CO2, aka Carbon Dioxide

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 marijuana plants ability to manufacture these sugars and 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 your strictly growing vegetatively indoors, (transferring your marijuana 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 used CO2 canisters from wielding 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 $100
Regulator $159
Timer or controller $10-125
Fill up $15-20
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Worst case = $395 for CO2 tank setup synced to a exhaust fan with a thermostat.

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 marijuana plants at all times the lights are on, dispersed directly over the marijuana plants during the time exhaust fans are off.
Opportunities exist to conserve CO2, but this can cost money. When the light is off you don’t need CO2, so during flowering, you will use half as much if you have the CO2 solenoid setup to 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 marijuana plants exposure times and amount of gas actually dispensed.
Environmentally, using bottled gas is better, since manufacturing it adds to greenhouse effect, and bottled CO2 is captured as part of the manufacturing process of many materials, and then recycled. Fermenting, CO2 generators, and baking soda and vinegar methods all generate new CO2 and add to greenhouse effect.
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 marijuana plants level. Fire and exhaust venting of the heat are 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 don’t 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’s 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.
Try 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 marijuana plants. A MUCH cheaper way to provide CO2 is 2 Oz sugar in 2 liters 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 next 2 weeks or so it will brew up about ½ 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 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 marijuana 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. It stands to reason this would work for only a small unvented closet, but may be right for some situations. It could get expensive with a lot of marijuana plants to spray. Use seltzer, not club soda, since it contains less sodium that could clog the marijuana plants stomata. Wash your marijuana plants with straight water after 2 or 3 seltzer sprays. It’s a lot of work, and you can’t automate it, but maybe that’s good! Remember, being with the marijuana plants is a beautiful experience, and brings you closer to your spiritual self and the earth. Seltzer is available at most grocery stores (I get it at Lucky’s @ .79 for a 2 litter bottle). 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 selzter water is it raises humidity levels. Make sure your venting humidity during the dark cycle, or you could risk fungus and increased internode length.
CAUTION: Don’t 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 air that is over 200 ppm CO2, the marijuana plants will have the required nutrients for photosynthesis. Most closets will need new CO2 coming in every two or three hours, minimum. Most citys’ 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.

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