Greenhouses are naturally humid from the respiration of plants, but there are times when more moisture must be introduced to maintain the humidity level required for rapid growth and optimum health of those plants. Commercial greenhouses are able to monitor and adjust humidity by misting plants at preset intervals. If you do not have the capability of an automated system, you can try some simple techniques to raise the humidity.
Place large buckets of water in your greenhouse. As the greenhouse warms during the day, water will evaporate and increase humidity. Clean the reservoir of water on a regular basis, as it can encourage the growth of algae or mold, and can easily become a breeding ground for mosquitoes and other insects.
Use a large mister to mist plants by hand on a daily basis. This will take some time, depending on how many plants you have, but it will raise the humidity in the greenhouse and provide individual plants with fresh water to encourage growth.
Place plants on pebble trays filled with water. As the water evaporates from below, the moisture will rise to the plants above, providing the added humidity they require.
Use a small humidifier or cool mist vaporizer for a home greenhouse that is too dry. These will add moisture to the air and increase humidity.