Since stem cuttings do not initially have roots, they must be placed in an environment that prevents them from drying out. This is accomplished by altering the cutting environment by high humidity tents, intermittent mist or by fogging systems.
Fog systems work by maintaining high humidity in the greenhouse reducing the difference between the vapor pressure in the leaf and the surrounding atmosphere.
Fogging systems generate small water droplets that are under 50 microns in size so they stay suspended in the air. They can be generated by high pressure atomizers, centripetal foggers or by humidifiers. In a true fogging system, the water particles stay suspended above the cuttings.
This raises the greenhouse humidity, but does not wet the leaves significantly compared to misting systems. Although fogging systems can be expensive to install and maintain, they provide an excellent environment for rooting cuttings.