What is the best humidity for indoor cannabis growing?

Glueberry OG field of green early flowering buds
Glueberry OG field of green early flowering buds

Many indoor growers don’t monitor relative humidity when growing cannabis. They just hope that their cannabis seeds will germinate and grow well. More experienced cannabis growers will routinely monitor humidity using a small, inexpensive device. They will understand the potential dangers of excessive humidity levels, especially around harvest. Those growers that understand their local natural humidity levels from one season to the next may even avoid growing in the most humid months or invest in a grow room de-humidifier to reduce humidity (often referred to as % Relative Humidity or %RH) to safe levels.

The basics of humidity control for indoor cannabis
What factors affect relative humidity control for cannabis?
What is the ideal humidity level for indoor cannabis?
Indoor cannabis growing humidity control is key

The basics of humidity control for indoor cannabis

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Humidity is a measure of water vapour in the atmosphere. The hotter the air, the more water vapour it can hold. Although normally thought of as a liquid at room temperature, water also easily forms a vapour which is made of airborne water molecules. When air has a high humidity, it may make you (and your plants) feel warmer than normal – the moisture helps the air to carry additional heat. ‘

  • Absolute humidity’ is the total mass of water vapor in a given volume of air, regardless of air temperature. It is usually measured in grams of water per cubic metre.
  • Relative humidity (%RH) is a percentage measurement of water vapor saturation relative to its maximum saturation at a particular temperature. This tends to be an easier value to measure.
  • 100% RH means at that particular temperature the dew-point has been reached, meaning that moisture droplets can form.

Many growers are aware that high humidity levels can be useful when growing cuttings, which may have incomplete root systems and therefore benefit from a humid grow environment. Conversely, high humidity levels may increase chances of dreaded bud rot/botrytis in late bloom.

That’s why many growers use an inexpensive RH meter (available from around $/£/€ 30) to monitor humidity during their grow. As your plants grow larger and use/transpire greater volumes of water, it may become more of a challenge to maintain low RH levels.

This pitfall can especially happen when growing indoors during warmer/wetter summer months. In these circumstances, maintaining efficient levels of grow room ventilation is of great importance.

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What factors affect relative humidity control for cannabis?

Relative humidity control for cannabis is complicated by the fact that it will vary from one season to the next. Even small geographical distances from one neighbourhood to the next can result in significant differences in the types of humidity experienced.

The quality conscious grower tends to monitor the relative humidity whether they grow their cannabis seeds indoors, outdoors or in a greenhouse/polytunnel.

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Climate factors:

Other factors:

What is the ideal humidity level for indoor cannabis?

Many growers will ‘get away’ without paying significant attention to %RH. But the more serious growers will monitor humidity and adjust conditions accordingly when growing their plants as well as when drying and curing the buds. The more you can optimise relative humidity during your plants development the higher your chances of pushing your cannabis seeds to their full genetic potential.

Bonus tip – drying/curing cannabis at the correct humidity:

You invested in the best cannabis seeds, pimped your grow room to the max with the latest LED tech and delivered a stellar harvest.

Now your premium quality buds deserve connoisseur drying and curing! This is often achieved by slowly drying the buds and patiently curing them. After cutting down the cannabis plant, many will dry the buds slowly, sometimes in brown paper bags to ensure a controlled drying.

Curing the cannabis buds involves slowly releasing the last remnants of moisture, opening the jars occasionally. The process may take the connoisseur a couple of months using humidity levels of around 52%-62% inside the jars. Around 55% humidity is ideal.

Those that strive for perfect buds often use the humidity regulating bags from Boveda or Integra Boost to maintain the perfect curing environment inside the jars.

Perfectly cured cannabis buds are one of the greatest rewards for the serious, quality-focussed grower. The buds deliver a pungent aroma thanks to the well-preserved terpene profile, they crumble easily into an oily slurry without being too dry. Best of all, perfectly cured buds deliver the ultimate vaping/smoking experience. Aroma, taste and effect should all be as good it gets!

Indoor cannabis growing humidity control is key

Glueberry OG grown indoors under led grow lights

Humidity control when growing cannabis is the sign of an experienced grower that understands the role of humidity at various stages of the plant life cycle. The final process of drying and curing the cannabis buds also has it’s own precise demands for those keen to optimise each and every stage.

If you haven’t bothered much with the finer points of humidity control when growing cannabis then an easy way to begin is with a basic humidity meter. Start by monitoring the humidity levels in your tent at various stages of growth, perhaps at different times of the year. You can then move on to trying to control/optimise conditions. And you will notice a significant improvement to plant health, growth and final bud quality if you can master humidity control in your grow room.

Remember, the pro-growers don’t invest so much time, effort and money in their climate/humidity control without good reason! With a small amount of additional focus on humidity control you may be able to significantly improve the quality of your own cannabis harvests.

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