Yes, you can absolutely keep your Lithops outside during the summer, and doing so can be highly beneficial for their health and growth cycle. However, this transition must be done with careful consideration of their specific needs. From the plant's perspective, moving outdoors provides ideal conditions that are difficult to perfectly replicate indoors.
Lithops are evolutionarily designed for intense sunlight. Their native habitat in southern Africa exposes them to many hours of direct sun daily. The high light levels outdoors are crucial for several physiological processes. It promotes compact, sturdy growth and prevents etiolation (stretching), which weakens the plant. Most critically, ample sunlight is the primary trigger for the initiation of flowering in mature plants later in the season. The sun also helps to properly harden off the outer leaf pair as the new leaves develop inside.
A Lithops that has been living on a windowsill is not prepared for the full intensity of the midday summer sun. Its tissues are soft and susceptible to severe sunburn, which appears as white or brown crispy patches. This damage is permanent and can be fatal, as it destroys the chlorophyll-rich tissue necessary for photosynthesis. You must acclimate the plant gradually over 1-2 weeks. Start by placing it in full shade outdoors for a few days, then move it to a location with only morning sun for another few days, and finally to a spot that receives the intended amount of light.
This is the most counterintuitive but vital aspect of Lithops care. For most species, summer is their dormancy period. During this time, their metabolic activity slows significantly to survive the extreme heat of their native habitat. The plant is not actively growing and its roots are highly susceptible to rot. From the plant's perspective, a summer thunderstorm is an anomaly. Therefore, you must be extremely cautious with watering. If the plant is outdoors and receives a light rain, it is likely fine. But you should never water it on a schedule. Only water if the plant shows signs of thirst (the top becomes visibly concave and wrinkled) and the weather is not excessively humid. In many climates, this may mean little to no water from late June through August.
The perfect summer spot for a Lithops mimics its natural environment: bright light with excellent airflow and protection from excessive rain. An east-facing location is often ideal, providing the beneficial morning sun but sheltering it from the harsher afternoon rays. A spot under the eaves of a roof, on a covered patio, or under a sheer shade cloth provides protection from torrential downpours that would waterlog the soil and certainly cause rot. Good air movement helps the soil dry quickly and prevents pest infestations. The plant's body is 90% water; it is designed to survive drought, not flooding.
