Lithops, often called "living stones," are fascinating succulent plants native to the arid, desert regions of southern Africa. Their unique survival strategy revolves around avoiding harsh conditions, not enduring them. To understand their winter hardiness, we must examine their physiology and annual growth cycle.
From a plant's perspective, the primary threat of winter is not just low temperature, but the combination of cold and water. Lithops are expertly adapted to store water in their thick, fused leaves to survive drought. However, if the water stored within their cells freezes, it expands. This expansion forms ice crystals that rupture the cell walls, causing irreversible damage known as frost damage. The plant's tissues become soft, mushy, and translucent, leading to rot and death. Their shallow root systems are also highly susceptible to cold, wet soil conditions, which can cause root rot even without freezing temperatures.
Lithops have a distinct growth cycle synchronized with the seasons. In their native habitat, their active growth periods are in the autumn and spring when temperatures are mild. The height of summer and the depth of winter are times of dormancy. During winter dormancy, the plant's metabolic processes slow down significantly. It is not actively growing and requires absolutely no water. This natural dormancy is a defense mechanism to conserve energy and resources during unfavorable conditions. Subjecting a dormant Lithops to cold, wet weather is a direct assault on its core survival strategy.
The USDA Plant Hardiness Zone Map is based on the average annual extreme minimum winter temperature. Most Lithops species are hardy only to approximately Zone 10b (35°F to 40°F / 1.7°C to 4.4°C). This means they might survive a very brief, dry dip to just above freezing. However, "survive" is not the same as "thrive." Even in zones 10 and 11, winter rainfall poses a significant threat. Prolonged exposure to temperatures even in the 40s (°F) while the soil is moist can be fatal. Therefore, the zones are a guideline for absolute minimums under perfect (dry) conditions, not a recommendation for year-round outdoor planting.
Given their physiological limits and dormant state, Lithops cannot survive winter outdoors in the vast majority of the United States. Only the warmest, driest parts of the country (such as parts of southern Arizona and California) might offer a chance, and even this requires perfect drainage and protection from any winter rain. For the plant's health and survival, it is universally recommended to grow Lithops in containers. They must be brought indoors to a bright, cool, and completely dry location once nighttime temperatures consistently drop below 50°F (10°C). This indoor wintering mimics the dry dormancy they experience in nature and is essential for their long-term survival and annual flowering cycle.
