Plant Growth and Carbon Dioxide for Maximum Yield

Plant growth and Carbon Dioxide are directly related. Plant growth requires a tremendous amount of Carbon Dioxide (also known as CO2). At the center of every plant cell is an atom of Carbon, which the plant has absorbed from the surrounding air. When all other growing conditions are kept ideal, Carbon Dioxide becomes the growth limiting factor. This means, as you increase the CO2 in your garden area, you will also increase the plant growth rates....and your yields.

When running your garden at an average temperature of 75 degrees.

The ideal level of CO2 supplementation to maximize plant growth in a well maintained garden is generally 1500 ppm

In one final twist of irony, as you begin to increase plant growth and Carbon Dioxide levels you will reach a point where temperature becomes the limiting factor. In order to benefit from the highest levels of CO2 supplementation (1500-2000 ppm), you actually need to run your garden area warmer than normal (80-85 degrees). On average, plants will also require an extra 30 watts of light/sq.ft. (70-80 watts/sq.ft.). Plants will use extra water and nutrients under these conditions, so make sure they are available!

This has one implication which may offer you (the indoor gardener) a very nice solution to one of your most difficult problems. If all other factors are perfect in your garden EXCEPT your temperature runs a little high, you may be able to maximize plant growth AND solve your heat problem at the same time with the addition of CO2.

When to Maximize Plant Growth and Carbon Dioxide in your Garden

CO2 should not be used in a garden where you have to constantly run your exhaust. CO2 should only be used in a well sealed garden area, and should only be used when there are decent periods of time between exhaust cycles. The most ideal circumstance is to use CO2 supplementation in a garden area that is cooled by air conditioning, and where the lights are cooled separately from the rest of the grow room (so you are not constantly exhausting your CO2).

The Best Ways to Produce
CO2 for Indoor Gardens

There are two good choices for efficiently increasing plant growth and Carbon Dioxide levels inside your garden. The first method is to use a CO2 generator. The second option is to use tanks of compressed CO2.

Gas powered CO2 generators will add a little heat to your grow room.

Generators are more convenient because they hook up directly to your natural gas line or propane tank, eliminating the need to periodically fill your CO2 tanks. They also include a dial to adjust CO2 levels. The drawback is that they produce extra heat as well as CO2. Most of the heat is ducted away, but can sometimes still cause a problem for plant growth.

Tank CO2 is cheap, and adds no heat to your grow room.

I prefer the second option, which is to use tanks of compressed CO2. My garden is only a little larger than personal size, so my CO2 tank does not need to be refilled that often. Furthermore CO2 is cheap, and compressed CO2 does not add any heat to the environment when released. A 5lb tank will need to be refilled too often, and a 50lb tank is heavier than most people can handle, making a 20lb tank the ideal size for most home gardens.

...and your grow lights should be ventillated separately from the rest of the air in the grow room.

The most efficient use of CO2 is with air conditioning, otherwise you end up exhausting a lot of CO2 out of your garden

The General Setup for a
Tank Based CO2 System

A quality CO2 tank valve will have a pressure regulator gauge, a flow rate control gauge, and a solenoid valve attached to a 3-prong electrical plug.

The pressure in a CO2 tank is around 1200 psi. In order to run CO2 tanks, you need a pressure regulator. The regulator will step down the pressure to about 50 psi, so it can be managed by the solenoid valve. Some regulators have two gauges, in which case one gauge regulates the tank pressure, while the other regulates the release pressure (or flow rate).

CO2 from a tank system is released with tubing, perforated with microscopic holes. It is often called 'laser tubing'.

"Laser tubing" is strung above the tops of the plants and run to the solenoid valve. The tubing has many microscopic holes in it, which allow the CO2 to be evenly distributed through the garden. As Carbon Dioxide is released, it sinks down into the plant canopy because it is heavier than air. An oscillating fan placed on the ground will help keep circulating the CO2 through the growing plants.

How to Get CO2 Levels Right
With Your Tank CO2 System

There are two ways you can set up your tank system to release the proper amount of CO2. The first way is to figure it out using mathematical calculations and set timers that will control your equipment.

The other way is to use a CO2 controller with a sensor. Today's controllers have easy to use dials and digital display panels that make it very easy to set the desired CO2 level.

CO2 Calculations and Setting Timers

The normal atmospheric CO2 level is about 300 ppm. The ideal range for maximum plant growth is 1500 ppm. That means we need to increase CO2 levels by 1200 ppm (or .0012) in the garden area.

First we need the volume of your garden area. This is length x width x height. For our example we will use a 5 x 5 x 8 foot room, which has an air volume of 200 cubic feet.

Next, we calculate how much CO2 to add to the garden area by multiplying the volume of your garden by .0012. For our example this would be 200 x .0012, which equals .24. So, it will take .24 cubic feet of CO2 to bring the garden in our example up to 1500 ppm.

Now to your CO2 tank regulator. On the tank there is a pressure regulator valve and also a flow control valve. The pressure valve is usually set between 50 and 100 psi, depending on the recommendations for the equipment you have purchased. The flow control valve is adjustable, and cubic feet per hour (CFH) is used for the measurement.

In our example, if you set your flow control to .24 cubic feet per hour, and programmed a timer to leave the solenoid valve open for one full hour, you would slowly bring the room up to 1500 ppm by the end of the hour (assuming you didn't have to exhaust in the meantime to bring your temperature down). The following is a more realistic way to set this up...

Set your exhaust fan to kick on at 90 degrees, and kick back off at the ambient room temperature. This will cause the room to get a little warm for a short time during the cycle, but it should not do any harm. Watch it for a while and determine how long you have in between exhaust cycles (while the fan is off). For maximum plant growth, you want to achieve 1500 ppm in the garden area at least 2/3 of the way thru this time period. Let's use our example one more time...

Let's say you watch the system and determine you have 30 minutes between the exhaust cycles. You need to release .24 cubic feet of gas to bring the room up to 1500 ppm, and you need to do it in less than 20 minutes (2/3 of the way through 30 minutes). Releasing the necessary amount of CO2 within 10 minutes would certainly be within these guidelines, and would give your plants a little extra time in the fully enriched environment.

The relationship between time and your flow rate is inversely proportional. That simply means if you take 1/4 the time, you need 4/1 flow rate (that's 4 times). In this example, we want to release the required amount of CO2 in 10 minutes, which is 1/6 of an hour (60 mins x 1/6 = 10 mins). We therefore need to multiply our old flow rate by 6/1, or by 6 in other words. The result is...

.24 CFH x 6 = 1.44 CFH

Now you set your flow rate to 1.44 cubic feet per hour and program the CO2 timer to come on for ten minutes immediately following your exhaust cycle. All done, now you are ready to maximize your plant growth!

Advanced Atmospheric Control

A controller is used to make all your atmospheric control devices work with each other smoothly. Below is an example of how a controller setup might look.

Advanced atmospheric controller setup.

The controller itself consists of the center piece and the digital display panel, where you can set the desired CO2 level. The panel controls both the exhaust fan and the release of CO2. A built in thermostat will kick your exhaust fan or air conditioner on when needed, it will release CO2 when needed, and it will keep the CO2 turned off whenever the exhaust fan is running. In addition, most controllers have a light sensor that will prevent CO2 from being released at night (if not, a digital timer can be used).

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