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Humidity and your Grow room

The quantity of water vapour in the air is called humidity and it is measured as a percentage.

Humidity is a significant factor for a healthy grow room.

The lower the humidity, the higher the evaporation pressure and the more water and nutrients your plant can absorb. However, if the evaporation pressure becomes too high, the plant will close it’s stomata to protect itself against dehydration. Consequently the plant won’t be able to absorb water anymore, and the growth will stagnate.

More humidity is required during the initial growth period than blooming period due to the younger roots being smaller. An advisable level of humidity for vegetative growth would be approximately 70% at 22 to 26 celcius. During the bloom period plants prefer the same humidity levels as in vegetative growth, although this high humidity can result in water accumulating on the plants and causing mould and rot. For this reason, it is advisable to lower the humidity in your grow room once clusters of flowers have formed. If high humidity is unavoidable, good air movement within your grow space will help to prevent moisture from being trapped. (link to floor circulator and pedestal fans)

A hygrometer is an instrument used for measuring humidity. There are various hygrometers but our favourite is the Digital thermometer/hygrometer. These can be hung above the plants in a clear area which is well ventilated.

The temperature of an area directly influences it’s humidity. The air is drier in the winter due to the cold temperatures but more moist in the warmer months of the summer.

Another factor which impacts on the humidity of an area is ventilation. Due to the humid air being drawn from the area along with the warm/odorous air, the area requires additional moisture.


The numbers on the chart represent the transpiration rate of water from leaves on your plants. The higher the number the greater the rate of transpiration. To produce healthy lush growth it is important to maintain the ideal transpiration rate. If humidity is too high, your plants will struggle to exchange water with the air as it already close to saturation point (100%RH) and if the humidity is too low, the exchange rate of water from leaves to air will be too great for the plants to uptake water to keep up. The ideal level is the yellow area, the white is acceptable, the red levels are detrimental to the plant growth and health. 

Humidifiers and Dehumidifiers

In order to maintain the correct levels of humidity within a grow room, specific equipment can control the amount of moisture in the air.

A humidifier coverts liquid water into water vapour, which is basically a fine mist of moisture which increases the humidity of the room (increases the % of water in the air). There are various kinds of humidifiers and accessories. Ours can be found here.

A dehumidifier draws moisture from the air making the air dryer which is preferential during the blooming period.

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