Chapter 5

Know Your Temperature + Humidity Goals

New home growers are often surprised that temperature and humidity are important factors for an indoor home grow. Managing temperature and humidity in your grow tent is actually is extremely important, and it can be a challenge. To get readings of both, make sure you have a hygrometer. Get more than one hygrometer because it’s nice to know what the temperature and humidity are inside the grow tent, and in the room your grow tent is in.

Below you’ll learn the acceptable temperature and humidity target ranges, along with tips on how to be successful reaching them. Usually the main challenges are lowering temperature and lowering or raising humidity.

VEG, FLOWER and HARVEST temperature and humidity ranges for indoor cannabis cultivation:

● VEG humidity: 50 – 60% RH is your target range (this humidity range helps prevent powdery mildew)
● VEG temperature: 70 – 80 F is your target range; but do not go above 85 F ideally

● FLOWER humidity: 50 – 60% RH is your target range (this humidity range helps prevent powdery mildew)
● FLOWER temperature: 70 – 80 F is your target range; but do not go above 80 F ideally

● HARVEST humidity: 50 – 60% RH is your target range (if it is 70% + RH, then you might have to dry your cannabis for 3 – 4 + weeks.)
● HARVEST temperature: 60 – 70 F is your target range; but do not go above 75 F ideally (80 F isn’t going to RUIN your harvest, but 85 F – 90+ F will)

Now that you know your grow tent temperature and humidity targets, it is important you don’t stress out if you don’t reach them every day all the time. It’s hard to manage them if you don’t have air conditioning or a dehumidifier and humidifier. 

It can be hard to manage temperature and humidity, but it is worth your best effort. If the targets are not being met, then do your best to adjust, and grow on. When you don’t meet the ideal guidelines, just keep going. You’re doing your best and there’s no better way to learn about cannabis cultivation than by experience after all. Watch, observe, and keep a grow journal.

In a perfect world, you’ll achieve 55% humidity and stay below 80 degrees Fahrenheit, which is relatively easy to do when you have central air conditioning or an AC or large dehumidifier in the room your grow tent is in. It is also easier to do in cooler months, vs. the hot summer months. For example, in the summer if it’s 80+ F outside, and inside your home it’s 77 F, then your grow tent is going to heat up to 80+ F extremely fast with your grow lights on. You can turn the lights to the weakest setting and open the tent doors to combat the heat if you don’t have AC as an option. You can also add fans to exhaust the hot air out of the room your grow tent is in. Lastly, you may want to run your grow lights at night when it’s cooler, instead of during the heat of the day.

Keeping your grow tent temperature under 80 degrees F when it’s hot outside is challenging to those without air conditioning.

TIP: Air conditioners and dehumidifiers are the golden keys to growing indoors 365 days a year. You don’t need them, but they make a world of a difference.

What happens if it is too hot in your grow tent?

When it is too hot for too long, there can be different signs of heat stress on your plant. Leaf edges might fold and “taco.” Plants also will wilt when it’s too hot. Too much heat in FLOWER phase can cause the aroma to evaporate and it degrades the potency. Will cannabis plants fall over and die at 85 F degrees? No, but it can stress them out and they might not grow as strong or yield as much if they are exposed to 85 F + regularly.

Lower Humidity Helps Plants Avoid Powdery Mildew

In both VEG & FLOWER, when you stay below 60% humidity, you help your plants avoid powdery mildew, as well as other plant fungal diseases like blight, rust, botrytis. There are many ways to prevent powdery mildew, but you should start by getting the humidity in your grow tent between 50% and 60% RH.

Managing Temperature in a Grow Tent

When you need to lower the temperature in your grow tent and don’t have air conditioning, start by opening the doors of your grow tent.

Then double check that your exhaust fan is blowing air out of the tent.

If your LED light is dimmable, then dim it down as low as it goes. Or turn the light off for a couple hours (as long as you’re in VEG phase, not FLOWER phase.)

You can also add more fans to blow hot air out of the tent now that the tent door is open. Or use fans to blow cooler air in.

If all else fails, and you can’t get below 85 F, just do your best and ride it out. Some strains are more resistant to heat stress than others, so your plants might survive just fine. Better safe than sorry though, so pay close attention to heat, especially in FLOWER and HARVEST. Too much heat can evaporate and burn away the terpenes (the smell). Cannabis without a strong aroma is undesirable, so don’t ignore the temperature factor.

If you ever need to increase the temperature in your grow tent, it’s important that you do NOT put a heater inside of one. Only use heaters safely in a room, NOT inside a grow tent.

Managing Humidity in a Grow Tent

If you need to lessen the humidity just a little, use a small dehumidifier in your tent. You’ll need to empty it daily of the water it captures. A big humidifier will do a much better job at lowering humidity than a small one. The small dehumidifiers are around $20 – $40, and the big one costs around $200. Small dehumidifiers would go inside your grow tent, but a larger one can be set up in the room the grow tent is in.

You can also open the windows in the room the grow tent is in, and try blowing fresh air in or blowing air out. Monitor your hygrometer to see which is more effective.

If you need to increase the humidity, you can use a small humidifier that fits inside the grow tent. Or you can put a bowl of ice cubes wrapped in a dish towel on the floor. You can also try blowing fresh air in from windows if the humidity is higher outside.

Small dehumidifiers and humidifiers will help to some extent, but don’t expect drastic changes. Use them along with opening/closing windows and running fans. They are helpful if you don’t have central air conditioning or a large dehumidifier in the room your grow tent is in.

Marc Eden’s DIY Cannabis Cultivation Book For Beginners
Published by Green Carpet Growing, Inc.
San Diego, CA 92103
www.GreenCarpetGrowing.com
© 2021 Green Carpet Growing, Inc
All Rights Reserved

No portion of this book may be reproduced in any form without permission from the publisher, except as permitted by U.S. copyright law.

For permissions contact: marc(@)greencarpetgrowing.com

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