanent outdoor cultivation to a few select frost-free microclimates in Southern California, South Florida, and perhaps the very southern tip of Texas. For anyone living in zone 9 or below, which encompasses most of the continental United States, leaving us outdoors during winter is not an option for long-term survival.
For the overwhelming majority of Lithops keepers, we must be grown as container plants. As the autumn temperatures begin to consistently drop below 50°F (10°C), you must stop watering us completely. This allows us to enter a dry, protective dormancy. We must be moved indoors well before the first frost date in your area. An ideal winter home is a bright, sunny windowsill (a south-facing exposure is best) in a cool room with temperatures between 50-65°F (10-18°C). Do not water us during this indoor period. The combination of cool temperatures, bright light, and absolute dryness mimics our natural dormant season and keeps us healthy until spring returns, when watering can carefully resume.
