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5 Secrets To Controlling Heat Indoors-Part II

Heat control in grow room is an important aspect to take care of your cannabis plants. Like we discussed in the last blog, we can apply the supplements which can protect against heat damage. Would like to find out more? Keep reading:

2.) Environment Hacks

Add CO2 to the Air

Plants can easily survive in temps up to 95°F (35°C) when the air is being supplemented with extra CO2. However, for CO2 to make a difference to your plants, you need relatively strong grow lights (i.e. big LEDs). But for those with strong grow lights, adding extra CO2 can result in faster growth as well as making it so plants can better handle the heat.

Choose Soil or Coco (Hand-Watered Grow) Over DWC/Hydro

Growing cannabis hydroponically can give you some of the fastest growth rates of any style of growing, but hydro does not really play well with hot temperatures because too much heat tends to trigger root problems and slower growth.

We talked about supplements that help prevent root rot for hydroponic plants, but the next step is to cut out the water reservoir altogether and go for a hand-watered grow. That’s because hand-watered plants are inherently much more resistant to high temperatures. Not only are the roots relatively safe from the heat (unlike in hydro) but the growing medium can hold a lot of oxygen no matter what the temperature (a lack of oxygen at the roots causes major problems in hydro). Plants grown in containers or in the ground tend to be more resistant to heat compared to hydro.

Soil in regular containers is probably the most common way to grow cannabis indoors, and that combo works great.

But when it comes to growing in hot temps, coco coir may be a better choice than soil. In particular, coco coir inherently has properties and hormones that encourage healthy root growth and promote hardiness against heat.

Soil is a good choice, but coco coir is even better when it comes to heat.

Keep Roots Cool

Your plant is much more resistant to heat if the roots don’t cook too, whether you’re growing in a pot or in a hydroponic reservoir. If you can find a way to help keep the roots around 70°F (21°C), your plant will suffer less and recover more quickly from a hot spell.

Watering your plant regularly when it’s hot will keep roots from drying out and help them stay cool. If the sides of the pots are getting hot from the grow light, put your potted plant inside a bigger pot (or find some other way to block the pot from direct light. This can help keep roots significantly cooler.

If the sides of pots are getting hot from the light, put the whole plant in a bigger pot to create an air gap and keep the roots out of direct light.

Use Fan Placement to Reduce Electricity & Bring Down Temps

Air conditioners can easily control the heat in any room but they use a lot of electricity, which can get expensive really fast! Unfortunately, a lot of growers are using an AC (or just dealing with too much heat) when they could actually reconfigure their fans to be able to maintain the right temps without needing an AC at all.

Proper fan placement can also help save costs because if you use fans more effectively you can use less of them. Fans use a surprising amount of energy that can really add up over time.

Being smart about fan placement and investing in electrically efficient fans can save you a lot of money. 

Exhaust Fans

The most effective way to control heat is to use an exhaust fan to vent hot air out of the grow space. Make sure your exhaust fan is set up to make sure you have the most cooling power. Here are some tips

  • Your grow space should be mostly sealed for an effective exhaust system. We highly recommend getting a grow tent for smaller grows to make this super cheap and easy. A grow tent with a strong exhaust fan creates a breezy plant environment that probably doesn’t even need any other fans. As an added bonus, a grow tent makes it easy to control smells with a carbon filter so no one can smell your plants even in the same room.
  • Your exhaust fan should be strong enough for the size of your space or tent. Avoid cheap ducting fans as they don’t move enough air to cool your grow light. 
  • Heat rises to the top of your grow tent/area, so make sure your exhaust fan is located at the top of the tent pointing out.
  • You want holes to intake air, about 4x the size of your exhaust hole if possible. When the intake hole is too small it’s hard on your fan, the exhaust won’t be able to vent as much hot air. With grow tents, the intake ports are already built-in so you just leave them open so air can get through.
  • The intake hole should have direct access to cool, fresh air. The inside of the grow space is always going to be at least a few degrees above the temperature of the intake air, so if the intake air isn’t cool the grow space will be even hotter. Cool air stays lower to the ground, so make sure your intake ports are as close to the floor as possible.


Keep all ducting as straight and short as possible. You want a short direct line to the window so that hot air is immediately vented outside. Try to avoid using anything in the window that restricts airflow. Though it’s often necessary to use some type of screen to make sure no one can see in, try to do the best you can to obstruct airflow as little as possible. 

Window Fan

It may seem simple, but a window fan can be surprisingly effective at venting some of the heat if the grow light is raising the overall temperature of your room. In some cases, a window fan can cool your room by several degrees, which can help keep your plants cool. If grow lights are raising the temperature of your room, a  window fan can help vent heat and cool the room down by a few degrees.

In order for a window fan to cool a room most effectively, there needs to be some sort of “intake” for cool air to get in. It’s like a bigger version of an exhaust system. A separate open window in the same room is perfect, though an open door can also help, especially if there’s an open window somewhere in the house. However, even in a completely closed room, a window fan can help keep temps down somewhat in the room because it helps vent out heat from the grow light. Although blowing air out is often the most effective, if the air outside is really cool sometimes it helps to blow air in. Experimentation is good.

Oscillating Fans

Oscillating fans are perfect for reducing “hot spots” that form in the grow space. Particularly “wing” type reflectors can cause major hot spots because the heat from the light is beaming directly down onto the plant. If it’s really hot directly under the light, using fans to blow that heat away from your plants can make a huge difference in how much heat they experience.

In addition to oscillating fans, using a strong-enough exhaust fan with a big intake hole helps keep everything moving so hot spots don’t form. In fact, a grow tent with a good exhaust fan often doesn’t need extra fans because the exhaust fan itself will make the entire tent breezy.

Just be aware that adding oscillating fans doesn’t bring down the overall temperature in the tent, they just move air around. So if the room is already hot, adding more fans isn’t going to help.

To be continued…

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