Elevating carbon dioxide levels can increase growth speed a great deal, perhaps even double it. It seems that the plant evolved in primordial times when natural CO2 levels were many times what they are today. The plant uses CO2 for photosynthesis to create sugars it uses to build plant tissues. Elevating the CO2 level will increase the plant’s ability to manufacture these sugars and the plant growth rate is enhanced considerably.
CO2 can be a pain to manufacture safely, cheaply, and/or conveniently, and is expensive to set up if you use a CO2 tank system. CO2 is most usable for flowering, as this is when the plant is most dense and has the hardest time circulating air around its leaves. If you are strictly growing vegetatively indoors, (transferring your plants outdoors to flower), then CO2 will not be a major concern unless you have a sealed greenhouse, closet or bedroom, and wish to increase yield and decrease flowering time.
For a medium-sized indoor operation, one approach is to use CO2 canisters from welding supply houses. This is expensive initially, but fairly inexpensive in the long run. These systems are good only if your area is not too big or too small.
The basic CO2 tank system looks like this:
20 lb tank
Regulator
Timer or controller
CO2 tank setup synced to an exhaust fan with a thermostat is best.
CO2 is cheaply produced by burning Natural Gas.
However, heat and Carbon Monoxide must be vented to the outside air. CO2 can be obtained by buying or leasing cylinders from local welding supply houses. If asked, you can say you have an old MIG welder at home and need to patch up the lawnmower (trailer, car, etc.)
For a small closet, one tank could last 2 months. But it depends on how much is released, how often the room is vented, hours of light cycle, room leaks, enrichment levels and dispersion methods. This method may be overkill for your small closet.
It is generally viewed as good to have a small constant flow of CO2 over the plants at all times the lights are on, dispersed directly over the plants during the time exhaust fans are off.
Conserving your CO2
Opportunities exist to conserve CO2, but this can cost money. When the light is off you do not need CO2, so during flowering, you will use half as much if you have the CO2 solenoid setup on your light timer. When the fan is on for venting, CO2 is shut off as well. This may be up to half the time the light is on, so this will affect the plant’s exposure times and the amount of gas actually dispensed.
CO2 generation from fermentation and generators is possible. A simple CO2 generator would be a propane heater. This will work well, as long as the gases can be vented to the grow area, and a fan is used to keep the hot CO2 (that will rise) circulating and available below at the plant’s level. Fire and exhaust venting of the heat issues as well. A room that must be vented 50% of the time to rid the environment of heat from a lamp and heater will not receive as much CO2 as a room that can be kept unvented for hours at a time. However, CO2 generators are the only way to go for large operations.
Fermentation or vinegar over baking soda will work if you do not have many vent cycles, but if you have enough heat to make constant or regular venting necessary, these methods become impractical. Just pour the vinegar on baking soda and close the door, (you lose your CO2 as soon as the vent comes on). This method leaves a great deal to be desired, since it is not easy to regulate automatically, and requires daily attention. It is possible, however, to create CO2 by fermentation, let the wine turn to vinegar, and pour this on baking soda. It is the most cost-effective setup for most closet growers, for whom $400 in CO2 equipment is a bit much to swallow.
In fermentation, yeast is constantly killing itself; it takes a lot of space. You need a big bin to constantly keep adding water to so that the alcohol levels will not rise high enough to kill the yeast. Sugar is used quickly this way, and a 10-pound sack will run $3.50 or so and last about 2-3 weeks. This is also difficult to gauge what is happening as far as amounts actually released. A tube out the top going into a jar of water will bubble and demonstrate the amount of CO2 being produced.
Sodium bicarbonate mixed with vinegar
1 tsp: ~30cc- this will gush up all frothy as it releases CO2. do it just before you close the door on your plants. A MUCH cheaper way to provide CO2 is 2 Oz sugar in 2 litres of water in a bottle [sterilized 1st with bleach and water, then rinsed], plus a few cc urine[!] or if you insist, yeast nutrient from a home brewing supplier. Add a brewing yeast, shake up and keep at 25 deg celsius[~70 F]. Over the next 2 weeks or so it will brew up about 1/2 Oz CO2 for every Oz sugar used. Keep a few going at once, starting a new one every 3 days or so. With added CO2 growth is phenomenal!!! I personally measured 38cm growth in 8 days under a 250watt HPS bulb[tubular clear, Horizontal mount.
A good container is a 1-gallon plastic milk jug, with a pin-hole in the cap. Also, the air-lock from a piece of the clear tube running into a jar filled with water will keep microbes out and demonstrate the fermentation is working.
A variation is to spray seltzer water on the plants twice a day. This is not recommended by some authorities and receives great raves by people who seem to feel it has enhanced their crop. Use seltzer, not club soda, since it contains less sodium that could clog the plant’s stomata. Wash your plants with straight water after 2 or 3 seltzer sprays. It is a lot of work, and you can not automate it, but maybe that is good! Seltzer is available at most grocery stores. Club soda will work if seltzer water is not available, but it has twice as much sodium in it. A very diluted solution of Miracle Grow can be sprayed on the plant at the same time. One factor of using seltzer water is it raises humidity levels. Make sure your vent humidity during the dark cycle, or you could risk fungus and increased internode length.
CAUTION: Do not spray too close to a hot bulb! Spray downward only, or turn off the lamp first.
Even though CO2 enrichment can mean 30-100% yield increases, the hassle, expense, space, danger, and time involved can make constant or near-constant venting a desirable alternative to enrichment. As long as the plant has the opportunity to take in new CO2 at all times, from the air that is over 200 ppm CO2, the plants will have the required nutrients for photosynthesis. Most closets will need new CO2 coming in every two or three hours, minimum. Most cities will have high concentrations of CO2 in the air, and some growers find CO2 injection unnecessary in these circumstances.
Some growers have reported to High Times that high CO2 levels in the grow room near harvest time lower potency. It may be a good idea to turn off CO2 2 weeks before harvesting if you want to grow more Cannabis with CO2.