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Relative Humidity in Your Indoor Garden

By Sunny Datko


A hygrometer is a device used for measuring relative humidity

What is Relative Humidity?

Humidity is a term used to describe how much water is in the air. However, since warmer air can hold more water vapor, the maximum amount of water that air can hold will change based on the temperature of the air. As the amount of water air can hold changes with temperature, it is difficult to pin a fixed amount of water that can be held by air. Therefore we use Relative Humidity (RH), a percentage that measures the amount of water vapor in the air compared to the total amount of water vapor that the air could potentially hold.

To monitor relative their humidity levels, many growers use a digital hygrometer – a simple device that measures the humidity levels and temperature of its surroundings. It is very important to monitor relative humidity levels, as it has a direct effect on the plant’s ability to transpire at a healthy rate. Transpiration is very important for healthy plant growth because the evaporation of water vapor from the leaf into the air actively cools the leaf tissue. In fact, the temperature of a healthy transpiring leaf can be up to 5-10°F (2-6°C) lower than a non-transpiring leaf. This shows just how important it is to try and control your plants’ environment to encourage healthy transpiration and therefore healthy growth.

Air enters the leaf through the stomata, which are usually located in the lower surface of the leaf. Stomata are enclosed by guard cells that can close the stomata to reduce water loss. This exchange is regulated by a concentration gradient, which is the concentration that exists between the two different areas. Any reduction of water in the air increases the concentration gradient for water to move from the leaf to the atmosphere, therefore the lower the relative humidity levels, the greater the driving force for transpiration. When relative humidity is high, the atmosphere contains more moisture and reduces the force for transpiration. Though plants have some degree of control of their rate of transpiration, drier air will cause the plants to transpire more.

If relative humidity is low…

The air is dry so there is a large concentration gradient for water vapor between the leaf and the atmosphere. The rate of transpiration will be high.

If relative humidity is high…

The air is moist (humid) so the concentration gradient for water vapor between the leaf and the air is small.  The rate of transpiration will be lower.

For many crops it is suggested that RH should be kept between the following limits at the stated temperatures:

Temp (°C)

Temp (°F)

Min RH

Ideal RH

Max RH




















Low humidity is fairly easy to adjust upwards to high RH, as putting water vapor back into the air can be easily achieved by using a humidifier. You can also place trays filled with gravel and water near the pots or containers. As the moisture around the pebbles evaporates, the relative humidity in the vicinity of the plants is increased.  However, having high humidity is a more common problem, as large surface areas of foliage tend to lose large volumes of water through transpiration, which adds to the humidity levels of the surrounding air. This humid air, called the boundary layer, must be removed or it will begin to restrict transpiration. The best way to accomplish this is by providing a continuous stream of fresh, dry air to your indoor garden, which not only lowers the humidity directly surrounding the leaf surface but also replenishes carbon dioxide (CO2) required for photosynthesis.

The amount of airflow required to continually remove excess water vapor and bring in sufficient CO2 is higher than many growers realize – there should be sufficient airflow to keep the leaves gently moving most of the time. The warmer it is and the faster the plants are growing, the more the air needs to be shifted over the leaf surface for these processes to occur at optimal rates. In very humid climates, a dehumidifier may be necessary as if the outside air being brought in is too humid, it won’t be able absorb much more moisture from transpiration.

Another problem with having high relative humidity levels when gardening indoors it that carbon filters will become less effective. At levels higher than 60% RH, most carbon filters begins to lose their ability to adsorb contaminants, and at over 90% RH they stop working completely. For this reason, we recommend to keep your relative humidity levels no higher than 60-70%, with the upper temperature limit depending on your crop’s ideal temperature range.

Having the correct humidity is particularly important for successfully rooting young seedlings and clones. Their ideal humidity range is between 80-90%. Low humidity will cause the young cutting to transpire and lose more moisture than what it can absorb – which can kill young cuttings. You can also use a humidity dome to prevent your clones from losing excessive moisture.  (NOTE: Cuttings need some ventilation and a couple of small holes in the dome will usually do the trick).

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One thought on “Relative Humidity in Your Indoor Garden

  • Those relative humidities in the chart appear to be on the high side. From experience I know that running above 70% for any extended time exposes you to fungi, particularly powdery mildew. I like to run at 75 degrees F and 50% humidity for best results. I will tolerate temperatures as high as 86 degrees and a corresponding increase in relative humidity to 65-70 percent on the high end, and as low as 40% before I increase it. While they have not been a problem for some time now, running humidity levels below 50% is just perfect for spider mites that would otherwise not be a problem at 65%. I believe the ideal range is in the 40-70 percent range depending upon the temperature. Getting above 70% is not a good idea. As I said, the charted relative humidity levels seem high to me for growing cannabis.

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