If you want to grow sticky, high-potency buds, your cannabis will demand attention to its many needs.

One need of the growing cannabis plant – that is commonly overlooked – is humidity.

Believe it or not, humidity is actually very important to your plant! Humidity will help determine your plants resilience against mold/mildew in addition to how much your plants need to drink. Getting it just right can improve plant growth and increase your yields.

Luckily, there isn’t much to learn, so you can be the master of humidity in your grow with very little effort! Today we will give you the information you need to have complete mastery over humidity and thus, give your plants a boost in production.

Why Humidity Matters To YOU?

In all stages of cannabis growth your plants will have a constant need to intake water, and the amount of water they need fluctuates with the humidity in your marijuana grow tent. When the humidity is high, cannabis plants use their leaves to absorb moisture from the air which causes them to drink less water from their roots. Conversely, when the humidity is low, they will pull more water in through their roots.

Since humidity changes how much water your plants drink, and the water you give your plants have nutrients in them, being in control of humidity gives you increased control over your plant’s nutrient intake.

Clones

Optimal Humidity is 70% RH

A new cannabis clone showing her first rootsClones need time to develop a root system to intake water. Young clones are solely dependent on getting water through their leaves from the air and this is only possible with high humidity.

Maintaining high levels of humidity will dramatically increase the chances of clones rooting successfully, and will increase young clone growth.

This is why many growers use a humidity dome for new clones!

Vegetative Plants

Optimal Humidity is 40-60% RH

40-70% Relative Humidity is ideal for vegetative cannabis plants - This pictures shows a humidity monitor (called a hygrometer) in the correct rangeSeedlings and young cannabis plants in the vegetative stage grow a lot of leaf mass in a great growing environment, and plants can sometimes grow foliage at a faster pace than the roots.

But the roots aren’t the only way your plant gets water. Leaves can actually pull water vapor directly out of the air, and higher humidity in the air allows the leaves to get more water to the plant if needed. Until your plant has fully developed roots, low levels of humidity in the air can cause growth to be slower.

The ideas is to create a comfortably warm environment that mimics springtime or summer.

Warning Signs:

Don’t give seedlings a humidity dome unless it’s very dry where you live. They need springtime conditions, but it can be easy to go overboard with seedlings (compared to clones which love high humidity).
The RH is too high if the plants are forming wet spots on their leaves even when they’re not touching each other
As seedlings get older and become young plants, they want the humidity around 50% RH
If the RH drops below 25%, the dry air tends to limit growth and cause nutrient problems to appear on leaves, especially for young seedlings. Some seedlings are fine, but other plants are particularly sensitive to humidity.

Flowering plants

Optimal Humidity is 40-50% RH

Ideally, the relative humidity of your grow room should be lowered to around 40-50% at the beginning of the flowering stage. As plants approach harvest, some growers lower humidity down below 40% or even less to force cannabis buds to produce more resin, though I’ve seen that can majorly stress some plants, so that technique should be used with caution. Definitely lower the humidity just a little at a time to see how plants react before putting your plants in an extreme environment.

By the beginning of the flowering stage, your cannabis plants have created a large root system to fulfill their water needs. They will still be able to take in water through their leaves, but maintaining a relatively lower humidity than the vegetative stage helps prevent molds or mildews from forming.

Mold is especially dangerous in the flowering stage because it can form on or inside your buds or colas without you knowing. If you find that you have mold growing in any of your buds, or buds are rotting from the inside, you should immediately throw all of the contaminated buds away. Bud rot can ruin entire harvests, so if one bud is infected thoroughly search your grow room for more. If you can’t lower the humidity and already have a rampant mold problem, the best choice might be to cut your losses and harvest immediately to avoid any other buds getting bud rot.

Even if the humidity is low, it is still important to have air moving over and around your plants. Small fans blowing over and under the plant canopy will help keep air moving so that wet spots don’t form around any parts of the plant.

Temperature and How It Relates to Humidity

Temperature and relative humidity are closely related to each other.

Temperature and Humidity Monitor - Digital. Relative humidity is measuring how much water is “being held” in the air compared to the maximum amount of water that can be held at that temperature.

Warm air can “hold onto” more water than cool air. Plants will tend to thrive at different relative humidities depending on the temperature of the air. Once the air becomes too saturated with water, it will tend to form dew or films of water over leaves, which leads to mold.

For those interested in learning more about the science behind this, there is a term used by greenhouse growers known at VPD, or Vapour Pressure Deficit, that roughly measures the temperature and relative humidity.

As a general rule, most plants grow well at VPDs of between 0.8 to 0.95 kPa.”

When growing plants, temperature is just as important as the Relative Humidity (RH) of the air, if not even more so! You need to control both temperature and RH at the same time to get the best results when growing cannabis.

If the air is too hot and dry (high VPD), plants will tend to have stretched growth.

If the air is too cool and humid (low VPD), plants are more prone to problems with mold or fungus.

How to Control Humidity in the Grow Room

A high-quality humidifier is a convenient and relatively inexpensive way of misting your grow room. Just set the dial on a specific humidity.

The humidity changes to the desired level as soon as the humidifier is able to mist enough water into the air.

humidifier

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