The ideal humidity levels for growing cannabis


Humidity is an important topic for cannabis growers. But why is humidity so important for cannabis plants, and what are the ideal humidity levels to maintain in your grow room?

To better understand why humidity is important to plants, it helps to think about how plants absorb air from the environment. Remember, all plants respire carbon dioxide from the environment through their leaves. In doing so, they naturally lose some of the water retained in their foliage.

Dry air causes plants to lose more water when respiring than moist air, and hence, drives down the overall moisture content in the plant more quickly. Once the environment becomes too dry, plants lose more water than they are able to regain via their roots.

When this happens, plants will almost entirely close the pores in their leaves in order to minimize moisture loss. This works, of course, but also means the plants receive less carbon dioxide from the environment. This causes cell death within the plants and leaves them looking limp and ill.

Now, you might think to water your plants more regularly in order to combat moisture loss in dry conditions. And while it seems logical, it won’t work. As you overwater the soil, you’ll only reduce the amount of air in the soil, further asphyxiating the plant and also increasing the chances of the plants developing root rot.

When plants have the right amount of humidity, however, they flourish. The correct amount allows plants to open their pores more fully, respire more carbon dioxide, and grow rapidly.

However, overly moist conditions can also be harmful to plants, especially cannabis, and puts them at risk of developing bud rot and other diseases as water builds up in the thick foliage and creates the perfect environment for bacteria and fungi.

UNDERSTANDING RELATIVE HUMIDITY

When we talk about the perfect humidity for any plant, it’s important to realize that we’re talking about “relative humidity.” This is a complex term, so we’ll try to explain it in the simplest terms possible.

Humidity simply refers to the amount of water vapor in the air. However, it’s critical to note that the amount of water the air can hold depends on temperature. Hot air can naturally hold more moisture than cold air.

Relative humidity is simply a term that refers to the amount of water air can hold at a given temperature.

Once relative humidity levels reach over 100%, the surplus of water can no longer be held by the air and will condensate into water droplets, such as fog, morning dew, or rain.

Let’s imagine the temperature in your grow room is 30°C (86°F) with a relative humidity (RH) of 33%. If you were to rapidly cool down the environment to 10°C (51°F), the RH would rise to 100% (also known as the Dew Point). Should you reduce the temperature further, the water in the air would begin to condensate.

WHAT IS THE IDEAL RELATIVE HUMIDITY FOR CANNABIS?

The ideal relative humidity level for plants vary. Plants native to hot, tropical areas are well-adapted to these temperatures and will grow best at higher RH levels than plants from colder, more temperate climates, for example.

History suggests that cannabis is native to various parts of the world. Indica varieties seem to originate and grow best in drier, mountainous regions and can also deal with lower temperatures. Sativa varieties, on the other hand, tend to grow naturally in warm, subtropical climates, and prefer warmer, more humid conditions.

Since cannabis varieties have been heavily crossbred, most of the strains bought through seedbanks are some kind of indica-sativa hybrid. Hence, it’s not enough to simply “guess” what the ideal humidity level might be for a particular strain based on its genetics. Instead, most growers will assess their plants and their morphology, and experiment with RH levels to find that sweet spot for each grow.

Nonetheless, there are some basic “rules of thumb” you can follow:

  • Seedlings and clones like RH levels of 65–80%. Remember, these young, fragile plants have weak root systems. By increasing humidity levels, you’ll allow them to take up more water from the environment and focus on developing strong roots. At this early stage, you’ll want to keep temperature levels at around 25°C during the day, and around 21°C at night (77°F and 70°F, respectively).
  • Vegetative plants, on the other hand, tend to prefer moderate humidity levels. There’s no exact figure to follow here, but you’ll want to stick somewhere between 55–70% depending on the strain. Temperatures should sit between 22–28°C (71–82°F) during the day and roughly 18–24°C (64–75°F) at night. Vegetative plants have strong root systems and will absorb more water from the soil, which is why most growers drop humidity levels slightly during this stage.
  • At the early flowering stage, most growers agree that plants benefit from lower humidity levels. Again, there’s no ideal figure to aim for, but many will stick between 40–50%, occasionally pushing to roughly 55%. You’ll also want to drop temperatures to about 20–26°C (68–78°F).
  • During the late flowering stage, try to drop humidity to about 30–40% and keep temperatures around 18–24°C (64–75°F) during the day, followed by slightly cooler nights (16-20C in the last few nights).

Remember, the figures listed here are guidelines. There’s no shortage of growers who’ll tell you they achieve great results with high humidity levels, especially during the flowering stage.

Just keep in mind that thick, bushy, indica-like plants with dense buds can develop bud rot in overly humid conditions. The best way to avoid this is to check on your plants regularly and use a quality thermometer and hygrometer (or an all-in-one thermohygrometer) to keep an eye on the temperature/humidity in your grow room.

If you’re looking to drive down humidity in your grow space, try the following:

  • Run your exhaust vent on higher velocities to push more hot air out of the room (remember, hot air holds more water).
  • Increase the cool air supply.
  • Water your plants right at the beginning of their light period.
  • Use a dehumidifier.

If, on the other hand, you want to increase the humidity levels in your grow space, try this:

  • Turn down your exhaust fan to keep more warm air in your grow space.
  • Increase the temperature in your grow space.
  • Mist your plants (don’t do this to flowering plants to avoid bud rot).