Greenhouses are designed to trap heat and protect tender plants, allowing for year-round cultivation. Our Growing Domes are typically 20–30°F warmer than the outside temperature, which impacts the humidity in a greenhouse differently in summer and winter. In summer, greenhouses are very warm but less humid because warm air can hold more water vapor. In winter, the air inside the greenhouse is cooler, reducing its capacity to hold water vapor. This results in higher relative humidity, as the air becomes saturated more quickly.
When air becomes saturated, it reaches its dew point. Dew point is the temperature at which water vapor in the air condenses into liquid water. The dew point is crucial in understanding condensation. If your greenhouse air is 70°F with 50% humidity, the dew point is about 50°F. This means water won’t condense unless the air temperature falls to 50°F or lower. During cold winter nights, the cooler air inside the greenhouse is more likely to reach its dew point, which is why condensation is more common during this time.
Why is Humidity in a Greenhouse Higher in the Winter?
Besides the temperature factor, humidity levels in a greenhouse are typically higher in the winter months due to a lack of ventilation. During colder months, trapping and maintaining warmth becomes a priority. This leads many gardeners to disengage fans and vents to conserve heat. While this strategy traps warm air, it also limits the intake of fresh and often drier outdoor air. This lack of air exchange creates an environment where moisture from plant transpiration, soil evaporation, and condensation has nowhere to escape, resulting in elevated humidity levels.
What is the ideal Humidity in a Greenhouse?
Different plants have different humidity requirements, however most mature plants thrive with humidity levels between 50% – 70%. Seedlings tend to thrive in slightly higher humidity of 70% – 80%. The area around and above the above-ground pond in the Growing Dome is a great place to start seeds for this reason. Higher humidity helps to prevent dehydration of young plants, but airflow is crucial to prevent fungal issues. Tropical plants will thrive in humidity of 75% or higher, but succulents prefer dry climates with humidity levels of 50% or lower.
Why is Controlling Humidity in a Greenhouse Important?
Controlling humidity in a greenhouse is important to help prevent diseases, pest infestations, and mold. High humidity levels and thus condensation can cause plant leaves and soil to become excessively wet which allows for spread of diseases.
How to Reduce Humidity in a Greenhouse
We wish we could give you a definitive answer as to how to manage humidity, but unfortunately there is no one-size-fits-all approach. There are many different factors that contribute to humidity levels, therefore there are many different potential solutions. In our experience a combination of the solutions below yields the best results.
Monitoring Water Usage
Watering practices can significantly impact humidity levels in a greenhouse. Any water applied in a greenhouse will evaporate and increase humidity. This can be water in the soil or the pathways of the greenhouse. Avoid standing water to prevent excess humidity from building up. A moisture meter or hygrometer can help determine when it is necessary to water to avoid overwatering. Simply place the meter 2” into the soil around mature plants and 1” into the soil for your seedlings and take a reading. Only water when the soil levels read as “dry”.
Increase Ventilation
Plant spacing is imperative at all times of year, but especially when humidity levels are heightened to promote proper air circulation. Adding an internal fan or horizontal airflow fans (HAF) to circulate air within the greenhouse can help with creating active ventilation without decreasing temperature. Circulating the air can help to reduce condensation on the greenhouse covering by preventing cold spots where condensation might occur.
During the winter it may sound counterintuitive to open the door on your greenhouse for any extended amount of time. However, opening the door or a vent on a relatively warm, sunny day for a few hours can help immensely to reduce humidity levels. Be sure to close everything back up at least 2 hours before sunset to trap as much heat as possible. We like to stick to a 2-3 hour window right in the middle of the day on days when temperatures are above 40°.
Use of Heaters and Dehumidifiers
Adding heat to a greenhouse during the winter months can, in theory, help reduce humidity levels. However, the type of heating system you choose may actually increase it. For years, we relied on propane heaters due to their off-grid capabilities. While effective for heating, propane heaters release water vapor as a byproduct of combustion, which can raise humidity levels inside the greenhouse. In contrast, electric heaters do not contribute additional moisture to the air, making them a better option for controlling humidity.
Dehumidifiers work by removing excess moisture from the air to reduce humidity in a greenhouse. They pull warm, humid air in, pass it over cold coils where moisture condenses into water droplets, and collect the liquid in a reservoir. The air is then reheated and released back into the environment, now drier. This can be an effective solution if the others mentioned do not work well for your circumstances.
How to Tackle Issues Related to High Humidity in a Greenhouse
If high humidity levels have persisted for an extended period, you may begin to notice mold and mildew buildup on surfaces like the polycarbonate glazing and greenhouse structure. A diluted vinegar solution can be used to clean these surfaces and kill any spores. We recommend mixing 1 part water to 1 part vinegar and allowing it to sit on the surface for 10-15 minutes before scrubbing or wiping it away. Be mindful not to get this mixture directly onto plants or soil.
Powdery mildew and other fungal diseases thrive in humid environments. To reduce the risk of spreading, it’s important to remove any diseased leaves and prune plants to improve spacing and airflow. A diluted potassium bicarbonate solution can also help prevent the spread of powdery mildew and other fungal diseases on the plants themselves. We recommend mixing 1 tablespoon of potassium bicarbonate and ½ teaspoon of Castile soap per 1 gallon of water and applying it with a sprayer once a week in the early morning for best results. If the powdery mildew is prevalent, increase the application rate to twice a week and if you have other soft bodied insect pests, such as aphids, increase the amount of soap.
Always test any products on a small, inconspicuous area first to ensure there are no adverse reactions.
Key Takeaways for Managing Winter Humidity in a Greenhouse
Managing winter humidity in a greenhouse is crucial for the health of your plants and the longevity of your structure. While winter conditions naturally lead to higher humidity, understanding the factors that contribute to this increase, such as reduced ventilation and cooler temperatures, can help you develop an effective strategy for control. By monitoring water usage, increasing ventilation, and selecting the right heating methods, you can maintain optimal humidity levels for your plants. Additionally, taking proactive steps to address mold, mildew, and fungal diseases will help keep your greenhouse healthy. With careful attention and a combination of solutions, you can create the ideal conditions for year-round growth in your greenhouse.
