How to reduce humidity in my grow room?


Many growers experience or have experienced an excess of humidity in their grow room which can if left unchecked turn into devastating results. A grower recently submitted a question regarding humidity :

“In my home state the humidity is fairly high, frequently over 70%.  I am just beginning my first flowering session and am 2 weeks into flowering. I have a large number of colas that are packed tight, and they are looking great… but I fear that I may attract bud rot, what can i do?  This week the humidity outside has been maintaining 90%”

In a case like this there are a few steps that need to be taken, first and foremost the room needs to be sealed off as much as possible. Ideally this will already have been done in an attempt to maintain proper temperature, but if not now is a good time. For this task I suggest rigid foam insulation board, it is fairly standard and used by many growers indoors when trying to maintain an artificial climate.

Once you have a room sealed off for the most part the second step is to add a properly sized dehumidifier into your grow room. During the flowering stage it is important to maintain a humidity level around 45%.  Based on the size of your grow room you can determine what size dehumidifier would be best for you. A dehumidifier is rated based on their total moisture removal capacity in a 24 hour period. For example a dehumidifier rated at 45 pints can remove a maximum of 45 pints of moisture every day.

Cannabis plants transpire roughly 95% of all of the moisture they take it. To get a rough estimate of how much moisture that is putting int the air you can compare the amount of water you are providing them. As an example if your garden consists of 12 plants  you are providing your plants with 1 gallon of water per day, per pant than your daily water input will be roughly 12 gallons. Each gallon provided to the plants is equal to a maximum of 8 pints of moisture that can be built up within the room, with a total maximum moisture level of 96 pints.

Taking into consideration that some of this moisture is consumed by the plant as well as some of the moisture is absorbed by the soil you can get a rough idea of the amount of moisture that can be pulled from the room. I estimate 20% of the water provided is “consumed” by the plants and soil and the remaining 80% should be addressed with a properly sized dehumidifier. In this example a dehumidifier that can is rated for 70 or more pints is an ideal. and cost effective solution.

Additional measures to take in order to reduce humidity include:

Defoliate below the canopy. Defoliation is the process of removing unnecessary leaves from a plant. Additional unnecessary leaves can cause moisture build up as well reduce air flow. By removing leaves that are shaded or otherwise hidden from canopy you will increase air circulation and reduce mold build up.
Mop up any water. This is a basic but often overlooked simple solution. Keep your grow room as dry and as clean as possible. Standing water or wet material can act as a breeding ground for problem causing bacteria and molds, as well as increasing the relative humidity of a grow space.
Increase air circulation. Once you have defoliated where you can, you may consider the integration of  additional measures such as wall mounted fans.

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