Greenhouse humidity levels tend to vary with the season. They can be very damp and humid in winter, but are more likely to be dry and arid in the hotter months.
As with everything pertaining to greenhouses, however, there is a fair amount of variation. A range of elements can affect how humid a greenhouse becomes. Here are the most important factors to consider…
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Temperature
Humidity is caused when warm temperatures combine with water. The water is slowly turned to steam and evaporates into the air. This is the very definition of humidity. It therefore follows that higher temperatures can lead to greater evaporation, and hence higher humidities.
However this doesn’t tell the whole story. Just as important is how warm the inside of your greenhouse is in comparison to the outside ambient temperature.
Even in the depths of winter a sunny day can rapidly increase the temperature inside your greenhouse. In doing so, humidity can rise.
Furthermore if the outside air temperature is low enough this humidity may condense on the greenhouse glazing, leading to moisture and condensation on inner surfaces.
In extreme situations may even feel like it’s raining *inside* your greenhouse as these drips give in to gravity and fall off the roof of your greenhouse
While in theory you would think that higher temperatures in the summer months should lead to higher humidity there are also a number of other factors at play that means this isn’t always so…
Water Levels
The amount of water available to evaporate also affects how humid your greenhouse will get. In winter there tends to be far more water around thanks to higher rainfall, sleet, snow and ice.
As this moisture soaks into the ground the earth can become sodden. Only a slight increase in temperature is needed before this starts to evaporate into the air. This can be a particular problem on poor-draining soils like those rich in clay. Sandy soils, in contrast, tend to drain away excess rainwater better.
In the summer, while you will no doubt be watering your greenhouse more regularly, the effects on humidity levels are likely to be far smaller. This is partly because there is less water overall inside your greenhouse. It is also because actively-growing plants are likely to absorb more of the available water. And lastly, of course, it is affected by how well-ventilated your greenhouse is. Which leads us to…
Ventilation
The more air movement there is, the lower the humidity typically is. This is because warm, moist air is able to escape, rather than being trapped within your greenhouse.
This helps to explain why greenhouses tend to be more humid in the winter months. In cold weather it is far more likely that your greenhouse will remain shut up for long periods of time, trapping the damp air.
In the summer, however, most greenhouses get far too warm for comfort. As a result doors and roof vents are left open. Any rise in humidity after watering is only temporary, as any excess water quickly evaporates in the heat and rises up out of your greenhouse.
It is quite possible to feel this difference on a summer’s day – especially if you wear spectacles. Within a few minutes of watering your greenhouse on a hot day you’ll often find your glasses steaming up. You’ll be able to feel the moisture on your skin. You’ll feel clammy.
Within a matter of minutes, however, this sensation will begin to fall away, as the moist air escapes from your greenhouse.
Indeed, this removal of moisture in hot weather and with good ventilation can be a real battle. Plants can quickly dry out and begin to wilt; something that rarely happens in the summer months.
Is a Humid Greenhouse Bad?
High humidities can cause problems in greenhouses, especially if combined by a lack of air movement. While tropical greenhouses growing plants like bananas and papayas tend to remain warm and humid year-round, they also make use of fans to gently move air around the space.
Damp, still air is best avoided as far as is possible. Just as it can lead to mold in homes, so fungal spores are also able to take hold within the confines of a greenhouse.
Formerly-healthy plants can deteriorate quickly, as microorganisms attack their leaves, stems and even roots. An environment that combines high humidities with a lack of air movement can result in the loss of plants, especially over an extended period of time.
Overly humid conditions can also affect the greenhouse panels themselves. Moss and algae can begin to grow on their surface. This not only looks unsightly but of course also cuts down on the light being allowed into your greenhouse. In these circumstances it is necessary to remove the algae using a scouring pad or (if your greenhouse is strong enough) a jet wash.
All this means that it is crucial to try and ventilate your greenhouse whenever the weather allows it – even in the winter months.
It is best to wait till a still, sunny day where the temperatures are comfortably above freezing. In such conditions it is normally safe to open your greenhouse doors and roof vents, allowing the moist air to escape, while the sun warms your plants inside.