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Of all of the technologies and methods that exist in order to improve and prolong plant production, the incorporation of CO2 is probably the quickest, and most simple of all of them in marijuana cultivation. Each species has its own maximum level of CO2 absorption, but according to the species and amount of CO2 necessary, growth can increase by as much as 40%.

Two concepts to touch on in marijuana cultivation

It’s essential that everyone knows what we are taking about at any given moment. For that reason, to avoid confusion in the rest of the text, it’s worth clearing up a few concepts:

  • Air replacement: When we say air replacement we are referring to when we change the air inside of the room with new air, in other words, removing old air and changing it for pure air.
  • Air recirculation: We remove the same air that is found in the room, but with movement through the use of ventilators.

Why does CO2 help?

Photosynthesis is the process by which plants inhale CO2 from the air and combine it with water, and then using light energy, produce sugars and oxygen.

The plants can’t move themselves to find new air, and so air must be replaced periodically so that the plant doesn’t stop its process of production of the sugars necessary to grow.

When we replace the air, C02 is added in great quantities and the plant responds with vigorous growth. Thus, it’s as vital to have good air replacement through entry/exit, and also a good interior ventilation system.

Lack of CO2

Many beginners commit the error of not considering the importance of C02 in a closed area. In a grow room, plants rapidly consume CO2 and convert it in oxygen. When plants have absorbed a third of this CO2, the growth is quickly blocked, and the plant stops growing.

This situation is most serious in areas without any type of air re-circulation, due to the fact that the plants create a micro-climate around their leaves. This small area remains free of CO2 very quickly due to the leaves’ direct contact with it. If we re-circulate the air constantly, we will avoid the leaves having contact with this air that lacks CO2.

C02 replacement is always necessary, even in a room with good re-circulation. A closed room full of healthy plants can lack C02 in as little as an hour, more or less. Clones, cannabis sees or small plants don’t waste as much C02, but they still need good air replacement.


Measurement of CO2 content

C02 is calculated and measured out in parts per million (ppm). Air tends to contain around 300 ppm and city air, up to 450 ppm.

A lot of research places the ideal maximum level for vital growth around 1500ppm, almost 5 times the quantity of C02 in the air.

Before beginning


Before choosing a system of CO2 production, have in mind that CO2 is not the only important factor for good growth. If some other factor is ignored, all your efforts with CO2 increase will be in vain.

The other factors are:

  1. Ventilation
  2. Temperature
  3. Humidity



  • Circulation and ventilation of air: If we inject CO2 in a room without re-circulation, the CO2 will fall to the bottom of the room and stay there, since CO2 is heavy. Even with moderate ventilation, the CO2 will not make it to the leaves.


  • The key to maintaining internal CO2 levels is circulation: Plants gain the benefits of constantly blowing CO2. Many growers use a double layer of circulation. A layer of ventilation at the height of the plants and another at ground level to move heavy CO2.

  • Vertical air circulation: Moving the air vertically helps to circulate CO2 and avoid weak points (micro-climates) in the grow room. Cold air from the ground level is moved towards the hot lamps, lowering the temperature, and at the same time as it lifts CO2.

  • Air replacement and ventilation: Even when we supply CO2, ventilation is necessary to move the plants’ wasted air, as well as air replacement in order to get rid of the excess oxygen created by them. It’s a good idea to alternate cycles.

  • Basic concepts of ventilation: Assuming that the C02 is supplied through replacement, high velocity and regularity of evacuation is desirable in order to change air as quickly as possible. If CO2 is not renewed, ventilation doesn’t work.


Temperature and humidity control

It’s necessary to use climate measurement tools, as much for temperature as for humidity. The optimal ranges are:

1 – Temperature: the best range is between 19-24ºC. Some tests indicate that the best CO2 absorption temperature of CO2 is 24ºC with 1500ppm. Other experiments show that it shouldn’t fall below 18ºC. The ideal temperature would be an average, around 22ºC.

2- Humidity: humidity should ideally be situated between 40% and 65%. With 65% humidity everything works, but problems can arise at higher humidity levels. Mold propagation stops when humidity is low and constant.

Some growers ignore the fact that humidity rises during dark times, and turn ventilators on only at the same time as the lights. This worsens mold growth during the nocturnal cycle.

In order to prevent this, ventilators should be connected during dark periods as well. A dehumidifier helps to lower humidity and maintain temperature. If there is a sharp fall in temperature, relative humidity rises. The same amount of water present in cold air creates more humidity than when it is present in hot air. A humidistat connected to the dehumidifier can aid us in maintaining a constant humidity.

Temperature and humidity controls with maximum and minimum help us to control parameters during periods of darkness without having to enter the grow room and interrupt the photo period.

Temperature control

Heat created by the high pressure lamps can be a problem for some cultivators. It is difficult to maintain temperature between optimum levels, above all in summer. There are solutions to this problem. Some are easy and expensive, and others are resourceful and thrifty. Here are the most frequently used: 

  • Air conditioning installation
  • Reflector installation (Cooltube)

Air Conditioning

Installing air conditioning is one of the fasted ways to avoid problems. It’s easy to install it in a window, position the ventilator, and adjust the temperature. Some people worry about the cost, but you also have to consider the benefits. The electric bill can really be a problem if an appliance is on for several hours. If that is the case, it would be best to think about isolating it to the grow room.


Cold Reflectors

Cold reflectors tend to be a good choice for small grow rooms, allowing the temperature to be lowered to 5-10ºC, although the refraction abilities of these aren’t as good as some professional reflectors.


Humidity control

Many growers use an electric dehumidifier to control humidity. A dehumidifier makes everything simple. You only need to set the humidistat to your desired level and the dehumidifier will connect and dry the room when the humidity goes above your set parameters.

It’s simple and economic, although expensive if it is connected all day. However, water is also saved, since the dehumidifier will also provide us with distilled water that can be used for growing, as long as we maintain clean machine filters.

In any case, there are several techniques that are used in the absence of a dehumidifier:

  • Increase the frequency of air replacement. Assuming that the outer humidity is lower that that which is in the room, this is simple. Air only needs to be more frequently replace, so that through connected vessels the exterior humidity level is reached.
  • Increase ventilation frequency, or its strength, assuring that the ventilation process is complete throughout the entire room, and to avoid pockets of undesired humidity formed by poor air circulation. If the humidity isn’t consistent in all parts of the room, we see different growth patters due to the different micro-climates in the room as a result of poor air circulation.

  • Capturing air at the entrance of another area. The interior air in a house, or air that comes from a non-sunny zone, will contain more water than outdoor air that comes from a sunny zone, thhough we also need to think about the fact that indoor air contains a higher level of CO2, because of our own presence, the presence of pets, and use of fuel.

The air absorbed from lower areas tends to have more humidity than air that comes from high areas.

The grow room’s situation is determined by its temperature and humidity. Basements tent to have more humidity, but a more consistent temperature. Attics tend to be hotter in summer and colder in winter, although they don’t have humidity problems.

We hope that this information is useful to you. Soon we’ll explain a little bit about the importance of CO2 in the cultivation of marijuana.