If your greenhouse is wet and dew forms on plants’ leaves, it means that you are not controlling humidity. Most of the humidity related problems arise in fall and spring times. Let me explain why does greenhouse steam up. On a sunny day evaporation of water from soil surface increases. Also, leaves loose more moisture in transpiration.
The warm air holds the moisture during the day in a form of vapor. Then at night when the air cools it changes water from vapor to liquid in a process known as condensation. As a result, water droplets form on cool surfaces like leaves and glazing.
Why does my mini plastic greenhouse steam up?
However, if you are germinating seeds in a mini greenhouse and it steams up, that is totally normal as seedlings need humidity to thrive. Nevertheless, you need to monitor the temperature inside mini greenhouse and open vents or unzip cover occasionally. To learn more information on how to sow seeds in a greenhouse click here.
How humid should a greenhouse be?
The ideal humidity in a greenhouse should be around 50-70% relative humidity, although 60-80% is usually tolerable. Relative humidity (RH) is the measure of amount of moisture in the air. RH is a ratio between the amount of water dissolved in the air and the total amount of water air can hold at a specific temperature and atmospheric pressure.
In a greenhouse atmospheric pressure doesn’t really calibrate so we can assume that it is constant. So, to find a relative humidity we need to divide the current concentration of water in the air by the total amount of water the air can hold at a specific temperature. The finding is then expressed in percentages. As a result, RH depends on the temperature.
Warm air holds more moisture than cooler air. That is because when temperature increases, air expands and can hold more moisture. Subsequently, if temperature goes down, air contracts and holds less moisture. As a result, when temperature rises, RH decreases and when temperature goes down, RH increases.
Let’s look at the example. Temperature in a greenhouse changes from day to night, therefore, RH varies too. Let’s say that a temperature in a greenhouse is 70º F during the day with RH 40%. If during the night temperature goes down to 60º F, the RH will increase to 50%.
That leads to a conclusion that a 10º F increase in temperature will increase the capacity of air to hold moisture and will decrease the RH. Understanding this relationship is key in managing humidity inside greenhouse.
Now, the day temperature in a greenhouse is 75º F and the RH is 50%. If the temperature at night drops to below 45º F, the RH will increase to 100%. If the temperature keeps going down, the amount of water in the air will exceed maximum, so moisture will condense as water droplets.
Should my greenhouse have condensation?
The temperature at which water begins to condense out of moist air is called the dew point. So, the condensation that you see on the leaves happens when the temperature of the leaves in below dew point. For example, when RH is 95% and air temperature is 60º F, a plant’s leaf temperature can only be 59ºF for condensation to form on it.
You should aim to reduce condensation in your greenhouse. Condensation means that there is too much water in the air to remain in the vapor state. So it falls out and forms water droplets on surfaces inside greenhouse that are at or below the dew point. These cold surfaces are aluminum frame, piping, door knobs, roof and plants.
Condensation on the surfaces inside greenhouse can damage them. Moreover, high relative humidity can lead to plant diseases including Botrytis and powdery mildew. Condensation on the plants’ leaves provokes these diseases. However, if you already have mold in a greenhouse, here are the top tips how to get rid of it.
How do you stop condensation in a greenhouse?
- Provide constant air circulation
Providing constant air circulation in spring and fall helps to stop condensation on leaves in a greenhouse. The best way is to heat a greenhouse in the evening for a short period of time when ventilation fan is running.
Ventilation ensures that dry air from outside enters greenhouse structures and exchanges with moist air. Greenhouse with natural ventilation can take up to 30 minutes or longer to exchange air.
- Adequate watering
Right watering helps to prevent condensation in your greenhouse structure. Water your plants just enough so excess water doesn’t stay on the floor. Also, it is very important to water plants early in the morning so they can dry before evening.
- Proper plant spacing
Sufficient plant spacing improves air circulation at the plant level. In fact, the highest relative humidity is inside plant canopies, the above ground portion of a crop. Plants produce moisture during transpiration and if there is not enough space it gets trapped. That also leads to insufficient air movement.
- Good drainage
Good greenhouse floor and drainage definitely help to regulate humidity. It prevents the formation of standing water which evaporates and increases amount of moisture in the air. Moreover, if the condensation forms, the water droplets will go down from the greenhouse glazing and frame and simply flow away through drainage.
How do I lower the humidity in my greenhouse?
- Heating
Combination of heating and ventilation lowers down the relative humidity in a greenhouse. Ventilation lets in dry air from outside and exchanges it with the moist air inside greenhouse. In a meantime heating system raises the temperature of the dry outdoor air and also expands air to hold more moisture, so it prevents condensation from forming.
You need to ventilate and heat your greenhouse 2 or 3 times per hour in the evening after sunset and in the morning at sunrise. If all of the above don’t reduce condensation completely, you can consider using anti-condensation coating.