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Carbon dioxide is very important to a plant. A high concentration of CO2 in the air makes a faster growth possible. It’s vital that your grow room contains a lot of CO2, so the plants can keep growing properly.

Luckily, CO2 is present in regular air. But since your plants consume a lot of it, you’ll have to refresh the air in your grow room with fresh air that’s rich in CO2. There are even ways to artificially add CO2 to your grow room, but this requires much growing experience, and the installation is very expensive.

CO2 and growth

A plant creates its own building materials through the process of photosynthesis. The solar energy is caught by chlorophyll in the leaves and if then used to create chemical bonds, which are mainly sugars. The building materials for the sugars are made from CO2 and water. There’s usually enough water and light, so the CO2 is ultimately the variable that determines the rate of photosynthesis. The more CO2, the more sunlight can be caught and can be processed into sugars. This concept is valid up to a certain limit, after which more CO2 no longer makes a difference, since other parts of the photosynthesis can’t speed up any more.

Regular outside air contains 0.035% CO2, indicated as 350 ppm (parts per million). It’s possible that the concentration in a grow room is much lower, since the plants consume CO2. The CO2 therefore has to be supplied from outside, because a prolonged shortage of CO2 leads to slowed down growth and development of the plants.

CO2 and growth
CO2 and Marijuana Plant

You can do this by simply extracting CO2-depleted air and blowing fresh air that’s rich in CO2 into the room. In this way you keep the right CO2 level in the room and your plants will never experience a shortage. You must do this to have a successful harvest. Read the part on Ventilation to see what’s needed to refresh the air in your growing room.

Getting the CO2 to where it needs to be

So there has to be sufficient CO2 in your grow room to ensure optimal photosynthesis. In addition, the CO2 has to properly arrive at its destination, which is the chlorophyll in the leaves. In order to get there, three obstacles have to be overcome:

  • the stagnant layer of air around the leaf
  • the stomata
  • entry into the cell
How leaves absorb CO2
How absorb CO2

Every leaf is surrounded by a tiny layer of air that barely moves, especially when the leaf has hairs. This forms a significant obstacle for CO2 to reach the cavities in the leaf, where absorption takes place, through the stomata. This layer of air is thinner when there’s more circulation of air, so there’s less resistance. It therefore makes sense to create circulation in your growing room, for instance with fans or a ventilation system.

The CO2 gas obviously needs to go inside the leaf for the process of photosynthesis. This takes places through the stomata. These also have a certain resistance, depending on the size of the opening. The stomata will fully open if the plant has sufficient water. If there’s not enough water, the stomata will partly or completely close, making it very hard for CO2 to get into the leaves. Lastly, CO2 needs to go into the cell, towards the chlorophyll. This takes place in dissolved form.

Adding CO2

As mentioned before, air contains 350 ppm CO2, but plants grow a lot faster and bigger if they receive more CO2. The rate of photosynthesis at a CO2 level of 1000 ppm is 30 to 50% higher than at 350 ppm. You can therefore significantly increase your yield by adding CO2 to your growing room. Unfortunately, this is not that easy. It requires a lot of growing experience and a major investment to make this possible.

First of all, you need equipment that measures and controls the CO2 level in your room; a CO2 sensor, a CO2 controller and a CO2 generator. The sensor measures the amount of CO2 in your room. If this drops below the value you set on the controller, CO2 needs to be generated. This CO2 then has to be distributed throughout your growing room so your plants can absorb it.

You can generate CO2 by burning gas with a gas heater in your grow room. This also generates a lot of heat, increasing the demand for extraction. Furthermore, you don’t want to have an open flame in your grow room, since this greatly increases the risk of fire. A good solution for this is the use of a carbon dioxide bottle, one of those that connect to a beer tap. Attach a pressure reducing valve with a solenoid valve that you connect to the CO2 sensor. If the CO2 level in the room decreases, the solenoid valve opens until the desired CO2 level is reached.

This isn’t that difficult and a set like this will cost about $1000, but the problem is the extraction. By extracting the heat from the lamps you’re also extracting the CO2 you add to the room. You would need a bottle of carbon dioxide a day to achieve the desired level of CO2.

In order to solve this problem you will have to install an air conditioner. Instead of extracting the warm air, you should cool off the air. Use an AC with the same cooling capacity as the wattage of your lamps, so if you have 2x 600 watt lamps, you need 1200 watt cooling capacity.

The climate also has to be a bit different when using this setup. The humidity and the temperature should be higher and you’d have to provide your plants with more water and nutrients. You can also give them more light by hanging 1000 watt lamps for instance. I will create a growing report with CO2 over the upcoming year, so you can see the things involved in practice. This can be a fun next step for experienced growers who are interested in a higher yield.

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