The Importance of Air Humidity in the Grow Room

In short: Keep relative humidity at 60-70% during vegetative growth and 40-50% in flowering to limit mould and pathogens. Cuttings root best near 90%, seedlings around 60%. Temperature matters too: at 50% RH, a rise from 20°C to 22°C drops relative humidity by roughly 3%, so measure with a thermometer and hygrostat.

You know those really hot days when, no matter what you do, you just can't cool down? That's mainly due to high humidity! When the surrounding air becomes saturated with water vapor, it prevents you from cooling down because the sweat on your skin cannot evaporate.

Then there are those really hot days when you can lie in the sun all day and still remain relatively comfortable... That's because of low humidity! The air is relatively dry, which allows skin evaporation, and that cools you down.

However, extremely low humidity (25% and lower) can have harmful effects on us humans, such as dry skin, irritated eyes, and breathing difficulties. And the same applies to plants. Humidity is one of the most underestimated environmental aspects of indoor growing, and it's definitely something we need to pay attention to.

Before we gain expertise with relative humidity and why it's essential to monitor and control it, let's emphasize that we want to avoid extremes. A good humidity range during vegetative growth is around 60-70%, because before a good root system develops, plants will more easily maintain the balance of water absorption and discharge. During the flowering stage, it's good for humidity to decrease to 40-50%, thereby preventing the formation of molds or pathogens, and also ensuring a good supply of water, nutrients, and minerals through the plant to the flowers or fruits. Cuttings need special conditions. To develop new roots, they require 90% humidity, while seedlings do well with humidity around 60%.

That's the basic information we all need to know. Now we can dive a bit deeper into the technical characteristics of humidity.

Humidity - Key Terms

• Transpiration - is the release of water through the surface of plants, specifically the leaves. This process is the culmination of the so-called transpiration stream, which carries water, nutrients, and minerals from the roots through vascular bundles to the leaves.

Figure 1 Transpiration - release of water through plant surface

• Stomata - these are pores that regulate moisture inside the plant. They help protect plants from dramatic humidity changes.

Figure 2 Stoma on a tomato leaf

• Saturation - when gas (or space) carries the maximum possible amount of water vapour at a given temperature, it is said to be saturated. If you add extra water to saturated gas or if its temperature drops, some water vapour will condense.
• Relative Humidity (RH) - this is the ratio of actual water vapour content to saturated water vapour content at a given temperature and air pressure, expressed as a percentage (%).

When measuring relative air humidity, the ambient air temperature is very important. This is because the "relative" part is essentially related to temperature and how saturated the air is at its current temperature. For example, if the temperature in a grow room rises, the relative air humidity will drop. So at 50% relative humidity, an increase in temperature from 20°C to 22°C will cause the relative humidity to drop by approximately 3%. You can get a digital thermometer that includes a hygrostat. It could be said that this is at the very least quite an essential piece of equipment.

To understand how plants function under different humidity levels, we need to know how a plant works. All plants allow carbon dioxide (CO²) to pass through their leaves via small openings called stomata. They use this gas during photosynthesis. The plant regulates CO² intake by opening and closing its stomata, which also allows moisture in the leaf to escape.

If humidity in the grow room is low (dry), it will cause plants to dry out much faster than in an environment with higher humidity. When this happens, the leaves become limp and start to wilt, and after some time the plant will close its stomata, thus reducing water output from the plant. This is very effective at stopping water loss, but unfortunately it also reduces CO² intake. Without sufficient CO² supply, cells begin to die and the plant will look tired and sick.

The key element to remember is that dry air removes water from leaves faster than the roots are able to supply water. Under these conditions, it no longer matters how much you water the plant, because it won't help. And excessive watering will remove oxygen from the root zone (rhizosphere) and create additional problems.

When a plant has the right humidity for its growth phase, it will thrive. The stomata will open fully and the plant will appreciate a good supply of fresh CO², along with controlled water drainage from the leaves.

This release of water from the plant into the atmosphere is known as evapotranspiration (a physical process by which water transforms from liquid or solid state into water vapour). Plants regulate this water loss by opening and closing guard cells, sometimes also called the vapour pressure gradient, which is the difference between the water vapour content in the atmosphere and the vapour pressure in the substomatal cavity (located immediately proximal to the stomata). This interesting fact leads us to another point, which is air movement around plants.

A layer of saturated or partially saturated air will now form around the leaf. Gentle air movement will shift this saturated air and help cool the leaves through heat transfer by convection from the leaf surface. This water movement in plants allows more water molecules to penetrate through the plants, stem and roots, creating negative water pressure in the root zone and enabling the plant to "drink". This process is known as osmosis.

However, it's important to note that high airflow speed when the fan is switched on will move all air away from the leaf edge, resulting in a dry atmosphere that increases water loss. This is not beneficial during vegetative growth. Therefore, we switch the fan to its lowest setting and avoid directing it straight at the plants - somewhere between the plant tops and below the lights is the golden zone. After all this information, it may seem difficult to know where to start. Here are a few top tips.

1. Remember that when the lights go off, the room temperature will drop and relative humidity will rise. It would be wise to ensure moisture extraction after lights-off or invest in some heaters. This way, the temperature drop won't be too drastic. This will prevent water from settling on plants/walls and creating a breeding ground for pathogens.

2. After switching on the lights, temperature rises and relative humidity drops. To prevent this drop, it's good to get a timed air humidifier. One that uses ultrasonic vibrations is better, as the released water is cooled and not in vapour form.

3. Keep these humidity levels under control, higher in vegetative growth and lower during flowering.

As mentioned at the beginning of the article, plants, like humans, prefer environmental stability in their lives. Whether it's relative humidity or temperature fluctuations, prevent dramatic changes in short time periods and your plants will metaphorically thank you.