Lithops, often called "living stones," are fascinating succulent plants native to the arid, rocky deserts of southern Africa. Their survival strategy is a masterclass in water conservation, evolving to mimic the pebbles they grow amongst to avoid being eaten. To understand their relationship with frost, one must first appreciate their fundamental physiological makeup and the environment that shaped them.
The greatest threat frost poses to a Lithops is the formation of ice crystals within its tissues. Unlike many hardy perennials that have evolved mechanisms to tolerate freezing, Lithops are extremely susceptible. Their leaves are essentially two highly specialized, water-filled storage tanks. This high internal water content means that when temperatures drop below freezing, the water inside the plant's cells freezes. The expanding ice crystals rupture cell walls, causing irreversible damage. This damage manifests as soft, mushy, and translucent spots on the leaves, which quickly lead to rot and the death of the plant. Even a brief, light frost can be fatal if the plant is fully hydrated.
To understand their cold intolerance, look to their origin. Lithops thrive in regions with very specific climatic conditions: hot, dry summers and mild, dry winters with abundant sunshine. Frost is an exceptionally rare event in their native ranges. They have no evolutionary pressure to develop frost resistance. Instead, their entire biology is fine-tuned for extreme heat, intense sunlight, and prolonged drought. Their growth cycle is perfectly synchronized with the mild winter and spring rains of their homeland, a period of growth that is never interrupted by freezing soil or air temperatures.
Officially, Lithops are classified as winter hardy only in USDA plant hardiness zones 10 and 11, where average annual extreme minimum temperatures rarely, if ever, fall below 30°F to 40°F (-1°C to 4°C). This means they can only survive outdoors year-round in the very warmest, most frost-free parts of the United States, such as southern Florida and some protected coastal areas of California. For the vast majority of the country, Lithops must be cultivated in containers and brought indoors long before the first autumn frost. They are not a plant for the outdoor garden in zones 9 and below.
For Lithops to survive the winter months in colder climates, their care must mirror the dry winter conditions of their native habitat. The single most important rule is to keep them absolutely dry. From late autumn through early spring, they should receive no water at all. Watering a Lithops during cold, dim conditions is a sure path to rot, as the plant cannot use the moisture and the cold, wet soil will damage the roots. They must be placed in a bright, sunny location indoors, such as a south-facing windowsill, and kept at temperatures comfortably above freezing, ideally between 50°F and 65°F (10°C and 18°C). This cool, bright, and dry period is essential for their health and encourages flowering in the following season.
