Taking Humidity In Your Grow Room Under Control

Last updated: 16 April 2020

cannabis humidity

The difference humidity can make to your grow environment is phenomenal. Dialing in the correct cannabis humidity for your grow will mean the difference between a bumper harvest of buds, and plants that are at a higher risk of succumbing to mold and other problems. In this article, we’ll be diving into all of the aspects of relative humidity and its importance in cannabis cultivation.

What Is Relative Humidity?

When growers talk about humidity, they’re usually referring to relative humidity (RH), which is the ratio of how much water vapor is in the air at a given temperature compared to how much of it could potentially be in the air at the same temperature. Because of this, relative humidity is expressed as a percentage. If the relative humidity reaches 100%, it means the air cannot hold any more water at its current temperature, and that rain, fog or dew may start to form.

Why Is Humidity So Important For Cannabis?

humidity for cannabis

We all know that plants drink through their roots, but did you know they also absorb moisture through their leaves? This means the humidity impacts how much nutrients in the water are taken up by the plant. This, in turn, has a drastic effect on how your cannabis plant grows – and when in the flowering stage, humidity only becomes more important. Controlling your humidity in this phase results in denser buds that can develop more resinous trichomes.

The Relation Between Humidity And Temperature

If you’re having issues with humidity in your grow op, it would be a good idea to change the temperature in the room. Why? Because Temperature and relative humidity are inversely related. It means if the temperature goes up, the relative humidity goes down and vice versa. It ties back to the fact that relative humidity is a measurement taken at a specific temperature, not of a specific water weight.

Dangers Of Poor Humidity Control

As with any environment you have total control over, your grow room is completely dependent on your ability to keep the humidity in check. The dangers of poor humidity control range from losing a bit of weight at final harvest to losing your crop completely.

What If Your Humidity Is Too Low?

humidity for weed

If the relative humidity in your environment is too low, you’ll see visible signs of plant stress, including leaves curling upward and branches drooping. An overly dry environment will also cause your plant’s pores to close, resulting in stress from lack of transpiration. This means your plants are not expelling enough moisture, resulting in them taking up less water (and nutrients!) through their roots.

… Or Too High?

best humidity for marijuana

If, on the other hand, your relative humidity is too high, you have a whole other world of worry to deal with. First off, high humidity will cause your plant to grow wider apart, resulting in airy buds that lack structure and firmness. High humidity during flowering is especially dangerous, as it can create damp spots in the bud that lead to mold.

If things get really out of hand, molds and mildew can set in, wiping out all of your effort. Bud rot is one of the most common problems when growing in high humidity, which can completely decimate your harvest. Although less common, issues such as stem infections can also arise if the humidity is too high, so it’s vital that you keep it in check.

Best Humidity For Marijuana Plants

But what exactly is keeping it in check? What numbers should you be looking out for on your newly purchased hydrometer? As with people, cannabis has different requirements depending on its stage of life. Fortunately, we’ve got information for every part of the plant's life cycle so you can know exactly what your humidity should be and when.

Clones And Seedlings

high humidity during flowering

As clones or seedlings have either just been cut from the mother plant or popped from a tiny seed, they have yet to develop a robust root system that will deliver water and nutrients to the plant. To get around this, the tiny marijuana plants use their leaves to absorb moisture from the air. Because we want the best for our new plants, it’s important to keep them in an environment with a high relative humidity of around 70%.

Vegetative Stage

humidity for seedlings

Once your plants have moved into the vegetative stage, they’re ready to start packing on the foliage and get ready for flowering. In this stage of their life, the relative humidity you’re aiming for is around 40-60%. This leaves enough moisture in the air for the cannabis plant not to be stressed, without having it so damp that there’s no reason for it to drink through its roots. A humidity level of around 40-60% will also help to promote good branching and stacking, which will result in a better harvest.

Flowering Stage

best humidity for cannabis

How high should humidity levels be during the flowering phase? Most growers aim for relative humidity of around 40-50% during the flowering phase. Although not much different from vegetation, dropping the humidity even 10% also helps ensure that there’s no danger of mold or mildew setting in as your buds start to form.

A technique sometimes used by more experienced growers is to drop the humidity even lower in the final few weeks of flowering, sometimes down to 20-30%. The reasoning behind this is to stress the plant into developing more trichomes in an attempt to protect itself from the harsher environments.

Drying And Curing

ideal humidity for cannabis

When setting up your drying and curing environment, it’s important to maintain full control of the humidity, as having it rise and fall will result in poorly dried and cured buds that smoke harshly and taste terrible.

Most growers try to keep their drying rooms at around 55% humidity, although some like to raise this to around 60%, which results in a slower drying process that can sweeten and improve the flower.

Keeping The Humidity Level Right At All Times

Even if you have a completely sealed grow room, fluctuations can still occur. Depending on the temperature outside and in, as well as the natural cycle of the seasons, you may have to adjust your grow room’s humidity at some point.

How To Raise Humidity

Using a sprayer or mister when the light cycle starts will help increase the humidity in your grow room, as the heat from the grow light will evaporate the water droplets into the air. Lowering your light a bit can also help to increase the humidity of your plant’s canopy.

how high should humidity levels be during the flowering phase

The more high-tech option is buying a humidifier, which is a machine that sends out a constant, fine mist to raise the environment's humidity. Higher-end humidifiers have inbuilt humidistats, which allow you to set your humidifier to turn off at the desired humidity.

How To Lower Humidity

Exaction fans are a fantastic way to lower your grow rooms humidity considerably. If you’re running intake fans, make sure these are pulling the coolest air you can find. Within the room itself, adding extra airflow with fans can help improve humid environments. Raising your grow light will also reduce evaporation from the canopy, which will in turn lower humidity.

humidity in grow room

If all that fails, you may need to invest in a dehumidifier, which is a machine that pulls moisture from the air. As with the humidifiers, good ones will usually have an inbuilt humidistat, allowing you to set a precise humidity you want to keep.

Taking Care Of Humidity Outdoors

what are the optimum humidity levels for indoor cannabis growing

Like with the humidity of a grow room, the humidity of the outdoor environment will have a direct impact on the plants that are grown. If you’re growing cannabis outside, make sure to choose a location with a good amount of airflow that’s in full sunlight.

The airflow around the plants will help to remove any stale air that could cause problems, while having the plants in full sunlight not only ensures they get maximum growing time, but also helps to dry the plants out every day from the morning dew.

Getting Humidity Right

Like with many environmental factors, when it comes to growing cannabis, setting the correct humidity can be a bit of a challenge at first. It can move up and down seemingly without rhyme or reason, and letting it run amok will have serious effects on your crop.

Once you’ve perfected the art of humidity control, however, you will notice enormous improvements in your harvest as well as in the overall health of your plants. So what are you waiting for? Go for it and get growing!

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