CO2 supplementation. Is it worth it?

So, you’ve got your grow system sorted, you’ve done two or three successful grows and you’ve learned how to nurture a plant from a baby all the way through to being a huge flower or fruit laden beauty. Now you are looking for that next step which will take your harvests to a new level.

Your standard setup will ensure that you get the most important environmental factors optimised for good plant growth. Your HPS lamp provides the light, your extraction system provides the fresh air and your hydroponic system or pot-culture provides the root-space, the water and the nutrients for the plant to grow.

Once a grower has got all the basics correct, and has learned how to consistently get a good yield from a standard setup, usually one of the last environmental factors to consider is that of CO2 (Carbon Dioxide) supplementation.

Photosynthesis is the light-powered process that plants use to produce sugar from water and CO2 (with oxygen being given off as a by-product). There are actually several steps in the process, but the basic equation is at the top of this page.

A lot has been talked about in the press about the rising levels of CO2 in our atmosphere. As a “greenhouse gas”, it traps heat from the sun more effectively than the major components of our air.

The vast majority of our air is made up of a mixture of Nitrogen and Oxygen. In fact, air is made up of approximately 78% Nitrogen and 21% Oxygen. However, there are some other components which are found in relatively trace amounts. These trace gases are mainly Carbon Dioxide (0.03%) along with some Hydrogen, Argon, Neon, Helium, Krypton and Xenon.

With a background level of only 0.03% - 0.04%, it is clear that the actual amount of Carbon Dioxide in our air is relatively little. However, when it comes to the greenhouse effect, a little bit of difference goes a long way!

The 0.03% - 0.04% Carbon Dioxide in our air is usually described as 300-400ppm (parts per million). The precise amount varies a little bit across the surface of the planet. However the natural background range is usually between 330ppm and 450ppm.

Getting back to growing, plants require a minimum of around 160ppm of carbon dioxide in the air to grow at any decent rate. Below this level, plants cannot effectively absorb enough to be of any use and will stop growing.

This is one of the reasons why an extraction system is so important in a plant growing environment such as a grow-tent. If the air in a grow-tent is neither continually refreshed nor has CO2 added back into it, the plants will quickly use up the small amount of CO2 in the air within the grow-tent, and then their growth will slow until they eventually stop.

As already discussed, the photosynthesis equation above shows us that plants require CO2, Water and Light to photosynthesise. This means that each of these 3 resources can be a limiting factor for growth rate. CO2 is just one of the limiting factors.. The other two limiting factors, of course, are Light Intensity and Water. However, as long as your watering schedule is good then water should never become the limiting factor in your grow.

With normal background levels of CO2 plants cannot make use of much more light than about 800 µmols/m2/sec. In fact, diminishing returns kicks in at anything above around 500 µmols/m2/sec.

However, plant growth speed goes up with an increased concentration of CO2, all the way up to at least 900 to 1000ppm, and for some plants, the optimum level can be as high as 1500ppm but there is a catch - for plants to be able to use the extra CO2 they also need to be given more light. One without the other will not result in the maximum possible increases.

Growth rate chart vs CO2 level for many popular plants:

Source: Roger H. Thayer, Eco Enterprises, Redrawn by Vince Giannotti.

This chart clearly shows that growth is virtually non-existent at CO2 levels below 160ppm. Also it shows that good growth speed gains can be had up to 1200ppm or so. Above 1500ppm, the CO2 level begins to become distinctly counter-productive. At about 1950ppm it actually becomes toxic, and growth is slower than in normal unenriched air. It should be noted that at levels above 4000-5000ppm, CO2 becomes toxic to people.

If the CO2 level is increased to around 1000-1200 ppm then the other limiting factor (the light intensity) should also be increased to around 1100 µmols/m2/sec so that the plants can make full use of the higher concentration. Also, the grow space temperature should be increased to speed up the plant's metabolic rate. With background levels of CO2 and about 800 µmols/m2/sec of light, a grow-room temp of 24-25 °C is about perfect. If CO2 supplementation and increased light levels are employed then the grow-room temp should also be increased to around 30 ºC.

The graph above clearly shows that the plant growth rate in a well-controlled CO2 enriched environment with increased light levels can be up to 2.5 times that of a plant in normal unenriched air. This is huge!

CO2 supplementation can definitely make a big difference. But this increased growth rate is at it's best during the vegetative stage. During flowering/fruiting, CO2 supplementation is slightly less effective. However, worthwhile gains can definitely be had with extra CO2 in the first 2 weeks of flowering fruiting, and may be effective (although slightly less so) up until the end of flowering/fruiting.

Changing your grow to include CO2 supplementation

There are a few different ways to increase the level of CO2 in your grow-room. These will be investigated in the next article.

However, no matter what method you choose to use to increase CO2 levels, there are a few things that you will need to do to make sure that your project is worthwhile and effective:

Your extraction system will need to be set up so that it is only on occasionally during the lights-on periods. There is not much point setting up a CO2 enrichment system, only to suck all that lovely CO2 enriched air straight out, and replace it with normal air again. Either that, or run a closed loop system with an Opticlimate.

Fortunately, plants grown in a CO2 enriched environment actually need a higher temperature than the standard 28C maximum for them to use the extra CO2. With a CO2 level of 1200-1500ppm the temperature needs to be between 30C and 35C. Along with the higher concentration of CO2, the higher temperature is conducive to faster growth. As long as the temperature remains in the correct range and the humidity stays reasonable, it is best to only have the extraction system running as infrequently as possible (say 15 minutes out of every 1-3 hours).

There is not much point increasing the CO2 level for sick or unhealthy plants. Only healthy plants will be able to make use of the extra CO2.

One last thing - there is no point adding CO2 during the lights off period. Plants consume no CO2 during this time.

In the next article, I will be discussing the different methods of CO2 supplementation along with their effectiveness and cost. Don't miss it!

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