As your plants "breathe" CO2 and "exhale" O2, the balance of
these two critical gases begins to shift. In nature, this exchange fits
in perfectly as animals "breathe in" O2 and "exhale out" CO2. Of
course, a perfect world this is not. Modern industry and the
burning of fossil fuels has somewhat "unbalanced" this effect.
However, in your greenhouse or grow room, you will need to help
your plants breathe by supplying a constant exchange of fresh air,
which by nature contains about 2% CO2. If you have already
employed a thermostat and humidistat in combination with a vent
fan, there is a good possibility that these two mechanisms will
provide a good exchange of fresh air. However, if your fan is not
operating frequently enough, you may be starving your plants of
their most important atmospheric gas, CO2.
Generally speaking, it is best to exchange the entire contents of
your growing area about once an hour during daylight hours. To
do this efficiently, you can use a fan that either runs continuously
at a slow speed, or a fan that runs at high speed in short bursts. To
determine the size of the fan that is necessary, simply multiply the
length of your growing area by its height and then by its width.
This number (use feet as a measurement unit) will be the Cubic
Feet of your area. When buying a fan, you will notice that they are
sold according to "Cubic Feet per Minute," or CFM ratings. What
this means is the amount of air this particular fan will move in one
minute. Therefore, if your greenhouse or growing room is 10 feet
x 10 feet x 8 feet, that’s a total of 800 Cubic Feet. You will need
an 800 CFM fan to exchange the air in the entire greenhouse in
one minute. That’s a big fan and you certainly don’t need to move
it all out in just a minute’s time. I would suggest using a 100 CFM
fan and running it for 4 minutes every half hour. You can do this
with a cycle timer.

CO 2 And You
These days, CO2 is best known as the "greenhouse gas" that
traps the sun’s heat in earth’s atmosphere. It is responsible for
global warming and a host of environmental changes that include
altered weather patterns and rising tides. CO2 causes these
problems by insulating the earth from heat loss and reflecting some
of the sun’s heating rays back onto the earth. From the previous
information, you know that plants require CO2 to manufacture
food within their leaves. Many of you may have also heard that adding CO2 to the growing environment can
significantly increase the growth rates of most plants. This is 100% true. However, managing CO2 is tricky
because of the factors preceding this topic. For example, if you are constantly exhausting the air from your
greenhouse or grow room, how would you supply a never ending supply of CO2? You could add a CO2
cylinder with a regulator as shown on the previous page. The regulator can be set to slowly "leak" CO2 into
the air flow of a reciprocating fan in order to evenly distribute it across the growing environment. You could
hook the regulator up to an electrical valve called a "solenoid" which is then controlled by either a timer (timed
to go on when the exhaust fans are off), or to release every X minutes for X minutes (another use for a cycle
timer). You could hook the solenoid valve up to a CO2 measurement and delivery system that would deliver
CO2 once the levels dropped below those you set as minimum. There are many crafty ways to add CO2 to
your garden. The trick is to make it cost effective and safe. CO2 is not a gas you want to inhale in high
concentrations. Plants will only benefit from so much before you wind up choking them with too much.
CO2 is measured much the same way as nutrient in solution, that is, PPM (Parts Per Million). Most gardens
and crops will benefit significantly when the concentration of available CO2 is kept between 1000 and 1600
PPM. You will need a CO2 test kit or meter to accurately monitor this value. However, you can use the charts
that come with CO2 injection systems to determine how to achieve these levels using their equipment. Without
using an integrated measurement/injection system, you will need to determine the size of your room in cubic
feet, and using this volume, ask the CO2 injection system manufacturer to specify the right setting along the
lines of "set the regulator to "X" PSI and open the valve for "X" minutes every "X" minutes between exhaust
cycles. Since every CO2 system is inherently different, you will have to rely on the manufacturers
recommendations to insure accuracy and proper delivery of this growth boosting gas to your growing area.
CO2 can also be generated by using propane and natural gas burners, since these gases result in the discharge
of CO2 and water vapor when burned. Of course, keeping an open flame in any unsupervised area is
dangerous, so these kinds of CO2 generation systems must be operated with caution according to the
manufacturer’s recommended operating procedures. The advantages to using a natural gas CO2 generator
include lower operational costs and they can often double as heaters for colder area applications. If you are
growing indoors, the heat generated by these units is usually a problem that neutralizes their effectiveness
since, to exhaust the additional heat, you will also wind up exhausting the additional CO2. If you are a
beginner, I strongly advise leaving CO2 until when you gain experience and have your garden completely
under control. There are a number of excellent books on the use of CO2 for gardening.
Do-It-Yourself CO2
One of my readers informed me of a simple way to create and distribute CO2 indoors using a few
inexpensive parts. You’ll need a one gallon milk jug, a pound of sugar, enough water to dissolve the sugar, a
packet of yeast, and some tubing. Begin by drilling a small tight hole in the cap of your one gallon jug, then
pass a length of 1/4" air tubing through it just enough so that it hangs inside the bottle. The other end should
be placed near your plants, preferably behind a fan that will evenly distribute the CO2 throughout your garden
area. Fill your container with one pound of sugar, add warm water and stir until completely dissolved (make
sure you leave an air space at the top of the container so the tubing doesn’t go under water) Add the packet of
yeast, replace the cap and stir. CO2 will be released gradually as the yeast begins to digest the sugar. Recharge
your "CO2 Generator" with fresh water, sugar and yeast once per week.