Cacti are xerophytes, a classification of plants supremely adapted to arid environments. Their evolutionary history is defined by strategies to conserve water, not to seek it from the air. Unlike tropical plants (e.g., orchids, ferns) that absorb moisture through aerial roots or leaves, a cactus's primary organ for water uptake is its root system, designed to quickly absorb deep or widespread soil moisture after rare rainfalls. Its famous spines are modified leaves that minimize surface area to drastically reduce water loss through transpiration. The plant's thick, waxy cuticle (skin) further seals in moisture. From a physiological standpoint, a cactus is engineered to hold water inside, making external humidity not just unnecessary but often detrimental.
Misting a cactus introduces several direct threats to its health. The most immediate and severe danger is rot. Water droplets can pool at the base of the plant or in its crevices, creating a microenvironment of sustained moisture against its stem. This compromises the protective cuticle and provides a perfect breeding ground for fungal and bacterial pathogens, which rapidly lead to soft, mushy, and irreparably rotten tissue. Furthermore, in environments with poor air circulation or lower light levels (like many indoor settings), the water evaporates very slowly, prolonging the plant's exposure to wet conditions. This directly contradicts the cactus's need for a cycle of thorough watering followed by a period of complete, fast-drying drought.
The core of the question often stems from a misunderstanding of how cacti hydrate. Misting to "increase humidity" implies the plant will use that airborne moisture, which it is not equipped to do. A cactus hydrates through its roots. The goal of watering should always be to simulate a natural desert downpour: completely saturate the soil until water runs from the drainage holes, and then allow the entire pot to become bone dry before even considering watering again. Humidity is irrelevant to this process. In fact, many cacti thrive in very low-humidity environments typical of heated homes, as this mimics their native dry air and helps prevent the fungal issues that thrive in stagnant, humid conditions.
While the rule is to avoid misting, a minor exception exists for a specific purpose: propagation. Some gardeners lightly mist the soil surface of newly planted cactus pups or seeds to encourage root development without dislodging them or causing rot from a heavy pour. However, this is a targeted, infrequent practice, not a routine humidity boost. For a mature cactus, the only proper way to provide moisture is through the soil. If your home environment is extremely arid, focusing on a consistent and correct deep-watering schedule is infinitely more beneficial than any attempt to alter the ambient humidity around the plant's stem, which will do nothing to quench its thirst.
