Temperature In Your Marijuana Grow Room

The temperature in your grow room is very important for the photosynthesis of your marijuana plants. For example, low temperatures reduces evaporation through the leaves. The result is that the suction force, which takes up nutrients via the roots, becomes smaller.

The nutrients that are necessary, but aren’t absorbed, remain in the growing medium and eventually disintegrates in the root environment. A high acidity in the growing medium reduces the working of the roots which makes the plant absorbs less water and nutrients, and the growth can even come to a halt.

So what posed as a small external imperfection at first, can have serious consequences in an entirely different part of the plant. Therefore it’s very important to create a good climate in your indoor cannabis grow setup. In this article I will explain what the ideal temperature is, and what you can do if the temperature is too high or too low.

The perfect temperature

The temperature in your grow room depends on several factors. The location of your room in the building is an important one; in a basement it’s a lot cooler than in an attic with a flat roof.

Aside from that, the size of your room, the airflow, the number of lights and the extraction rates play an important role. Keep this in mind when building your grow room.

When the light is turned on, an ideal temperature for the cuttings and seedlings is between 68ºF and 77ºF (20ºC and 25ºC). As the plants get older they can evaporate a little bit more and the temperature may increase to a maximum of 82ºF (28ºC).

Download my free marijuana grow guide at this link for more about temperature

When the lights are off, the temperature should lie between 62ºF and 72ºF (17ºC and 22ºC). Another important rule is that the temperature differences between day and night cannot be too high, a maximum difference of 10ºC. So when it’s 82ºF (28ºC) during the day, it cannot go below 64ºF (18ºF) at night. A temperature difference of 5ºC is ideal.

Measuring the temperature

You measure the temperature in your marijuana grow room with a thermometer. There are analog and digital thermometers, and they’re for sale everywhere for a few bucks. I always use a digital thermo/hygrometer (something like this one) with a built-in memory, so I can see what the maximum and minimum temperature was.

It’s also a good idea to get one with a temperature sensor on a wire, so you can hang the display outside your grow room, and can see the temperature when the lights are off.

Always measure the temperature in the shade, and at various places in your growing room. Provide good air flow by placing several fans. The lamps emit radiant heat which does not affect the air temperature. Therefore it will always be a few degrees warmer right under the lamp than a shaded spot. That is not really an issue, but make sure there’s enough distance between the lamp and the plant, so the tips of the plants won’t literally burn. Radiant heat does affect the leaf temperature.

Lowering the temperature

Heat often becomes a problem in your grow room. This has to do with the fact that the lamps we use produce a lot of heat. Fluorescent lighting is not really a big deal, but HPS lamps can heat up your room to soaring temperatures of 122ºF (50ºC), which is fatal to your marijuana plants. First of all, the dimension of the room is important. For a 600 watt HPS lamp always use a minimum space of 100cm x 100cm x 200 cm (3.2 ft x 3.2 ft x 6.5 ft).

For the extraction, use the following simple formula; number of watts divided by two = extractor in cubic meters (feet). So 2 x 600watt HPS is requires an extractor fan of 600m3 (22000 cubic ft.). The extractor fan blows the hot air outside, and sucks in fresh air inside. You can put a carbon filter on it, so it doesn’t blow marijuana smelling air out. You can also put ballasts etc. outside your grow room because they generate a lot of heat.

Download my free marijuana grow guide at this link for more about temperature

Perhaps the easiest way to keep the temperature low is by running at night. Your lamps will turn on for a few hours after the sun has set, and turn off a couple of hours after sun rise. This way you’ll have your lights on at the coolest period of the day. You can imagine that if it’s hot weather outside, you’ll also suck this hot air into your grow room with your extractor. So, the room temperature never gets below the hot temperature outside. There are professional growers who use an air conditioning unit so they can run it during the day and at night.

Increasing the temperature

When the lights are on, it’s not necessary to increase the temperature. The lamps themselves will take care of that. However, it is important to distribute the air over your room evenly so that you’ll get the same temperature everywhere. Use swivel fans for this, and aim them between the lamps and the plant. Fresh air from outside should also be well distributed over the growing area, so that there won’t be any cold spots. Especially in winter when temperature can get below freezing.

When the lights are off, it can get cold in your grow room. Luckily there are plenty of things you can do to increase the temperature. A simple space heater with thermostat is usually sufficient enough to heat your space. However, they do consume a lot of electricity. A radiator with a thermostat works fine too. You can also turn off the extractor fan (that provides the fresh air) on the moment the lamps turn off. This can be done by a so-called fan controller with thermostat, or with a timer.

Thanks for reading. Please leave comments or questions below and don’t forget to download my free grow bible

Robert Bergman