Keeping Things Cool In Your Growroom

By Sara Elliott
Published: September 29, 2019 | Last updated: April 21, 2021 11:34:42
Key Takeaways

Beating the heat is a chore many indoor gardeners face, but it’s a problem that can be tackled in a number of ways, some of which are even budget-friendly! Let’s take a look at a few ways to shift the thermometer out of the red zone and into the optimal grow zone.

Plants are like children. They respond best to a consistent environment. What constitutes the best growroom environment can vary from plant variety to plant variety, but will often conform to a temperature range of between 70 and 80°F for aerial growth, and around 66°F for root growth.


When plants become overheated, they begin to show signs of stress. Heat stress can result in spindly young plants and mature plants that fail to flower or fruit properly. Excessive heat also makes plants more susceptible to disease.

A hydroponic system can provide the proper conditions necessary for destructive micro-organisms to thrive. Beat the heat with these 10 proven strategies to keep things cool in your growroom.


An oscillating fan inside a cannabis grow roomRedistribute heat by positioning a fan just above the canopy to blow hot air away from plants.

Invest in an Air Conditioner or Fan

Nothing manages heat better than an air conditioner. When you think about an efficient cooling strategy that can be monitored and controlled easily, an air conditioner with an on-board thermostat tops the list.

A window unit might not break the bank, but do your homework before you buy. Growroom size, heat-gain figures from lighting or other fixtures, and seasonal outdoor temperature limitations all have an impact on an air conditioner’s efficacy and operating costs.


If you can’t raise the heat source, or simply don’t want to, redistribute the heat by positioning a fan just above the canopy to blow hot air away from plants.

Read also: The Best Temperatures for an Indoor Grow Room


Use a Swamp Cooler

Swamp coolers, or evaporative coolers, use one of the basic principles of air conditioning to reduce the ambient temperature. When converting from a liquid to a gaseous state, substances like refrigerant and water absorb heat. In the case of a swamp cooler, a fan propels hot air through water-moistened media, evaporating the water.

This results in a temperature drop of up to 10 degrees. Misters perform a somewhat similar function by producing minute water droplets that evaporate almost immediately. But as there is a risk of swamp coolers and misters increasing humidity, they might not work well for your set-up.

Insulate or Relocate Your Growroom

Lighting might be the main culprit causing dangerous heat spikes, but it may not be the only offender. Poor insulation on a south-facing exterior wall, or a growroom located directly under a hot roof, can worsen heat problems too.

Read also: How to Use a Climate Battery to Heat and Cool Your Grow

Adding insulation and moving the growroom to the basement are both solutions that can help. Below ground level installations are typically cooler because the surrounding soil provides natural insulation.

Monitor and Control Humidity

Low relative humidity increases the impact of heat on plants. Plants use transpiration, exhaling water vapor through their leaves, to help stay cool in hot weather. If their efforts aren’t strong enough, they become stressed, and important functions like photosynthesis slow down or stop.

Increasing the relative humidity creates an environment where plants feel the heat less. You can mist your growroom a few times a day, or invest in a humidifier. A good rule of thumb is to keep the humidity above 40% for mature, flowering and fruiting plants, and a bit higher—above 60%—for young plants and greens.

Lettuce plants under an LED grow lightUnlike other types of grow lights, LEDs do not burn a filament, but instead pass light through semiconductors to create their spectrum.

Rethink Your Lighting

In an enclosed space, lights can get hot fast, but there are ways to minimize the impact of heat from grow lights and fixtures. Of course, among the most effective choices are to upgrade to lights that emit the least amount of heat, like LEDs, or use air- or water-cooled reflectors.

The only downside is that these options can be expensive, though, especially if you have a substantial investment in lighting already.

Read also: 10 Mistakes to Avoid When Using LED Grow Lights

Raise the Lights

Raise your lights by 8 to 10 in. as a temporary measure if you’re unable to switch your lighting system right away. This will reduce their effectiveness somewhat, but also quickly decrease the heat at canopy level and below.

Dimming your lights or turning some of them off is also an option.

Install a Light Mover

To help better manage light resources, install a light mover. Motorized light movers transport grow lights back and forth along straight or curved tracks, distributing illumination across a broader area and reducing hot spots and problems with canopy overgrowth.

Using a light mover can also reduce the need for additional lights and the heat they are sure to generate.

Run Lights at Night

Evening and overnight temperatures are cooler than daytime temps, so it makes sense to use hot grow lights when the ambient temperature is naturally lower.

Another advantage is that many electrical utilities charge lower rates during off-peak times, usually from 7:00 p.m. to 7:00 a.m. When you run your lights at night instead of during the day, you save energy and money. You also create a favorable environment for your indoor garden.

Photo of a man installing ducting in a grow room.For fully enclosed growing areas, warm air may have to be pumped out and fresh air brought in to achieve proper growing temperatures.

Improve Your Venting System

Install or increase your use of input and exhaust fans to vent hot air and replace it with cooler, fresher air. This process is called air exchange, and it’s a good way to help control high humidity, too.

Maximize the effectiveness of your fans by placing your input ductwork in the coolest spot you can find. Also, consider investing in an automated system that raises and lowers fan speed automatically in response to temperature fluctuations. It takes some of the guesswork out of heat management.

Read: How to Perfect your Grow Room's Ventilation System

Promote Good Air Circulation

Keep the air inside your growroom moving around and among plants to avoid hot spots, aid in photosynthesis and help produce specimens that have stronger stems and leaves.

In nature, plants aren’t motionless. A little movement is exercise for them. Use multiple fans to ensure good airflow or invest in an oscillating fan.

Adjust Nutrient Concentrations

Transpiration is a type of plant air conditioning. To perform this feat, plants increase their water uptake. This can upset the delicate balance in a hydroponic set-up as roots take up not just more water, but more nutrients, too.

To avoid problems with toxicity from common minerals like boron, copper, manganese, nitrogen and phosphorus, adjust the nutrient-to-water ratio in the reservoir. Refer to nutrient labeling for instructions on the product you’re using.

Some growers recommend reducing nutrient concentrations by about 20% in hot conditions. It may also be necessary to increase air circulation in the reservoir to raise the amount of dissolved oxygen.

Grower filling a scoop with silica Silica in the form of potassium silicate can help plants develop stronger cell walls, protecting them from heat fluctuations.

Make Use of Micronutrients

Adding or increasing the concentrations of micronutrients may help improve your plants’ natural defenses. For instance, silica in the form of potassium silicate can help plants develop stronger cell walls, protecting them from heat fluctuations. It’s useful to note potassium silicate may not be included in your nutrient mixture, but it can be used as a foliar spray.

Read: 8 Crop Micronutrients Growers Can't Ignore

Boost CO2 Levels

Another popular method of dealing with the heat is to increase the amount of carbon dioxide in the air. CO2 is an essential component of plant photosynthesis. When levels spike from an average of 400 ppm to between 1,200 and 1,500 ppm, plant photosynthesis increases, which helps plants perform more efficiently, maintaining normal function in hotter conditions.

Plants absorb CO2 better in bright light, like the illumination provided by high-pressure sodium (HPS), metal halide (MH) or the newer and more expensive LED lights. Small growroom operators can use dry ice or the fermentation of common yeasts to produce CO2. Other forms of CO2 can be brought into the indoor garden using bags, bottles, buckets, pads and so on.

Rethink Your Reservoir Placement

In nature, heat fluctuations are less pronounced in the soil than they are above ground. As a result, plants are more sensitive to overheated roots than they are to elevated temperatures on their stems and leaves. Beyond stressing plants, a nutrient solution that is too warm can become a breeding ground for molds, mildews and dangerous bacteria. If your nutrient reservoir is located inside your hot growroom, you can eliminate potential problems by moving it to a cooler location.

Consider a Reservoir Chiller

Finally, if moving your reservoir isn’t an option, you can retrofit your set-up with a water chiller. There are numerous hydroponic and aquarium chillers on the market at a number of price points. Most are easy to install.

Read next: Humidity 101: Basics for Your Indoor Garden


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Written by Sara Elliott | Gardener, Writer

Profile Picture of Sara Elliott
Sara Elliott is a professional writer with extensive horticultural knowledge acquired through theoretical study and practical experience. You can find her gardening and lifestyle pieces in print and online.

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