Humidity refers to the amount of moisture in the air. Humidity monitoring is vital to a healthy garden, because too much humidity can cause mold. The amount of humidity can be measured by using a device such as a hygrometer that can be installed in the grow space to provide a constant reading of the dampness of the air. If the amount of humidity is too high, it can be reduced by use of an extractor fan. Proper humidity monitoring will help to ensure a productive and mold free garden.