Growing cannabis at home is a rewarding experience. One issue new growers often face is keeping the temperatures inside their grow tent in check. Cannabis needs to be grown in a specific range of temperatures to maximize yields. Too cold and the plant’s growth will slow or stop completely, too warm and the plant will be stressed and the quality will be adversely affected.

Keeping your tent cool when the temperature begins to rise is crucial to your marijuana plant’s overall health.

Let’s talk about why heat is bad, signs your plants are getting too warm, and how to control your grow tent temperature to grow better weed.

Why heat is bad for your cannabis grow

The ideal temperature for growing cannabis is between 70°F and 85°F (20°C and 30°C). This is why growing indoors inside a grow tent is so popular; the weather is no longer a factor. How to keep your grow tent in this range is the part that requires a little bit of thought.

The external room temperature and the amount of heat your lights give off are the two most important factors to what the temperatures will be like inside the tent.

If the temperature falls outside this range by just a few degrees, it’s not going to be a big issue. When the grow room temperature stays below or above this range for longer periods of time, you will run into problems.

When cannabis plants get too warm they will begin to get stressed out and growth will slow down or stop completely. Plants need to breathe and perspire to keep their biological processes going and feeding on nutrients. Keeping temperatures lower during flower is also key to nice smelling flowers and buds. Terpenes are oils in your plant that give it its signature flavor and smell. These compounds will evaporate if temperatures get too high which will diminish the quality of your harvested buds.

So what are the symptoms of heat stress to look out for?

A cannabis plant that is stressed and drying out.

Symptoms of Too High of a Temperature in Your Grow Tent

Curled up crispy leaves

Just like humans, plants will sweat to cool down. But, when your plants begin to run out of moisture, the leaves begin to dry out and could potentially die off. If you see your leaves curling up around the tip and edges you should begin looking for problems.
If the temperature stays too high, the leaves will continue to dry out, lose color, and then die.

Foxtailing cannabis flowers

Foxtailing is when the plant starts new flower growth as a reaction to heat stress. These new flowers will grow ontop of the more mature buds which isn’t ideal. You might think “more flower yay!” but these new buds will be less potent than the ones that were already growing. Your final harvest will be a mix of mature and immature buds which might not be what you want for your final product.

Fixing Heat Stressed Cannabis

There’s not much you can do except finding a way of cooling your plants. First, we need to go over the anatomy of a grow tent to find out the easiest way to fix the problem and prevent it from happening in the future. Once your plants have cooled down they should begin to grow normally again.

Anatomy of an Indoor Cannabis Grow Tent

Grow Tent or Grow Room

Some growers with enough space might be using a dedicated room. But for many small growers, grow tents are the best way to grow weed indoors. Grow tents are meant to maintain the ideal environment for growing cannabis. A quality grow tent will keep in humidity, heat, and light; preventing damage to your home or rental.

Types of Grow Lights

The grow light you choose will have the biggest impact on the temperature inside your tent. There are two main types of grow lights; HIDs and LEDs.

Choosing HID Lights for Your Grow Tent

(High Intensity Discharge) HIDs are the most popular lights for cannabis cultivation. One of their downfalls is that they produce a lot of heat while they’re on. They will warm up your grow room, but things will quickly cool once they turn off for the night.

Choosing LED Grow Lights for Your Grow Tent

(Light Emitting Diodes) LEDs are a newer and usually cheaper alternative to HIDs. LED grow lights also produce much less heat than other lights.

Both types of lights will increase the temperature of your grow space. Having an exhaust fan will be necessary to prevent the room from getting too hot.

An exhaust fan in a small grow environment.

Cooling with a Mini Inline Fan

Also called exhaust fans or ventilation fans, will help circulate the air inside your grow space. It’s like an air conditioner for your grow tent.

For smaller grows, one exhaust fan might be all that is needed. In a larger grow you will need to be able to bring in plenty of fresh air and having an intake fan will be necessary. The goal of these products is to replace the entire volume of air in your grow space once every minute. This helps replenish CO2 which plants need to breathe. These fans also help keep your grow tent cool. Ideally, these fans will run at a maximum of 75-90% power while the lights are on and then adjusted to a lower power setting while the lights are off.

If you plan on growing indoors you may be worried about the smell of several flowering cannabis plants. In this case, you will need to attach a carbon filter to your exhaust fan to eliminate most if not all smells outside the grow room. There are many kits available online that include the fan, carbon filter, and ducting.

Operating a grow tent without one is possible but you will have very little control of the cooling of the grow tent.

Equipment for cooling air inside a tent

The number one piece of equipment you will need to cool down a hot tent is an inline fan. These fans have two jobs. Firstly they help circulate air by bringing in fresh air filled with CO2 and expelling oxygen. Secondly, they help control the temperature inside the grow space by bringing in cool fresh air and expelling warm air.

Having simple oscillating fans within your grow room can also help keep your plants cool. Keeping an oscillating fan inside the tent will help prevent any “hot spots” within the grow space. They also help keep your plants hardy. A constant breeze blowing on your plants is good for your plant’s health. As it dances in the breeze, the stalk becomes stronger. You will want to avoid having a fan blowing directly and constantly on a plant since it could affect how it grows especially if it’s still small.

Keeping the fan moving or aiming indirectly at the plants is the way to go. 

Keeping a Cool Grow Tent

Your inline fan will come with a CFM rating. This will let you know how many cubic feet of air can be moved in one minute. CFM stands for Cubic Feet per Minute, get it?

The first thing you need to do is calculate the volume of air inside your tent. This is done with the formula Length x Width x Height.

Here’s How:

For practical example let’s take a fairly compact 3′ x 3′ x 6′ grow tent.

3 x 3 x 6 = 54 cubic feet

For effective ventilation, your best choice will be a fan that has a higher CFM rating than the volume of your tent or room. The reason for this is that you don’t want your fan running at full power all the time, this can lead to your ventilation fan getting hot and wearing down prematurely. Another reason for using a higher rate fan is making up for a loss of efficiency from a carbon filter. A carbon filter is typically attached to the intake of your ventilation unit on the inside of the grow tent to help lower and eliminate smells escaping your grow tent. The fan needs to work a little harder to pull air through this filter.

The two most common sizes for fans designed for cooling down a grow tent are 4″ and 6″. The measurement refers to the size of the duct on the ventilation fan itself. A bigger size means it can move more air than its smaller counterpart. 

Most 4″ ventilation fans have a CFM rating of around 190. So in the example above, that grow tent would need a 4″ fan running at 30%-40% power to keep air moving. Many fans today will come with some sort of speed control so you can adjust the power used. When you notice your tent getting a little warm after the lights go on, you can increase power to the fan so that it can exhaust warm air even faster.

Choosing the right spot for your indoor grow tent

If you’re using a grow tent inside your home, where you place it will make a big difference to the temperature inside your tent. An uninsulated garage might sound like a good spot to keep it out of the way but you will want to make sure temperatures inside the tent don’t go below 60°F or your plants might stop growing.

Alternatively, an attic might get warmer than the rest of your home. While the grow lights are on, the temperature will usually get higher than the ambient room temperature outside the tent.

Tips for Keeping your grow cool

Be sure to have a thermometer inside your grow tent. This will let you record the inside temperature and make adjustments as you go. Keep your grow cool!

Keep your tent cool with these tips.

Think about potential automation. If you have the budget, there are products out there that will measure temperature and humidity and adjust the setting on your hardware to automatically keep your grow tents running at the proper levels.

Keep an eye on the humidity inside your grow tent. Hot air can hold more water than cool air. As the temperature gets higher the relative humidity decreases. When the temperature cools down humidity goes up. When the grow lights turn off in your grow tent the humidity will rise. Relative humidity above 60% can lead to mold growth.

How to Maintain Your Grow Room Temperature

Now you should have a good idea of how to regulate the temperature of your grow space. This can be a challenge to even the most experienced growers so don’t feel too intimidated. Every grow space will be a little different depending on your particular style of growing whether it be hydro or in soil.


Are You Looking to Grow Good Weed?

Before you go, check out this video for the best all-in-one solution for beginners:

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