As Temperatures Rise...Don’t Forget About Humidity

By Lacey Macri
Published: July 9, 2019 | Last updated: April 21, 2021 11:06:48
Key Takeaways

Humidity levels can sometimes get overlooked in the summer months, especially when the heat starts soaring, but after a few mishaps, most growers will begin to realize how important monitoring and controlling humidity really is. Here are some general tips for optimizing humidity indoors, and how to avoid its associated issues throughout the summer months.

For gardeners who grow indoors in the summer, there are a few crucial guidelines to keep in mind to give plants a fighting chance against potentially hot conditions. Humidity levels are often overlooked by novice growers, but after some trial and error, most growers start to realize how important monitoring and controlling humidity really is.


Just like every other controllable environmental condition in the growroom, there is a sweet spot for humidity levels. Let’s start by exploring how humidity interacts with your plants throughout their natural course of development.

Humidity is a lot like osmosis—plants interact with their environment to maintain an equilibrium between the water levels inside and outside of their bodies. If the relative humidity in the growroom is too low (too dry), plants will transpire more water vapor back into the environment in an attempt to stabilize humidity levels.


As a result, plants may compensate for the higher-than-normal water loss by taking up more water and nutrients through their roots. This can set the stage for leaf burn if plants take up too many nutrients, or even nutrient lockout.

On the flipside, if the relative humidity inside a growroom is too high, transpiration will be slowed and plants may not be able to absorb any more water or nutrients. Transpiration is controlled by the plant’s stomata, which are similar to the human lungs in the sense that there is an exchange with the environment to carry out healthy survival and development.

Water vapor is released into the environment as a cooling mechanism for plants (much like sweating), and CO2 is brought back in through the stomata to encourage growth. If the relative humidity is too high, plants will struggle to make this exchange. High humidity has a choking/drowning effect on the transpiration process.


High humidity levels come with a few other risks as well. Many growers fear molds, mildews and the dreaded bud rot that happens as a result of high humidity levels during the flowering stage of growth.

Read also: It’s (Probably) Not pH Lockout: Troubleshooting Common Problems in the Growroom


Because of their size, flowering plants may absorb more water than normal, which can contribute to higher humidity levels in the growroom, fostering a favorable environment for bud rot development. Keep a close eye out for signs of bud rot, and adjust your environmental controls to suit your plants’ needs, especially during the final few weeks of flowering—in this case, lowering the humidity.

f you are a novice growers, be careful not to overcompensate for these risks by lowering your humidity to dangerous levels. This can be equally detrimental, so it’s wise to find the sweet spot tailored to your particular situation.

There are a few easy ways to set up your growroom from the start that will lessen the chances of encountering problems associated with inadequate humidity levels. To start, space your plants appropriately. Many growers just starting out are under the impression that the more plants they grow, the greater yields they will get.

However, overcrowding plants predisposes the environment to high humidity levels, especially since most plants maintain a humidity rating of close to 100%. As a general rule, give each plant at least a 2- by 2-ft. area to flourish.

Another added bonus to spacing your plants generously is it opens up the benefits of pinching plants. Growers who overcrowd their tents also have to be careful, as an overly dense population of growth may promote even higher humidity levels.

Deep water culture may be particularly useful to accommodate these standards, as the nature of the set-up allows for robust root growth, along with maximizing vertical growth and providing a strong foundation to support prolific horizontal growth when spaced appropriately.

Oscillating fans are another no-brainer when it comes to stabilizing humidity levels in your growroom. While they may not be enough to deter all of the problems associated with excess humidity, they will improve air movement, which vastly contributes to healthy transpiration.

Determining the equipment you need to deal with humidity depends on the region where you’re growing. If you live in a humid climate, additional fans, dehumidifiers and exhaust systems may be necessary. In areas where arid climates are prolific, you might need a humidifier.

Determining your target humidity level based on your region’s climate and the garden’s stage of growth is vital for maximizing growth potential. Once you do these two things, you can determine what type of equipment you need to maintain your targets. You can implement all of the tricks in the world, but having your humidity out of whack will likely negate all of your other efforts, wasting precious time and money.


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Written by Lacey Macri

Profile Picture of Lacey Macri
Lacey Macri works as head of sales at CleanGrow, focusing her time on business development within the company. She received a bachelor’s degree in communications and psychology from the University of California, Davis, in 2011, where she worked at the California Aggie student newspaper on campus.

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