Carbon Dioxide is an odorless gas that makes up about 300 parts per million (ppm) of our ambient atmosphere, yet a plant can utilize up to 1600 ppm. The concentratino of CO2 in the atmosphere has varied greatly over the past millions of years—meaning plants hold the inate compacity to thrive in myriad environments leaving ample room for experimentation. Dried plant material contains an average of 40% carbon that comes entirely from CO2. Therefore, we need to consider CO2 to be a major plant nutrient, one that affects growth rate, yield, and one that needs to be supplied in adequate quantities if crop growth is going to be achieved and maximized.

CO2
The main plant process a grower needs to consider is 'photosynthesis '

Photosythesis : The process by which carbon dioxide and water are combined in the presence of light energy and chlorophyll to form carbohydrates. Photosynthesis takes place in the plant cell's chloroplasts. Inside the chloroplasts, chlorophyll absorb light energy from the sun. The chloroplasts then use that energy to jumpstart the process of photosynthesis. The carbohydrates/ sugars are the plant's internal energy storehouse; they are used to build and maintain plant tissue.
, as this is what drives growth, development, and production. Photosynthesis is a reaction that occurs within the leaf tissue and requires light of the correct wavelength, water, and carbon dioxide to produce assimilates which are used for growth and development. As a by-product, oxygen is released into the environment. When artificial lights are used to grow plants, the aim is to provide just the right intensity and wavelengths for optimal photosynthesis in an enclosed environment. Provided your indoor garden has sufficient water, light, and nutrients, the limiting factor in the process of photosynthesis in an enclosed environment then becomes the availability of carbon dioxide (CO2). In a well-sealed growing environment situation, CO2, under good lighting, begins to limit photosynthesis very rapidly. Since ambient CO2 levels in the air are around 300 ppm, this can be used up by even a small population of actively photosynthesizing plants within a couple of hours. When this happens, if the CO2 is not replaced, photosynthesis and plant growth is hindered or stops completely. This can be prevented with a good ventilation system. An exhaust fan will create negative pressure ensuring the influx of fresh air from without. CO2 pucks can be used to ensure CO2 presence in rooms that are not ventilated.

Think of light and CO2 like a seesaw. Ideal light levels for indoor gardening run anywhere from 25-100 watts/ sq.ft. Say you are increasing CO2 levels to 1600ppm and you only have 25 watts/sq.ft. of light on your garden. You will no doubt experience a faster and more significant rate of growth, but no where near what you would if you increased the light level on your garden. There is simply not enough energy coming into the plant to utilize the increased amount of CO2 available in the environment. Conversely, if you increase light levels, say upwards of 50 watts/sq.ft. and do not increase CO2 levels you are not doing the increased light levels justice due to the limiting factor of CO2 availability. A firm grasp of photosynthesis will allow you to get more bang for your buck in your indoor garden. If you have questions, ask PG.

CO2 Enrichment
Not only is it important to prevent CO2 depletion, but also beneficial to enrich to levels much greater than atmospheric levels. Higher than ambient CO2 levels are known to boost plant growth by over 40%. The level of enrichment and the timing of enrichment are crucial, since all consistent and reliable methods of CO2 enrichment have a cost involved. Obviously since plants only require, take up, and use CO2 when photosynthesizing in light, enrichment only needs to occur when the lights are on or during daylight hours. Enrichment at night is pointless since the extra CO2 will not be taken up by the plants and will just accumulate. Emphasis needs to be placed on regulating levels during photosynthetic periods. Secondly, enrichment levels need to be high enough to replace the CO2 used by the plants and to increase the levels of CO2 in the environment to a level where it will accelerate photosynthesis and therefore plant growth. Levels of 800-1600 ppm have proven to be optimal for plants grown using enclosed cultivation. CO2 regulation and monitoring equipment then becomes important to make sure this level is reached and maintained consistently. A fluctuating CO2 level will not allow the plant to acclimate itself to the given environment, in turn, not allowing the plant to use the higher levels of CO2 available to it. In other words, it will acclimate to the lowest common denominator. CO2 enrichment will have its greatest effect on accelerating photosynthesis and growth when all other factors are also optimal - when there is sufficient light for photosynthetic reactions (45-50 watt per square foot), nutrient for plant implementation, and temperatures and humidity are in desired range. In short, get your environment right before implementing CO2 in your growroom. Temperatures can be run a little higher where CO2 is enriched and light levels are at optimum levels - generally in the range of 80 F (27 C) to 92 F (32 C) day temperatures for most fruiting and flowering plants.

Supplying CO2
The two most commonly used methods used for CO2 enrichment of a growing area are burning of hydrocarbon fuels such as natural gas or propane and compressed, bottled CO2. But CO2 is CO2. There are actually a few other, less practical ways - these are dry ice, fermentation, burning of candles and oil lamps (PG NEVER recommends fire other than commercial generators for CO2 implementation), decomposition of organic matter, and respiration. Ever wonder why people talk to their plants!

CO2 generators are widely available for use in growing areas and are less expensive than using bottled CO2 due to the inexpensive availability of propane tanks and natural gas. The major problem with burning fuel to create CO2 is that heat is produced as a by-product. This may be useful under cooler conditions, but not if the growing area is sufficiently warm. This happening can be controlled by tweeking your environmental controls. As the CO2 is introduced, it needs to be thoroughly mixed with use of a circulation fan.

Compressed, bottled CO2 is a safer option for plant enrichment in that no toxic by-products or additional heat can be produced. Compressed CO2 comes in cylinders stored under high pressure (1600-2200 psi). CO2 is injected into the growing area via the pressure regulator and flow meter, which is controlled via a solenoid valve and timer. One pound of compressed CO2 gas contains about 8.5 cubic feet of CO2 gas at normal atmospheric pressure.

No matter which method of enrichment is used it is important to bring the environment up to the predetermined level and then constantly replenish to this level as the plants absorb the CO2 using an exhaust fan and a timer. The rate of CO2 absorption will change with plant size, temperatures, and light level. This is why constant monitoring of levels is important.

Measuring CO2
There are a few devices which can be used to measure and monitor CO2 levels in your growing area. There is a range of CO2 sensors available- from single 'syringe' test kits, which allow a grower to take a sample of the air in the growing area and determine the CO2 level, to using PPM devices. The most common way to ensure consistency with CO2 is to measure the cubic feet of your room and set your flow rate accordingly. In other words, a respective flow rate will inject CO2 at a certain rate than can be jived with the amount of space to fill, which results in a reliable PPM of the CO2 in the grow space.

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