Indoor Grow Temperature and Humidity, Stage by Stage

For an indoor cannabis grow, aim for 20–25 °C (68–77 °F) and 65–70% humidity for seedlings and clones, 20–30 °C (68–86 °F) and 40–70% humidity in veg, and a cooler 18–26 °C (64–79 °F) with 40–50% humidity in flower, running about 4–5 °C (7–9 °F) cooler when the lights are off. Plants like roughly the same temperatures we do, and many strains slow down below 15 °C (59 °F). That's why, when the weather turns cold, an indoor grow is the easiest way to keep your cannabis production on track, hot season or cold. 

So what exactly is an indoor grow? It's a setup that mimics, and even improves on, the natural conditions that make growing cannabis easy. Sunlight gets swapped for artificial light, airflow comes from exhaust fans and clip fans, and so on.

Before going further, you need to know that temperature and relative humidity are tightly linked. Relative humidity is the ratio between the actual water vapor pressure in the air and the maximum it could hold at that temperature. Sounds like a tongue twister, but the gist is that the higher the humidity, the better your plants handle temperature swings.

In other words, warm air holds more water vapor than cold air, and that's where the juggling act starts: the season, the humidity, the temperature and the stage your grow is in. Below is an indoor grow guide covering the ideal temperature for each stage and how to raise or lower humidity indoors.

Ideal temperature and humidity for each growth stage

1. Seedling stage

Temperature and humidity are crucial at this stage. Seedlings and clones love high humidity, around 65–70%, because their roots aren't developed yet, and high humidity lets them take in water through their leaves. A humidifier is ideal here. Keep the temperature at 20–25 °C (68–77 °F) with lights on and 4–5 °C (7–9 °F) lower with lights off.

2. Vegetative stage

In veg, the ideal indoor temperature can run warmer, between 20 and 30 °C (68–86 °F)  (lights off: 4–5 °C / 7–9 °F lower). You can lower the humidity by about 5% a week (acceptable range: 40–70%), because the roots are now pulling up more water and evaporation through the leaves cools the plant. Keep temperature and humidity in that sweet spot throughout this stage.

3. Flowering stage

In flower (when your plants start putting out buds), the grow should run a bit cooler, about 5% lower, somewhere around 18–26 °C (64–79 °F). That's generally done to get the best bud quality. Humidity needs to come down to 40–50%; you can get away with 55%, but 60% is asking for trouble. 

How to raise or lower temperature and humidity

There are a few ways to keep humidity and temperature at acceptable, even optimal, levels. Some are more sophisticated than others, and they vary in cost and effectiveness.

How to lower humidity in an indoor grow:

Run your exhaust fan faster or upgrade the system (make sure the air leaves both the grow tent and the room it's in); bring in more fresh, cool air; try watering right after the lights come on (some growers use cold water); use a dehumidifier. 

How to raise humidity in an indoor grow:

To raise humidity, keep the soil surface moist, not soaked; set containers of water inside the grow space; use a humidifier with a big tank. You can also mist the leaves with lukewarm water.

How to lower the temperature indoors:

Your light schedule matters a lot: run the lights at night and keep them off during the day. If you grow with HPS lamps, think about using a cool tube, although LEC or LED lighting is recommended since it puts out less heat. With MH/HPS lighting you can move the ballast outside the tent, which takes a heat source out of your grow; and use a cool tube, a fixture that isolates the bulb from the plants. 

How to raise the temperature in an indoor grow:

You can get bigger lamps, and a heater will help regulate the temperature in your grow during cold spells, especially if you grow with LED lights. If you have a cool tube or cool box hooked up to ducting and an exhaust fan, you can pull the ducting so you keep the heat coming off the bulb. And don't set your grow tent directly on the floor. 

Whatever method you use, always keep track of the temperature and humidity in your grow. 

Cannabis strains for a winter grow 

If you're about to start a new indoor grow, here are some strains that handle cold well and are easier to grow in winter:

1. Critical (feminized):

A vigorous, sturdy strain.  It's a tough plant and very easy to grow. It handles cold really well and flowers fast, so it won't give you headaches.

2. Mazar:

A seriously resinous strain praised for its quality, incredible vigor and resistance to cold climates. It's a hybrid of Skunk #1 and top-notch Afghan genetics. Hash lovers dig it for its pine-and-lemon aroma and insane resin production. If you're into a fully relaxed high, Mazar is your pick.

3. Moby Dick: 

A sativa-dominant hybrid that doesn't need much babysitting and can handle extreme climates. We recommend Moby Dick XXL Auto.

4. Royal Gorilla:

A balanced 50/50 indica/sativa hybrid with delicious earthy, sweet notes, rounded out by hints of lemon and pine, and a great option for a winter grow. 

If you're growing autoflowers, you can keep the grow lights on 24 hours a day, which helps keep the plants and grow space warm, especially in winter. 

One last tip: the first thing to buy for keeping these variables in check is a thermo-hygrometer, ideally digital with a memory function that shows the previous highs and lows. 

It looks like a lot to take in and put into practice, but once you've got that thermometer on hand, the theory takes care of itself, and in the end it's not as complicated as it seems. Before long, running your indoor grow this way becomes second nature.

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