Humidity 101

Some of you said I went too fast in my video, so here's the transcript. If you have any questions, please don't hold back. Here's everything new and aspiring indoor gardeners should know about HUMIDITY!

What exactly is humidity?

  • Invisible steam? 
  • Gaseous rain? 
  • Mist? 
  • Odorless unicorn farts?  

None of the above! It’s just moisture—or water vapor, to be exact—that’s “in” the air. Now, when I say “in” don’t think of it as being somehow attached to the air or joined at the molecular level. It’s not—it’s just hanging out in the gaps between the air’s molecules. However water molecules can only join this vapor party when there’s enough energyaround to release it from its bonds as a liquid, turn it into a vapor, and keep it there—bouncing around frivolously with all those air molecules. 

Humidity 101

Understand Humidity in your Grow Room in under 6 minutes.


Now—you may have heard that hot air can hold onto more moisture than cold air. Hmmmmmmyeeeeah---kinda. I don’t really like the ‘holding onto’ notion so, instead, think of it like this: The warmer the air, or the water, the more energy it contains—and the more the air and water molecules move and collide with each other. Bam! These tiny collisions transfer energy and, when a water molecule gains enough energy, it can free itself from the attractive forces that hold it together as a liquid and liberate itself as a vapor. 

Anyway…bear in mind that this water molecule could be located on the surface of your nutrient reservoir or maybe the Pacific ocean, or on a bead of sweat emerging from your armpit (mmmm nice!) or it could simply be moisture passing through microscopic holes called stoma on your plants’ leaves. It’s all the same deal—just thermodynamics in action.

An 18 degree Fahrenheit rise in air temperature will double the amount of moisture that can potentially mix as a vapor with the air.

Okay—so that’s humidity covered, I hope—basically it’s just water vapor in the air. You got that already—so let’s move on to “relative humidity”. Expressed as a percentage, relative humidity tells you how “full” of moisture your air is forany given temperature. There’s a limit—basically when there’s no energy left for any further evaporation. If your air is completely saturated with moisture (think Fort Lauderdale in August) then your relative humidity is one hundred percent—that is, your air is one hundred percent full. If it’s super dry and has next to no water vapor at all (think Arizona desert) your relative humidity is going to be close to zero percent.  

Humidity and Plant Relationships 

So what has any of this got to do with the plants you’re growing in your basement? Hmmmm? Well firstly, plants sweat, or transpire, like humans do—primarily to keep themselves and the air around them cooler. The lower the relative humidity, the more “room” there is in the air for evaporated moisture molecules to leap up and join the vapor party. That’s why Florida feels so hot and sticky. Our shiny, tanned bodies are trying to cool themselves by sweating buckets but there’s precious little space left for additional moisture molecules to evaporate from our skin and relieve us of some heat at the same time. Remember—most plants don’t like being hot and sticky and more than we do.

What are the Ideal Relative Humidity Levels for Healthy Plant Development? 

Getting relative humidity right throughout your grow is like hitting a moving target. In general terms, when your plants are just starting out, either as a cutting or seedling, they enjoy high relative humidity—80 percent or more. That’s why we use propagators to seal in the moisture. We want to minimize the air’s moisture-sucking effect on your plants as the last thing you want to be doing at this stage is putting undue pressure on their small, undeveloped root systems.

Moving on—when your plants are growing, that is—building stems and leaves—they prefer relative humidity to be around 55 to 70 percent.

And finally, when they are flowering and fruiting—the ideal is around 40 to 55 percent. This helps them focus on producing buds, flowers and fruit.

These guidelines indicate that as plants develop, their need for humidity tends to decrease. However, the bigger and more numerous your plants indoors, the more moisture they transpire and the more they contribute to your grow room’s humidity! It’s a sort of conflict of needs, isn’t it? This is why we ventilate our grow rooms or use air conditioners and dehumidifers.

Humidity and Temperature

Of course, relative humidity is dependent on temperature—and that should ideally be somewhere in the 70s for most species. Remember that any changes you make to your grow room’s humidity should be as gradual as the changes taking place in your plants.

Okay—so how do we make these changes? Well, the first thing you’re gonna need is a hygrometer so you can monitor relative humidity in your grow room. They start at around 30 bucks. Get a digital model—in fact, get several, and record minimum and maximum readings for different parts of your grow room. These are particularly important because when your grow lights switch off for your plants’ night-cycle, temperatures in your indoor garden will naturally fall. This, in turn, reduces the amount of water vapor the air can party with. Remember that an 18 degrees Fahrenheit drop in air temperature will halve the amount of moisture the air can hold. Where does the other half go? Good question! It condenses on to anything surface it can find. Leaves, buds, flowers, fruits, walls, floor—you name it. This can massively increase the risk of molds and mildews that could decimate some sensitive crops in a matter of days.

Controling Humidity in your Grow Room

Growers control humidity in a number of ways. Techniques differ according to geographical location. Growers in areas with moderate humidity use ventilation. That is—extracting away warm, humid air and replacing it with cool, moderately humid air. Growers in Miami don’t have this option because the air outside is already really humid. They would tend to use an air-conditioner, carbon dioxide supplementation, and dehumidifiers.

Lowering Humidity Levels

The most common problem faced by indoor gardeners is a grow room that is too humid, full of flowering, heavy fruiting plants. The simplest fix is to invest in a dehumidifer. This Ideal Air 50 pint dehumidifer can be plumbed straight into a drainage point. You can even re-use the water once the unit has been running for three days. It removes up to 50 pints a day in standard conditions, but is actually capable of removing significantly more moisture from a hot, damp grow room. As a general rule, your dehumidifier should be rated to remove at least the same amount of water as your plants drink in a day when mature. Just be sure to plug it into a suitable controller such as the EOS1 or the Kronus or Saturn product series by Titan Controls. Avoid plugging dehumidifers into electrical extension leads.

Another way of countering spikes in relative humidity and condensation falling on your plants after your lights go off is to deploy a thermostatically controlled block heater on a timer to keep temperatures closer to lights-on levels. A small difference between your lights on and lights off temperatures will also help to encourage squatter plants that are easier to light efficiently. If you are growing in a ventilated indoor garden, remember it’s vitally important to keep your extraction fans on 24 / 7.

Raising Humidity Levels

Growers wanting to increase humidity should consider grow tents, propagators, humidity domes, trays full of water on the floor, or just plug in a humidifier if you have to.