It’s a lot of fun growing your own cannabis indoors. One thing a lot of new growers have a hard time with is keeping the temperatures within the grow tent (or room) under control. Cannabis thrives in a particular environmental range. Having control of this environment will help you optimize your indoor grow and maximize yields.
Cold temperatures will slow down growth while higher temperatures can stress your plants and cause all sorts of problems.
So let’s discuss how to cool down the grow tent temperature and grow great weed!
Is excessive heat bad for cannabis plants?
It most certainly is. Plants will sweat to help cool themselves down, just like you. When the temperature begins to soar your plants will be unable to cool themselves down fast enough and begin to dry out.
The temperature range you want to keep your plants in is between 20°C and 30°C (70°F and 85°F). Going a little below or a little above won’t cause any major problems but in this range, your plants will grow their best.
When the grow lights are on, the air temperature will slowly rise. If this air has nowhere to go then it can get quite warm inside your weed grow tent.
Signs your weed plants need to cool down
Wilted Leaves
The water in the leaves will evaporate if the plant starts getting too hot. Water in plant cell help give the leaves rigidity and a lack of water will cause the leaves to droop. Drooping leaves can also be caused by a lack of watering which triggers the same symptoms.
Curled up crispy leaf tips
If the leaves continue to dry out they can become dry and brittle. When this happens the leaf tips and edges will begin to curl up. This is usually a sign that the temperature has spiked recently.
Fox Tails
As cute as they sound, this isn’t a really good phenomenon to affect your weed plants. Fox tailing refers to new buds growing from the top of existing mature buds. More buds might sound like a great problem to have but in reality, this isn’t ideal. Your plant is getting stressed out and trying to increase its chances to get pollinated. You want your plant to focus its energy on the existing buds and not producing more flowers. Since these new buds are immature they can lower the average potency of your harvest.
Fixing high temperatures and cooling down a weed grow tent
You need to find a way to lower the temperature inside the grow tent. The easiest way to do this is by using an inline fan. This inline fan works like an air conditioner for your grow tent. What it does is exhaust hot air from inside the grow tent and replaces it with cooler air from outside the grow tent.
If you grow indoors, you can get away with using just one of these inline fans to exhaust hot air and aid with cooling your plants. Using ducting attached to the fan you direct the outake to a different room or outside window. As the fan runs it creates negative pressure which draws fresh air into the grow tent from an open vent or ducting.
If the ambient air around your grow tent is also quite warm then you may run into problems. In this case you will need to route an air duct from the grow tent to a cooler air source.
The biggest mistake you can do is have your air intake and outtake in the same room. This is just no good since you will end up recycling the same air you’re trying to remove.
Having oscillating fans inside grow tents can be very helpful for two reasons. Firstly they help eliminate any “hot spots” inside the grow tent by circulating air. Secondly, having a breeze on your plants can make them stronger. Plants that are gently dancing in the breeze will develop stronger stems and branches from the constant movement of air.
What causes a high grow room temperature
Cannabis Grow Lights
The biggest culprit to increase temperature inside your weed grow tent are your grow lights.
There are two common grow lights used for growing cannabis indoors; HIDs and LEDs
HID (High Intensity Discharge) are the most common grow lights used for cannabis lighting cultivation because they have been around the longest. HID grow lights require a lot of energy but they give off a lot of high quality light. They also have a tendency to get quite warm while they are running. The bulbs themselves will get quite hot but the ballast that powers them will also get hot. The nice thing about ballast though is that they can be wired outside the grow tent and not add to the heat from the grow tent lights This excess heat will build up while the grow lights are on.
Led Grow Lights
The other type of light used by growers inside their weed grow tents are LEDs. LED grow lights have become much more common and popular in cannabis grow spaces. Because of their lower power requirements and cooler running temperature they can save on electricity costs.
One thing to keep in mind while looking for LED grow lights for your weed grow tent is that cheap usually means bad quality. Don’t go with the cheapest LED grow lights because they will be prone to failure and might not actually work as well as advertised.
When doing your LED grow light research you will find two common types. “Blurples” and full spectrum. Blurple is a slang term for the LED grow lights that give off a blue and purple color hue because they have blue and red (and sometimes UV) LED bulbs.
Full spectrum lights give off white light because in addition to blue and red light they also have green, yellow, and UV. These full spectrum lights usually do a better job at growing cannabis, but are a little more expensive.
To find out more about how the color spectrum affects cannabis growth you can read more here.
Ambient Air Temperature Outside Weed Grow Tent
If it wasn’t obvious, the room you place your indoor weed growing tent will have an impact on the inside temperature.
The temperature inside your grow tent when the lights are off will be the same as the space the grow tent is placed in.
If you’re growing auto flowering cannabis you can keep your lights on 24 hours a day. This will keep the air inside the grow tent slightly warmer than the ambient air outside. if your space bucket or grow tent needs to be placed in a cold room it could work. Grow temperatures under 18C 64F might result in slower growth. Lights on all day will result in higher energy costs and shorten the “light span” of the light.
Another factor to consider is where your tent is getting fresh air from. If you’re using an inline fan and the output is right outside the tent, you are slowly raising the temperature of the ambient air just outside the grow tent. This might not be a problem in certain situations but if the grow tent is already in a warm room it can make it a few degrees warmer while the lights are on.
How to Lower Temperature Inside a Weed Grow Tent
Increase Ventilation
An inline fan is the greatest weapon against rising temperature inside a weed grow tent. Inline fans are designed to cycle the entire volume of air inside once every minute, cooling down the grow tent. With cool fresh air coming in and warm air going out, your cannabis will be having a great time.
Inline fans have a rating of CFM which stands for Cubic Feet per Minute. When reading the spec on an inline fan for a grow tent these efficiency ratings should be shown. Keep in mind many manufacturers will embellish their numbers to a best case scenario value.
The size and power fan you use will depend on the total volume of your tent. Measure the grow tent’s width, length, and height in feet. Calculate the volume of your tent with this formula.
Width x Length x Height = Total Volume
When you have the answer to that equation you can compare it to the CFM rating of an inline fan. You will want to choose a fan that is rated a bit higher than your room’s volume.
First, you don’t want your fan running at full power constantly, this will wear out the motor. If you decide to add a carbon filter to your ventilation system it will lower the efficiency of your fan, making it work harder to cycle the air.
The next few suggestions won’t have the biggest impact compared to a good ventilation system but they might come in handy for some particular situations.
Lower Grow Light Power
More light usually means bigger buds, but it also means more excess heat. If you’re growing in a small space using a 1000+ watt light source might be overkill. Adding very powerful lights, especially HID type lights, will raise the grow room temperature quite a bit. A cannabis plant needs a minimum of about 300 watts of light power to survive. Doubling that power will grow a bigger plant but give off more heat. If you’re running into the problem of your lights heating up the grow space two much, consider replacing them with lower powered lights or LEDs.
Change Grow Light Schedule
If you’re growing autoflowers, you might be tempted to keep the grow lights on 24 hours a day. Doing this is absolutely fine if your ventilation is keeping the temperature under control.
The problem is that in some scenarios it may be increasing the temperature of your tent to a dangerous level. If this is your case then you might want to adjust your light schedule and give the cannabis a few hours of “lights off” and time for the tent to cool.
Declutter Gear Inside Your Tent
Everything you put into your grow tent takes up spaces and reduces the volume of air inside the tent. With less room in your tent the temperature will rise faster. Any piece of equipment that runs on electricity might also give off heat and add to an already warm tent. Inline fans and ballasts are the biggest sources of additional heat inside a tent besides the grow lights.
Removing hot equipment from your grow tent won’t have the biggest impact but it will help keep the temperature lower and make it easier to work inside your grow space.
Cool Your Grow Down Gradually
Wide temperature swings can throw your plant off and cause further shock.
Now You Have a Cool Grow Tent
This guide should have given you all the information you need for cooling down grow tents. Using an inline fan is the best piece of equipment for cooling down grow tent temperature. Once you master ventilation, grow tents can be held at different temperatures. This way you could have a veg grow and a flower grow going on at once. Each tent cooled or heated by adjusting your inline fan.
Reefertilizer sells complete indoor cannabis growing kits. We have paired up affordable professional grade growing equipment so you don’t have to. Each kit includes our grow nutrient which we think are the best on the market. But why believe us? Check out our customer reviews
Mike Drouin is the co-founder of Reefertilizer. He’s an experienced craft cannabis grower and a writer of many articles regarding the process. Mike lives on Vancouver Island and enjoys cycling and camping and will sometimes combine the two.