Mastering the Vicious Cycle of Humidity for Indoor Marijuana

Understanding vapor pressure deficit will improve how you grow

Humidity keeps growers awake at night—and with good reason. A too-high humidity level in an indoor marijuana grow can breed mold quickly, especially during the flowering stage, and that can mean an entire crop heads off to the incinerator. What’s more, messed up humidity and temperature conditions can reduce yield even if mold isn’t a problem.

But it doesn’t have to be that way. With an understanding of the vicious cycle of irrigation, transpiration and condensation, you can head mold off at the pass–and maximize your yield. This post explores the problem of excess humidity and the technologies and techniques that can offer a solution.

How Humidity Becomes A Problem

As we all know, plants need water.  Water is crucial for nutrient uptake, metabolic processes and cooling the plants’ leaves. But not much of that water actually turns into plant matter. If you think about it, there’s very little water comprising plants; they’re just cycling moisture into the surrounding air and taking up the nutrients from the irrigation solution or soil.

The process of plants “sweating out” moisture is called transpiration, and it’s how humidity builds in the grow room. Plants transpire to cool themselves, so the more heat is present, the more plants will steam up the air. And because the plants are releasing moisture, they need more water—which increases humidity. It’s a nasty, self-perpetuating cycle, and humidity gets out of control pretty quickly.

Why Humidity is A Problem, Relatively Speaking

In the closed environment of an indoor grow (or greenhouse) humidity is ever-changing and tricky to control. The trickiest part is the relativity of humidity. So, what does relative humidity (RH) mean?

‘Relative’ refers to the difference in how much water air can carry at different temperatures. At high temperatures, air can carry a lot of water; at low temperatures, not so much.

RH refers to the water-carrying capacity of the air at a given temperature. At 50°F, 1,000 cubic feet of air can carry just less than 9 oz of water. So, if it’s carrying 4.5 oz., it’s at 50% RH. But if the temperature is 70°F, it can carry almost twice as much. That same 4.5 oz would be just 25% RH at 70°F.

What’s the rub? A 90-degree day feels worse in muggy Georgia than it does in arid Phoenix because sweating doesn’t work as well in Georgia. Sweating is our evaporative cooling system, and it doesn’t work when the air around us is already full of moisture.

Plants, just like humans, can’t easily cool themselves when it’s humid. Transpiration isn’t as effective because the water won’t evaporate as easily. And with nowhere to release the water, the plants stop taking it in from the roots. That not only means they’ve stopped “drinking,” but that they’ve stopped “eating” too. With no nutrient intake, growth slows.

Then, the more dangerous side of the humidity problem comes into play. Mold spores, which are present on all surfaces everywhere around us, find the conditions they need to propagate. With a little extra moisture and warmth, grey mold and powdery mildew take hold and, once they do, there’s not much to be done. Bud rot ensues. Remediation techniques exist, but in all likelihood, a mold outbreak means crop loss.

When Humidity Bites

When the temperature drops at “night,” the relative humidity changes. The amount of water in the air remains the same when the lights go out, but the RH goes up. Relative to warmer air, the now-cooler air is beginning to max out its capacity to carry water. And because the air can’t so easily hold it—and because the surfaces of the plants are cooling too—condensation begins to build. Just like a pint of beer on a hot day the plants become covered in water droplets and dew. This is especially bad in the large-surface-area of the buds with all their nooks and crannies. And where there’s moisture, mold isn’t far behind.

Condensation and mold are detrimental, but on a subtler level, humidity can simply slow growth. The pressure exerted on the leaves by too-thick air causes the stomata of the plant to close, limiting the CO2 available to the plant. It also slows water and nutrient intake. Even if humidity isn’t causing a crisis, it can still be affecting your results.

What Humidity Levels Should You Maintain?

The ideal temperature and humidity depends on many factors, like the strain you’re growing, the phase of the growing cycle and if you’re getting fancy with growing room pressurization.

Ideal relative humidity levels can range from 40% for the flowering period up to 95% for clones. The ideal RH changes throughout the growing cycle, with lower levels being a better bet during the critical, mold-prone flowering phase. As a rule of thumb, humidity levels should be lowered every week throughout the plant’s lifecycle.

Clones 80-95%
Vegetative 60-70%
Flowering 40-60%

Sativa and sativa-dominant strains prefer higher humidity because they hail from coastal regions. Indicas, coming to us from inland regions, prefer the grow room a little dryer.

The pressure level in the grow room can change how humidity affects plants too. Air intake that’s more forceful than the exhaust will cause positive pressure; exhaust that’s more forceful than the intake will cause negative pressure. Just like the pressure exerted on the leaves by humidity, pressurization changes how the stomata of the leaves open or close. Open stomata allow greater air exchange, so many growers are manipulating the pressure of their grow rooms—along with CO2 levels—to maximize growth.

How to Control Grow Room Humidity

Irrigate Carefully

If excess humidity is a problem, refining irrigation practices is likely the first step in a solution. In an ideal world all of the nutrient solution would go into the root system with no runoff or excess. Regardless of your growing medium or technique, introducing less moisture into the grow room is the first step in reducing the moisture in the air.

Measuring Humidity

The first step in controlling humidity is measuring it. A hygrometer measures relative humidity fairly accurately, and a more expensive psychrometer gives very accurate information. Knowing your humidity levels is the only way to get a handle on them, so a psychrometer is the best choice for serious growers.

A humidistat is just like it sounds: a thermostat that controls humidity. When humidity rises, a humidistat kicks on the ventilation. Often, ventilation and the heat from lights are all that’s needed to keep humidity down.

Assessing Dehumidification Needs

The amount of dehumidification scales with the size of the grow room and the outside humidity levels. A large volume growing space naturally needs more dehumidification than a smaller one. Humid, coastal locales need more dehumidification too.

You can make sound equipment choices after your grow room’s maximum dehumidification needs are nailed down. Once the problem period of excess humidity is identified, you can calculate how much water you need to get rid of and ballpark a dehumidification system based on that. Dehumids are rated in liters per hour or gallons per hour of moisture removal, though their efficiency scales with humidity (i.e., it’s easier to collect water from humid air than super dry air).

Heating and Cooling

Larger operations nix those nighttime humidity problems with open ventilation and floor level heating. Because natural gas it so cheap, this can be a practical solution, though it’s a wasteful one. Heating in the canopy drops the RH and ventilation blows out the extra moisture.

During the day, air conditioners can serve as dehumidifiers. An air conditioner does this naturally because it cools the air by blowing it over a radiator-type cooling element. The moisture in the air condenses onto that cold surface and is collected in a reservoir. If it’s too hot, an AC offers the two-fold benefit of cooling and dehumidification.


A dehumidifier is a lot like an air conditioner. It passes air over a cooling element, condensing the moisture into a reservoir. But then, it passes the same air over a heating element to warm it up again. In an air conditioner, the heat moves to an outside radiator, like the metal fins of the type that mounts in a window. The “dehumid” unit retains the heat in the room while the air conditioner gets rid of it.

Dehumidifiers can be portable, floor units on wheels or large wall-mounted affairs the size of a kitchen refrigerator. They offer some odor mitigation because they remove airborne moisture that carries terpenes and flavonoid molecules (just like air conditioners). But unlike air conditioners, they won’t increase heating needs during the dark period. Some dehumids feature filtration as well, further countering the dangers of mold spores.

Desiccant dehumidifiers are a different animal altogether. They use a chemical reaction to collect moisture. The simplest example of a desiccant is the packet of silica beads that comes in a box of new shoes. The beads collect unwanted moisture. The industrial version of a desiccant dehumidifier passes the air over a similar material to collect water. But this special desiccant material, when heated later, lets the moisture falls exhaust away.

Controlling humidity is tough because it changes in relation to temperature. Wily RH values can be crazy-making, but with the right equipment and careful assessment, you can optimize growing conditions and maximize your yields.


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