How to Manage Humidity in a Greenhouse. One of the biggest challenges in maintaining a greenhouse is humidity control. Most plants enjoy a humidity of 50 to 60 percent, and this is a challenge to manage at times. The main goal is to avoid condensation on plants, which promotes mold and fungus growth. Low humidity leads to leaf dehydration, more frequent watering and pest infestation.
Vent your greenhouse to promote a good exchange of air. Good circulation is the key to humidity control. Hot, humid air exits the greenhouse, allowing cooler, dryer air to enter. If the temperature rises above 80 degrees F or the humidity gets close to 100 percent, there must be around 50 exchanges of air per hour to keep the humidity down. To accomplish this, a roof vent and a side vent close to the floor are the best solution.
Turn on a fan to circulate the air and force the humid air to move up and out through the vents. It doesn't take much air movement to make a difference.
Shade your greenhouse in the hot summer to keep it from overheating. Hot air escapes through the vents when the greenhouse heats up, causing plants to wilt as they lose valuable moisture. There are shade cloths available from a greenhouse or gardening supply store to drape over the top of your greenhouse if you don't have any trees to provide shade.
Mist your greenhouse to add humidity when it begins to warm up and vent the hot air out. Installing a misting system is one option, but there are a few precautions to consider if you do. Only mist during the day because as the greenhouse cools at night, the humidity is raised, encouraging disease. A timer, set to turn off a few hours before sunset, is a good idea if you install a misting system and make sure you place the misting nozzles under the plant shelves, to avoid soaking the plants. Spraying the floor with water in the morning will also raise the humidity levels during the day.
Avoid condensation when the weather is cold outside. It occurs when the plants are colder than the air around them. Venting and air circulation controls humidity in the warm months, but during the winter, venting alone causes the temperature to drop. Therefore, heat must be included with air movement and ventilation to control the humidity.