Use a moisture meter or humidity monitor to prevent excess moisture from damaging or killing your plants. Meters measure soil moisture so you know when to water. Controllers read relative humidity in the air and automatically adjust a dehumidifier. 

Excessive moisture can diminish nutrient uptake, and encourage root rot, mold and bacteria, endangering your plants and ruining your harvest. It can even endanger you and workers by releasing a high number of mold spores into the air. When absorbed by the lungs, mold spores can cause allergic and flu-like symptoms. By contrast, low moisture can stunt plant growth by causing photosynthesis to decrease, lowering yield.

Meters are a simple solution in soil, conveying moisture levels when inserted. They are extremely economical, with some as low as $7.49.

In hydroponic grow rooms, relative humidity must be held in check and adjusted for different plant cycles. A humidity controller wires easily to a dehumidifier to adjust humidity levels automatically to the percentage you set, day or night.