Lithops, commonly known as living stones, are succulent plants native to the arid and semi-arid regions of southern Africa. They have evolved over millennia to thrive in an environment characterized by intense sunlight, poor rocky soil, and, most critically, extremely low humidity and infrequent rainfall. Their entire growth cycle and physical form are a direct adaptation to conserve every possible drop of water. The plant's body is composed of two fused, succulent leaves that store all the necessary water to survive months, or even an entire year, of drought. In their natural habitat, relative humidity levels are consistently low, often dropping significantly between rare rainfall events.
For Lithops, any consistently high humidity level is problematic. While a specific universal percentage is difficult to pinpoint due to varying air circulation and temperature, a relative humidity level consistently above 60-70% is generally considered too high and poses a significant risk, especially when combined with cool temperatures. The core issue is not the humidity reading itself in isolation, but what that humidity prevents: the rapid evaporation of water from the soil surface and the plant's tissue. In their native habitat, any moisture from a brief rain shower is quickly evaporated by the dry, moving air. In cultivation, stagnant, humid air prevents this crucial drying process.
Prolonged exposure to high humidity creates two primary and often fatal dangers for Lithops: rot and fungal infections.
Firstly, high humidity drastically slows the evaporation of water from the potting medium. Lithops require a gritty, extremely well-draining soil that is designed to dry out completely within a few days after watering. In humid conditions, the soil remains damp for extended periods, surrounding the plant's sensitive root system and buried leaf base with moisture. This leads to root rot and stem rot, which quickly progresses upward, turning the firm, succulent body into a soft, mushy, and irrecoverably dead plant.
Secondly, a humid environment is the perfect breeding ground for fungal pathogens like Botrytis and other molds. These fungi can attack the plant directly, especially if there is any minor damage or weakness, causing lesions and decay. A common sign is the sudden appearance of a brown, sunken patch on the side of the plant that rapidly spreads.
Airflow is the essential counterbalance to humidity. Even in areas with moderately high ambient humidity, constant gentle air movement across the soil surface and around the plants can mimic the arid conditions Lithops require by forcing evaporation. This is why growing Lithops on a sunny windowsill or, ideally, under strong grow lights with a small fan gently circulating air is highly recommended. The fan does not lower the room's overall humidity but creates a microclimate at the plant level that prevents moisture from stagnating, effectively making the environment safer for the plant.
The danger of humidity is most acute during the plant's dormant periods in the height of summer and the depth of winter. During these times, Lithops should be kept completely dry, and any environmental humidity is an added threat. Watering should only occur during specific growth phases (typically in autumn and spring) and must be done with extreme caution. The golden rule is to only water when the weather forecast predicts several days of sunny, dry conditions that will facilitate rapid drying of the pot. Watering a Lithops plant before a period of humid, cloudy weather is a common and often fatal mistake.
