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