Low humidity in a greenhouse can be a particular problem during cool weather, when the capacity of air to carry moisture is greatly reduced. Typically, overly humid conditions in a greenhouse is the main problem and can cause plants to grow tall and spindly. A more serious problem of a high-humidity environment is disease, especially fungus and mould. Low humidity can cause plants to dry out or fail to thrive, so maintaining appropriate and consistent humidity throughout the designated growing season is essential.
Monitor indoor greenhouse temperatures using a wall-mounted thermometer. Higher temperatures increase air's capacity to hold moisture, so if low humidity in the greenhouse is a problem then raising the greenhouse temperature is the first step in increasing humidity.
Close windows or doors. Allowing the warm air in the upper levels of the greenhouse to escape lowers temperatures at the ground level and reduces the overall humidity of the interior.
Install a humidifier. Setting up a humidifier programmed to run for a certain time or once air moisture levels reach a certain threshold can help ensure that appropriate humidity is maintained.
Increase the amount of light reaching the interior of the greenhouse. Removing any items that shade the greenhouse, such as overhanging tree limbs, fabric roof covers or debris can help raise internal temperatures and relative humidity.
Cluster plants together. Arranging plants so they are near to one another is another method of increasing ambient humidity. The plants' natural respiration process releases moisture into the air. Humidity also rises as the population of plants is increased.
Allowing temperatures in the greenhouse to rise above 32.2 degrees Celsius may lead to severe damage or death of the plants located inside. Even during cold winter months, the internal humidity of a greenhouse must be regulated to prevent proliferation of disease or the undesirable growth habits of overheated plants. Venting the greenhouse with fresh air at regular intervals---along with the use of a heating device to maintain adequate temperature---helps balance the ill effects of an overly humid greenhouse.