Why is Grow Room Humidity Important?

This article is republished here from Issue 6 of Garden Culture Magazine. It originally appeared under the title, “What is Humidity”.

What exactly is humidity? How does it affect plants? As the controller of grow room humidity, having a keen understanding of what it can do to crop health is vital to your success as a grower.

You know those really hot days when no matter what we do we just can’t cool ourselves down? That’s mainly due to high humidity! When the air around us becomes saturated with water vapor, making it impossible to cool ourselves, as the sweat from our skin can’t evaporate.

Then we have the really hot days that we could just lay in the sun all day, but stay relatively cool… That’s due to low humidity! The air is quite dry, allowing any perspiration to evaporate off of our skin, which keeps us cool. However, extremely low humidity (25% and below) can have detrimental effects on us humans, such as dry skin, and irritated eyes and respiratory system.

The funny thing is that it’s the same for plants. Humidity is one of the most underrated environmental aspects of an indoor garden, and it’s definitely something we need to keep an eye on.

Before we get technical with relative humidity, and why it’s essential to monitor and control it, the bottom line is that we want to avoid extremes. A good humidity range during vegetative growth is around 60-70% because before a good root system has formed, the plant will find it easier to maintain the equilibrium of water uptake and water loss. During the flowering stages, it is good practice to drop the humidity to 40-50% to prevent any mould or pathogens from forming, but it also ensures good movement of water, nutrients, and minerals through the plant to the flowers or fruits. Cuttings are special, and require 90% humidity to form new roots, whilst seedlings do well at around 60%.

That’s the essential bit of information that we all need to know. Now we can delve a little deeper into the technicalities of humidity.

Humidity Key Words


The rate at which the plant expels, and absorbs moisture. This helps to cool the plant, and enables a flow of water, nutrients, and minerals.


These are the pores that regulate moisture within the plant. They help protect it from dramatic changes in moisture.


When a gas (or space) holds the maximum water vapour possible at a given temperature, it is said to be saturated. If you add extra water to a saturated gas, or if its temperature decreases, some of the water vapour will condense.

Relative Humidity (RH):

It is the ratio of actual water vapour content to the saturated water vapour content at a given temperature and pressure expressed as a percentage (%).

The air temperature is vital to know when measuring relative humidity. This is because the ‘relative’ part is essentially related to the temperature, and how saturated the air is at its current temperature. For example, if your grow room rises in temperature, the relative humidity will drop, so at an RH of 50%, a temperature rise from 68°F to 70°F will cause RH to drop by about 3%. The digital thermometer that you should be buying from the local grow shop usually comes paired with a humidistat, and it’s an essential piece of equipment, to say the least.

To understand how plants operate under different humidity levels, we need to understand how a plant works. All plants allow carbon dioxide (CO2) to enter through their leaves via tiny openings called stomata. They use this gas in photosynthesis. The plant regulates it’s intake of CO2 by opening and closing its stomata, and as it does this, moisture in the leaf can escape.

If your grow room humidity is low (dry), it causes the plants to transpire much more rapidly than in a higher humidity environment. When this happens, the leaves become flaccid and begin to wilt, and over a longer period of time the plant will close its stomata, and reduce the flow of water out of the plant. This is very effective at stopping water loss, but unfortunately, it also reduces the intake of CO2. Without an adequate supply of CO2, the cells will begin to die, and the plant will look tired and ill.

The key point to remember here is that dry air will remove water from the leaves quicker than the roots can deliver it. Under these conditions it doesn’t matter how much you water the plant, it won’t help. And over-watering will remove oxygen from the root zone (rhizosphere), creating further problems.

When a plant has the right humidity for its stage of growth it will thrive, the stomata will open completely, and the plant will enjoy a good fresh supply of CO2, along with controlled water loss from the leaves.

This loss of water from the plant to the atmosphere is known as evapotranspiration. Plants regulate this loss of water by the opening and closing of guard cells, but also something called the vapour pressure gradient, which is the difference between the water vapour content of the atmosphere, and the vapour pressure within the sub-stomatal cavity. The reason this is important is that it brings me to my next point, which is air movement around your plants.

A layer of saturated or partly saturated air will now have built up around the leaf if the air is still. Slight air movement will move this saturated air away and helps in the cooling of the leaves because of the transfer of heat by convection from the leaf surface. This movement of water away from the plant allows more water molecules to move through the plants’ veins, the stem, and the roots, creating a negative water pressure in the root zone which allows the plant to ‘drink’. This process is known as osmosis.

An important point to note though is that high wind velocity from clip fans will move all the air away from the leaf boundary, and result in a dry atmosphere that increases water loss – something we don’t want too much of during vegetative growth. Therefore we want our clip fans on the lowest setting, and we don’t want them pointing directly at the plants -somewhere between the tops of the plant and below the lights is the golden zone. After all of that information, knowing where to start may seem difficult. Here are some top tips…

1. Remember that when the lights turn off, the temperature will drop, and the relative humidity will rise. It may be wise to keep the extraction running after the lights are off, or invest in some heaters so the temperature drop isn’t too drastic. This will prevent the water from depositing on the plants/walls, and create a breeding ground for pathogens.

2. When the lights ignite the temperature will increase, and the relative humidity will drop. A good investment to negate this drop would be a timed humidifier. One that uses ultrasonic vibrations is better, so that the water released is cool, and not steamed.

3. So keep those humidity levels in check, higher in vegetative growth, and lower during the flowering stage.

As we said at the start of the article, plants, just like humans prefer environmental stability in their lives. Whether it’s relative humidity or temperature swings, prevent dramatic changes in short spans of time, and your plants will metaphorically thank you.

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  • Millie Hue says:

    Thanks for pointing out that the humidity level must be high in vegetable growth while low on flowering plants. I guess I would need to hire a professional to check our greenhouse since the roses are not growing. Some grow at a certain state then dies without reaching their full potential. https://southernclimatesolutions.com/about/


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