Humidity is really just the amount of moisture in the air, turned into a percentage. Among cannabis growers, it’s one of the most talked-about subjects. This is because it can truly make or break plants, letting them thrive or potentially destroying an entire crop. So why does it matter so much? What can you do to control it? And what exactly are the right levels of humidity you should have in your grow room?

Why is humidity important?

marijuana-grow-room-humidityHumidity in your grow room is important, simply because it means there’s more moisture in the air. The more moisture that’s in the air, the less your marijuana plants will drink water up from the soil, or water through their roots. This is not ideal, as the water has nutrients in it. The moisture in the air has very little. When the plants can take moisture from the air, they will stop doing so from the nutrient-rich roots. That means you probably wont get those huge, dense buds you’re looking for come harvest time. Simply because they haven’t been fed properly.

While controlling moisture in the air is a very good reason to make sure your grow room has proper humidity levels, there’s another reason, too. As  marijuana plants move into the flowering stage, they actually need even less moisture in the air. If there’s too much at this point, that moisture will cling to the buds.  For growers this can spell trouble because it means that dreaded bud rot can set in. Possibly destroying a entire plant or even an entire crop.

When a grow room has poor humidity, there are other problems that can arise as well.

What are the effects of poor humidity?

Bud rot has already been mentioned, but it can be so devastating to growers, it’s worth one more mention. There are times when growers cut a huge dense cola from their marijuana plants, only to find that the inside has become white or brown. That’s mold. Buds showing this type of mold are unusable, the only real option a grower has is to throw them away.

bud rot poor humidityHowever, this kind of rot isn’t the only type of mold that can show up in plants when the humidity in the room is poor. White powdery mildew is another type of mold that can show up on the leaves of the plants, as the leaves are so prevalent in the early stages of growth. Essentially making up the entire plant, and it’s at this young stage that plants love humidity. It can be a delicate balance for the grower to walk between levels that are too high, and levels that are not high enough.

Improper humidity levels can really mess with a plant’s nutrient intake. If levels are too high, they’ll stop drinking water from their roots, leading to nutrient-deficient plants. But levels that are too low will also cause problems. This is because if the plants aren’t getting enough moisture from the air, they’ll suck up all those nutrients from the roots, possibly getting much more than they need. This can cause the leaves to turn yellow, or even start to crisp around the edges. This is called nutrient-burn and it can be deadly for plants.

Lastly, anything that is not working properly in the grow room is going to negatively affect the plants within it. When the humidity levels are off, plants won’t grow to be the healthy abundant plants they potentially could have been. The plants also may not grow to be entirely healthy, which again, can be devastating for growers.

How to monitor humidity

So now you know just how important humidity is and how it affects your marijuana plants. But how exactly are you supposed to control it? It’s easier than you think.

Start by making a small investment in a thermometer that will also give you the humidity of the room. After all, you can’t fix a problem if you don’t know that it exists. These are little wall units that will tell you the exact environment your plants are in. So you’ll know whether to increase or decrease the humidity levels.


If the humidity is too high and you need to get some of that moisture out of that air, a dehumidifier works wonders. It’s all right there in the name. These machines literally suck moisture out of the air and collect it in a basin hidden within the machine. When the basin is full, you just need to empty it, restart the machine, and it will continue to do its job.

Along with a dehumidifier, fans and ventilation should be a part of every grow room. Fans will keep the moisture moving, and help to evaporate it off leaves and buds. Reducing the chances of finding any rot or mold when it’s time to harvest (or even earlier.)

Also avoid over-watering plants when the humidity levels are already high. This will not only leave the plant sitting in soggy soil, something that can cause root damage, but it will also only add to the high humidity levels of the room.

If the humidity levels aren’t high enough for your plants to thrive in, add a humidifier to the room. These are similar to dehumidifiers, but they do the opposite job, actually adding moisture to the room instead of taking it out. This small investment could do wonders for plants, particularly young ones, that love their humidity.

Lastly, always know what the best level of humidity is for your plants. If you don’t know, you can’t compare and fix problems when they arise.

What is the best level of humidity for your marijuana plants?

One of the reasons humidity is such a tricky subject for growers is because plants need different levels of humidity throughout their lives. Levels should start fairly high, and then start to taper off as the plant reaches its final weeks of growth.

Clones should start off in an environment that has 70 to 80 percent humidity. As they move into the vegetative stage this can be lowered slightly, around 40 to 70 percent is a good level. Once the plants start flowering, the humidity can be lowered even further, generally down to about 40 to 50 percent. Then, in the final weeks of flowering when plants are trying to get that last nutrient push, it should be lowered to about 40 percent.

There’s nothing that can trip a grower up, particularly one new to the game, than humidity. But follow these tips and you’ll see plants that thrive!

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