Maintaining an ideal growing environment in your grow space is probably the most challenging aspect of growing cannabis indoors. It’s also the most important aspect. It’s no wonder I get so many questions from growers asking for help lowering their grow tent temperature.
There are several practical steps you can take to lower your grow tent temperature. They include moving the tent to a cooler area of your house, switching from an HPS to LED grow light, or ensuring your grow tent fan is blowing air out the window instead of into the room where your tent is located.
For the record, an ideal temperature for growing cannabis is between 70 and 80 degrees Fahrenheit, or 21 to 27 degrees Celsius. Growers using CO2 can run slightly higher temperatures in their tent. Keeping steady temperatures in your grow tent is important to avoid plant stress. Frequent swings in temperature result in reduced yields.
I’ll start with the most practical methods to lowering grow tent temperature, and end with less practical:
Method 1: Relocate Your Grow Tent
Consider moving your grow tent to the coolest area of the house. For example, your basement, or maybe close to some air vents if you have central air conditioning. You should also consider placing it near open windows.
Being near open windows will allow your grow tent to take advantage of cool seasonal climates. It’ll also allow you to easily route your ducting from inside the tent straight out the window.
Method 2: Route Grow Tent Ducting Out The Window:
You don’t want to defeat the inline fan purpose by simply blowing the air from your grow tent into the room where your grow tent is located. Because the air in the room where your tent is located is the air that’s being sucked back into your grow tent to balance the negative air pressure created by your inline fan.
You should take your ducting, or buy additional ducting if necessary, and point it out a nearby window. This way the air that’s getting pulled back into your tent doesn’t mix with the hot tent air, and it becomes easier to maintain your grow tent temperatures.
Method 3: Turn on Your Air Conditioner:
Keep the room where your grow tent is located cool. The easiest way to do this is by opening a window during the cold season, or by purchasing a portable air conditioner to keep the room cool. If that doesn’t help, consider placing the portable air conditioner inside the grow tent.
The air in this room is what gets pulled back into your tent when the inline fan blows out hot air and creates negative air pressure in your grow tent. Cooler air gets pulled in through the screen windows on the bottom of your grow tent.
Remember that if your tent temperature is 85 fahrenheit and the temperature of external air being pulled into the tent is only 80 degrees fahrenheit, that’s not a large enough difference to cool the tent down. Replacement air needs to be low 70’s to get your 85 degree tent temperature into the mid-upper 70’s.
Did you know temperature and humidity are highly correlated? Read my post about maintaining proper humidity levels here.
Method 4: Add CO2 to Your Grow Tent
Adding this easy to use CO2 bag to your grow tent won’t actually lower the temperature, but it’s just as good. Under elevated CO2 levels, cannabis plants are able to withstand higher than normal temperatures.
How much extra heat can cannabis withstand under elevated CO2? A growing environment of 85 or 86 degrees fahrenheit would be just fine. That’s compared to a normal range of 70 to 80 fahrenheit without CO2.
So if you’re really struggling to reduce heat inside your grow tent, these affordable CO2 bags that last for the entire grow are probably a good choice for you. Click here to check them out on Amazon.
Method 5: Make an Ice Box (with ice or dry ice):
This method will require a bit of creativity, but definitely works if you’re up for the task. Using dry ice will require far less maintenance because you won’t need to replace the ice as often.
Using dry ice to lower grow tent temperature also has a dual benefit of producing CO2 which allows plants to thrive at higher temperature than normal (85 to 90 Fahrenheit)!
What you’ll need:
How to assemble:
This can be as simple as placing your ice into a container or chest and focusing an oscillating fan on it. If you want to get more creative, you may choose to seal the chest, cut a hole in each side, place a fan in one hole and allow air to escape through the other.
Note on Dry Ice/CO2:
CO2 is present at ~400 ppm in fresh air. Most gardeners find 1,200 to 2,000 ppm of CO2 to be an ideal level for significantly increased plant yields.
Keep in mind CO2 is dangerous for humans, and levels of 1,000-2,000 ppm while usually safe, can cause complaints of drowsiness. Levels above 2,000 ppm can become toxic. Do not allow levels to increase beyond that point.
CO2 is heavier than air, so you should have an oscillating fan running to circulate it around the tent. Plants only utilize CO2 during photosynthesis, so when your lights are off you can remove the ice box from your grow tent.
Using Regular Ice:
If you choose not to use dry ice, you may want to make salt-ice. You can do this by freezing salt water instead of regular water. The salt helps the ice stay frozen longer.
Method 6: Make Sure Your Inline Fan is Strong Enough:
The inline fan in your grow tent is a great first step to removing heat from the grow tent. You can read my article about choosing the right size fan for your grow tent by clicking here.
The rule of thumb is that your inline fan should be replacing the air in your grow tent once per minute. You also need to take into consideration reduced fan power due to bends in the ducting and using a carbon filter.
If your inline fan isn’t strong enough consider running it on a higher speed, or buying a more powerful one.
Method 7: Switch from HPS / HID Grow Lights to LED Grow Lights:
HPS and CMH grow lights run very hot. Consider switching to LED grow lights which run cooler and make your grow tent temperature easier to maintain.
I’ve spent countless hours researching and maintaining a current list of the best LED grow lights available, so check it out if you’re interested in switching.
Method 8: Place Your Grow Light Drivers Outside Your Grow Tent:
If you make the switch from HPS to LED grow lights, some LED grow lights like the Mars Hydro TS-3000 (4×4 ft) and TSL-2000 (2x4ft) models have removable drivers. This is a huge advantage to growers who have trouble lowering grow tent temperatures.
Method 9: Relocate Hydroponic Equipment Outside Your Grow Tent:
If you’re growing hydroponically, you may be using pumps, filtration, reservoir chillers, or other electronic equipment that can be relocated with the use of an extension cord or extended pipes. Consider moving as much of it as possible outside to help reduce grow tent temperature.
As you can see, there’s not one perfect solution for lowering grow tent temperature. One method may be easier or more effective for you than another. Lowering the temperature inside your grow tent is just as much of an art as it is science.
If the easier methods don’t work for you, my personal preference would be the dry ice method because the CO2 byproduct of dry ice actually allows plants to thrive at higher temperatures while the ice itself will act to cool the grow tent.
While it may not be the easiest method to execute, I think it’s probably one of the most effective for getting your grow tent environment to an acceptable temperature.