Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible

air by the same stomata the plant uses to absorb CO 2 during transpiration. Carbon dioxide enrichment affects transpiration by causing the plants’ stomata to partially close. This slows down the loss of water vapor into the air. Foliage on CO 2 -enriched plants is measurably thicker, more turgid, and slower to wilt than leaves on non-enriched plants.
    Carbon dioxide affects plant morphology. In an enriched growing environment, stems and branches grow faster, and the cells of these plant parts are more densely packed. Flower stems carry more weight without bending. Because of the increased rate of branching, cannabis has more flower initiation sites. Plants that sometimes do not bear from the first flower set are more likely to set flowers early if CO 2 enrichment is used.

Inexpensive electronic components have made it possible for manufacturers to lower the price on CO 2 monitors.
    With CO 2 -enriched air, plants that do not have the support of the other critical elements for life will not benefit at all, and the CO 2 is wasted. The plant can be limited by just one of the critical factors. For example, the plants use water and nutrients a lot faster, and if they are not supplied, the plants will not grow. They might even be stunted.
    To be most effective, the CO 2 level must be maintained at 1000 to 1500 ppm everywhere in the room. To accomplish this, the grow room must be completely enclosed. Cracks in and around the walls should be sealed off to prevent CO 2 from escaping. Enclosing the room makes it easier to control the CO 2 content of the air within. The room must also have a vent fan with flaps or a baffle. The vent fan will remove the stale air that will be replaced with CO 2 -enriched air. The flaps or baffle will help contain the CO 2 in the enclosed grow room. Venting requirements will change with each type of CO 2 -enrichment system and are discussed below.
Measuring CO 2
    Measuring and monitoring CO 2 levels in the air is rather expensive and often unnecessary for small growers. Monitoring CO 2 levels in grow rooms with ten or more lights really helps keep the levels consistent.
    Disposable comparative colorimetry CO 2 test kits are easy to use, accurate, and inexpensive. The test kits contain a syringe and test tubes and sell for about $30. To use the kit, break off each end tip of the test tube, and insert one end into the closed syringe. Pull 100 cubic centimeters into the syringe, and note the blue color change in the cylinder where the active ingredient reacts with the CO 2 in the air drawn through the cylinder. These kits are reliable to within 40 ppm.
    Electrochemical sensing systems measure electrical conductivity of an air sample in either an alkali solution or distilled or deionized water. These systems are relatively inexpensive, but they have drawbacks: limited accuracy and sensitivity to temperature and air pollutants.
    Infrared monitoring systems are more accurate and versatile. They correctly measure CO 2 and can be synchronized with controllers that operate heat, ventilation, and CO 2 generators. Even though the initial cost for a monitor is high, they can solve many CO 2 problems before they occur and can ensure optimum growing conditions. Specialty indoor garden stores sell the monitors for less than $1000.
    Growers who do not want to spend the time and energy required to monitor CO 2 can use a set of scales and simple mathematics to determine the approximate amount of CO 2 in the air, but this calculation does not account for ventilation, air leaks, and other things that could skew the measurement. It is easier to measure the amount of CO 2 produced rather than to measure the amount of CO 2 in the grow room’s atmosphere.
    To measure the amount of fuel used, simply weigh the tank before it is turned on, use it for an hour, and then weigh it again. The difference in weight is the amount of gas or fuel used. See the calculations (page 340) for more information on calculating the amount of CO 2 in the room.
Producing CO 2
    There are many ways to raise the CO 2 content of an enclosed grow room. The two most popular ways are to disperse it from a tank or burn a fuel to manufacture it. Carbon dioxide is one of the byproducts of combustion. Growers can burn any fossil (carbon-based) fuel to produce CO 2 except for those containing sulfur dioxide and ethylene, which are harmful to plants. Carbon dioxide gas is a by-product of fermentation and organic decomposition. The CO 2 level near the