Ideal Humidity Level for Cannabis

The humidity of your cannabis grow room plays a large part in how your plants function. It can effect the growth and final yield as well as having implications in the spread of disease and mildew. Through an understanding of exactly what your plant needs and how to make sure it has it, you will increase the quality of your grow and subsequently the marijuana you end up with. Knowledge is power!

Firstly, what is humidity? Well, humidity refers to the amount of water vapor present in the air (water vapor is simply water in its gas form). It is often referred to as a percentage, for example, a humidity of 75% means that the air currently contains 75% of the total maximum water vapor it can hold at that temperature. Also, as air increases in temperature it becomes capable of holding more water vapor, meaning temperature and humidity is related. As a result the percentage used to measure humidity is referred to as Relative Humidity (RH).

A lot of novice cultivators try to maintain a low humidity; this is a common mistake and is largely down to the idea that high humidity encourages disease amongst plants. Whilst partly true, this “play it safe” attitude can have its own adverse effects on how your cannabis plants grow.

The reason it is so important to your plants is because it will affect the rate at which they transpire. During transpiration plants release water vapor into the air, it functions along the same lines as osmosis – that the water levels inside and outside the plant will try to level out to equilibrium. This means that if you have a low humidity, your plants will rapidly transpire as water is drawn out into the air, reducing the amount of water within the plant and potentially having detrimental effects when water levels reach to low. If humidity is high then plants will transpire at a much slower rate and have a lesser potential for loss. It should be noted that cannabis plants have a “humidity” rating of pretty much 100%, so they will always transpire (which is OK as it is an essential part of its functioning).

So why do plants transpire? Well basically, they do it for a number of very important reasons. It is done for temperature control, it is how plants cool themselves and regulate their own temperature. It is also how the cannabis plants move minerals and nutrients about, as water leaves the plant it draws more up from the roots, allowing for the absorption of nutrients from the soil. Finally it is how your cannabis plants get the carbon dioxide they need out of the air – Plants open their stomata to let water vapor out, and in the process carbon dioxide gets in.

If the humidity is too low, then your cannabis plants are going to do a lot of transpiring and it will play havoc with their transportation systems. They will lose a lot of water and begin to exhibit the damage usually caused by dryness – stunted new leaves, shriveling old leaves and dying flowers.

As mentioned, whilst low humidity is usually the pitfall of most novice growers, it is for good reason – they have heard or read somewhere that high humidity is likely to cause the spread of disease, and they are not far off. High humidity has its own perils. In a grow room with excessive humidity and very little air movement (ventilation) you run the risk of exposing your plants to fungal disease, mildew and root rot. However, it is very easy to avoid, with careful grow room planning and management you should not find yourself in a situation where this happens.

The best way to monitor the RH of your grow room is to use a hygrometer, this should give you an accurate read out of the exact water vapor content of the air.

Humidity effect cannabis plants throughout their entire life cycle, below explains how it affects them at each stage and what the optimal humidity is.

Cutting/Seedling Stage:

At this stage the ideal RH is 70-80%. This RH will ensure that the seedlings do not put too much of their energy into their transpiration process, as there is not much of an imbalance. This will allow your seedling to focus their energy into root and leaf growth. Your little ones will also need to maintain some level of transpiration in order to draw up nutrients from the growing medium.

A great way to control the humidity of you grow room is with good ventilation, and the use of a humidifier when humidity are beginning to get a bit low. A humidifier is an easy to obtain, cheap bit of equipment that simply introduces more water vapor into the air. When used in conjunction with a hygrometer you should easily be able to keep RH within your cannabis plants' safe limit.

Vegetative Stage:

In this stage the RH can be 50-80%. Now that your plants have entered their main growing phase the RH can be a lot more varied. This is because your plant will now have a much bigger surface area with which to transpire, meaning less stress is put on it through faster transpiration.

However, this does mean your plants can transpire at a much greater rate, be sure to keep an eye on your plants. If the levels of humidity drops below a safe level then your plants will transpire so rapidly that they run the risk of over fertilization from their fast uptake of nutrients drawn in from the soil.

Flowering Stage:

Once your cannabis is flowering you will want to consider dropping the RH greatly. This is in order to reduce the risk of the dreaded rot. You ideally want you grow room to have a relative humidity of 40-50% now.

Keeping track and controlling your grow rooms humidity is important if you want to really get the most out of your plants. It is another important factor that is usually overlooked by less experienced cultivators. Now that you have a better understanding of its impact you should be able to utilize the knowledge to improve the quality of your grow.