by Ed Rosenthal
Once CO2 is absorbed into the plant, it is directed to the chloroplasts—the plant organelles that contain light-absorbing chlorophylls—where photosynthesis takes place.
Photosynthesis consists of a complex series of reactions in which light energy is used to convert carbon dioxide and water to sugar, releasing oxygen as a byproduct.
The amount of CO2 in the air has a profound effect on the rate of photosynthesis and plant growth. Photosynthesis speeds up as the amount of CO2 in the air increases, as long as there is enough light to power it. Conversely, as the CO2 content of the air falls, photosynthesis slows to a crawl and virtually stops at a CO2 concentration of around 200 ppm, no matter what the other conditions. Lacking CO2, plants continue respiration and growth for a short time, until their sugars are used up; then they slow down their metabolism to conserve energy. Only when more CO2 is available can the plant processes continue.
Outdoors, breezes and the exchange of gasses in the air constantly replace the CO2 that plants consume. This provides enough CO2 for vigorous growth, and outdoor growers rarely think of the gas as a limiting factor, even though growth of some plants, including cannabis, is not maximized in the Earth’s present atmosphere. In fact, the 380 ppm of CO2 found in earth’s atmosphere is on the low end of the continuum of most plants’ ability to use it as fuel for photosynthesis.
Outdoor plants growing in the bright light of summer grow heavier and faster when supplemented with CO2. Raising the level of CO2 up to 0.15% (1500 ppm), or a little more than four times the amount usually found in the atmosphere, increases plant growth rate significantly. Enhancing growth outdoors using increased CO2 is discussed in the supplementation section.
CO2 is not dangerous. It is a non-flammable gas. It is non-toxic at the low levels growers employ. CO2 can pose health risks in extreme concentrations (above 50,000 ppm), but this level is more than 30 times the maximum plants find useful.
When plants are growing in an enclosed area, there is a limited amount of CO2 for them to use. Under bright lights, CO2 is used up quickly. Enclosed gardens with no ventilation are also rapidly depleted to the point where the photosynthesis rate slows to a virtual stop at 200 ppm. Only when more CO2 is added to the mix does photosynthesis resume.
A closed closet or other small gardening space can be recharged with CO2 simply by opening the door or curtain to let in fresh air. This increases the CO2 content of the closet passively, as air naturally equalizes the concentrations of oxygen (O2) and CO2 inside and outside the growing space, exchanging the higher O2 levels with CO2. Adding a small fan expedites the air exchange.
The rate of photosynthesis has the greatest increase as the CO2 level climbs from 0-200. Under low-light conditions (150 mols or 1150 fc) (12,330 lux), the rate of photosyntheses increases as CO2 rises to 400 ppm. Increasing the CO2 concentration beyond that without increasing light intensity does not result in a higher rate of photosynthesis. The plant cannot take advantage of higher CO2 levels until the light intensity increases.
Light level, temperature and CO2 level must all increase for the plant to utilize resources most efficiently. At less than 100 ppm of CO2 photosynthesis does not take place and plants suffer a net loss of sugar due to respiration. At 100 ppm CO2 respiration and photosynthesis are equal so there is no net loss or gain. Photosynthesis increases quickly as the CO2 levels climb to 400 ppm. The increase in photosynthesis is more moderate as the CO2 concentration climbs to 800 ppm. The increase is more moderate, but significant between 800 and 1200 ppm.
At a light intensity of 600 mol (4600 fc) (49,310 lux), the photosynthesis rate increases more as CO2 concentrations are increased to 400 ppm. The rate of increase declines a bit after that, but the photosynthesis rate continues to increase as CO2 levels reach 600 ppm. Above 600 ppm of CO2, the photosynthesis rate continues to climb but at an even slower rate, until the rate increase levels off at about 1200 ppm.
PHOTOSYNTHESIS BRIEFLY
Chlorophyll pigments absorb light and convert it to electro-chemical energy. This energy is used to cleave water and combine the H with CO2 to form suger and release oxygen.
6(H2O) + 6(CO2) + LIGHT=> C6H12O6 + 6O2
COMMON MYTHS ABOUT CO2 DEBUNKED
•CO2 enrichment is like chocolate cake for your plants—you can’t give it to them all the time.
•Your plant can overdose on CO2.
•All you need is good ventilation—extra CO2 will not help.
•Plants need fresh air, keeping them in a closed system is imprisonment.
•The only time plants need CO2 is when other conditions aren’t right.
•Plants grow immune to CO2.
ALL FALSE!
By increasing the light intensity, you encourage your plants to absorb even more CO2 which increases growth and yield. When the plants receive between 4500-5500 fc (48,240 lux) of light, they can utilize between 1200-1300 ppm of CO2. While very few gardens are supplied with more than 7500 fc (80,400 lux) of light, at that intensity the plants can use up to 1500 ppm of CO2, the enrichment rate recommended by some manufacturers.
Marijuana uses CO2 only when it is receiving light. Enrichment during the dark period has no effect.
You can supply CO2 to your plants easily and cheaply. The most convenient way to do this is by using a meter, regulator, and tank kit. There are other ways, too. Instead of using a tank, you can use a meter that regulates a CO2 generator that burns propane or natural gas. You can also use metabolic and chemical processes to produce CO2, or obtain dry ice, which sheds CO2 as it evaporates.
CO2 TANKS
The easiest way to supply the gas is to use a CO2 tank kit. The kit consists of a CO2 meter, pressure regulator, and a solenoid valve. For most gardeners, 20 or 50 pound tanks (the weight of the gas), are the most convenient. Tanks can be bought or rented. Steel tanks weigh twice as much as aluminum tanks, so a steel tank that holds 20 lbs. of CO2 has a gross weight of 50 lbs., and an aluminum tank weighs about 40 lbs. filled. The 50 pound tanks weigh respectively 170 and 110 lbs. when filled.
A 20 lb (9 kg) aluminum tank weighs about 35 lbs (16 kg).
CO2 regulators attach to CO2 tanks and allow the grower to pre-set and adjust the amount of CO2 being released into the room. This CO2 regulator is adjustable from 0.5-15 cubic feet (0.014-0.42 cubic meters) per hour and is available from C.A.P. Controllers. The pressure regulator, flow rate valve and solenoid switch which opens and closes the valve are all regulated by the ppm meter.
You can calculate how much CO2 is needed to bring a growing area to 1,000 ppm by multiplying the cubic area of the growing room (length x width x height) by .001. The total represents the number of square feet of gas required to reach optimum CO2 range. For instance, a room 13’ x 18’ x 12’ contains 2808 cubic feet: 2808 x .001 equals 2.8 cubic feet of CO2 required. A room 3 x 4 x 3 meters contains 36 cubic meters and would require .036 cubic meters of CO2.
Although CO2 tanks are a bit cumbersome to lug around and are more expensive than other methods of supplying CO2, they are still the best solution for most gardeners. The reason is that they don’t run the risk of degrading the garden environment, as compared with CO2 generators. Tanks release nothing but cool CO2, while CO2 generators also release heat and water vapor, neither of which is helpful in most gardens.
CO2 tank systems use a regulator/emitter system to control the amount of gas being released, and consequently, the concentration of CO2 in the garden.
•A regulator that standardizes the pressure.
•An adjustable CO2 flow meter that controls the amount of gas released over a given time period.
•A solenoid valve that shuts the gas flow on or off.
All systems include a CO2 controller. The best units have a sensor that constantly measures the ppm of CO2 in the air and turns the flow on or off to maintain a ppm you set. These systems keep an accurate gauge of the CO2 in the air, eliminating guesswork and unwanted fluctuations.
The yield of your indoor harvest is dependent on the l
imiting factors. Many products automatically measure and regulate the climate of the room, freeing up the grower’s time. The Harvest Master can be set to control the light cycle, exhaust fans, CO2, temperature and humidity.
Another type of controller injects CO2 into the room on a timed basis. These units aren’t as helpful because they are set and release CO2 according to your guesstimates, not accurate measurement of conditions. Some units coordinate release with ventilation fans and lights so the whole system works automatically.
CO2 SYSTEM SET-UP
In most tank systems, CO2 should be released just above the plants. The gas is heavier and cooler than the air so it sinks. As it flows downward, it reaches the top of the canopy first. This is where most of the light touches the leaves and where most of the plants’ CO2-consuming photosynthesis takes place. A good way to disperse the gas is by using inexpensive “soaker hoses” sold in plant nurseries and gardening stores. These soaker hoses have tiny holes along their length that disperse the gas.
Some models of premade grow cabinets come with CO2 equipment.
www.actechwi.com
In this garden air is enriched as it passes through the tubes. The reflectors, with no bottom glass, draw in air as it exits through a carbon filter.
Some systems circulate air through the plant canopy, drawing it up towards the ceiling. In this kind of system, the CO2 should flow from the tube just below the canopy, so it is pulled up to the top. Another method of enriching a space with CO2 is to add it to the air intake, so all new air is enriched. This is especially useful when a space has constant or frequent ventilation. Whichever set up you choose, place the tank where it can be replaced easily.
CO2 REQUIRED FOR LIGHT USAGE
CO2 UPTAKE WITH SUPERCHARGING AND NO LIMITING RESOURCES
CO2 REQUIRED FOR LIGHT USAGE: The amount of light, CO2 and nutrients required for fast growth rise in direct ratio to each other. CO2 UPTAKE WITH NO LIMITING FACTORS: As the intensity of light increases the plant requires higher temperature and higher concentration of CO2.
1 pound of CO2 (0.45 kg) = 8.7 cubic feet (0.246 cubic m)
MANUAL CO2 CONTROL
Before CO2 meters and controllers were available, growers had to guesstimate the amount of CO2 required by figuring the cubic area of the space, then estimating the depletion of the gas over time. From this, they would calculate the flow rate adjustment and set a timer used to control the solenoid valve. While this method is not as efficient as exact measurement with a meter, and the CO2 levels will vary, affecting growth, this type of equipment is still available.
If you’re using such a system, the ratio of gas to space depends on the ppm of gas desired:
•600 ppm is 1:666 (multiply cubic area by .0006) A room of 1,000 cubic feet (28 cubic meters) requires 0.6 cubic feet (.0168 cubic m) of CO2.
•1000 ppm is 1:1,000 (multiply cubic area by .001) A room of 1,000 cubic feet requires 1 cubic foot (0.028 cubic m) of CO2.
•1200 ppm is :1,000,000 (multiply cubic area by .0012) A 1,000 cubic foot (28.8 cubic m) room requires 1.2 cubic feet (0.033 cubic m) of CO2.
To calculate how long the valve should remain open to reach that level of CO2 concentration in the room, divide the number of cubic feet of gas required by the flow rate of the tank’s valve. For instance, if the tank’s flow rate is 50 cubic feet per hour, 2.8÷50 gives us 0.024 hours or 1.44 minutes (0.024 hours x 60 minutes = 1.44 minutes).
In a warm, well-lit room with no outside ventilation, the gas should be replenished every 10 minutes when the plants have filled out the canopy. When the plants are smaller or in a moderately lit room, they do not use the CO2 as fast. The gas should also be replenished each time after the room is ventilated or opened, since the enriched air will have evacuated.
GENERATORS
CO2 generators are much less expensive to operate than bottled CO2 injection systems. They create carbon dioxide inexpensively by burning either natural gas or propane. They are safe to be around and burn cleanly and completely, leaving no toxic residues and creating no carbon monoxide (a colorless, odorless, poisonous gas).
Generators emit CO2, water, and heat. Each pound of natural gas or propane burned produces about three pounds (1.36 kg) of CO2, one pound (0.45 kg) of water, and about 21,800 British Thermal Units (BTUs) of heat. Other gasses and fuels produce different amounts of energy, per unit burned.
If your growing area is cool or cold, the heat from a CO2 generator can be useful in keeping the space warm, as well as supplying CO2 and humidity to the garden. In a warm space, the generator’s heat must be dissipated to maintain the moderate temperature levels necessary for optimal growth. Some CO2 generators use water-cooling to absorb the heat. The heated water is cooled outside the garden area, eliminating the temperature problem. However, the CO2 enriched air still contains the moisture that was created.
Nursery supply houses sell large CO2 generators especially designed for greenhouses. Indoor garden centers typically sell smaller generators more appropriate for indoor gardens. Even a small generator unit can raise CO2 levels very quickly.
OTHER METHODS TO ENRICH AIR WITH CO2
There are other ways of bringing CO2 into the garden space. They include water heaters and other gas appliances, dry ice, chemical reactions, and biological processes such as composting, fermentation, and animal respiration.
GAS APPLIANCES
Like CO2 generators, water heaters and other gas appliances produce CO2 as they burn natural gas or propane. If the water heater is in the garden space—or if its exhaust is vented into the room—it enriches the garden with CO2 whenever the gas turns on. Although the gas may be produced irregularly rather than on a schedule, each time the garden air is enriched, the photosynthesis and growth rate spurts. Other appliances that create CO2 output are gas-powered furnaces, clothes dryers, and stoves.
This is a single plant. A water heater (seen in lower photo) intermittently provided CO2 to the plant. Notice the difference in growth between the enriched and unenriched side. The plant grew 20’ high and 30’ wide.
A. During a sunny day in the winter period
B. As compared to outside air: CO2 concentration is too low for healthy plant growth.
ASK ED: Marijuana Questions
CAR FUMES FOR PLANTS
The only decent outdoor spots around here are near the highway. There’s a lot of traffic—four lanes each way. Since a portion of the fumes from vehicles contains CO2 would planting near a highway be beneficial or bad for my plants? I’ve been growing along the highway for the last few years. Some plants grew only a few feet (approx 0.9 m) tall, but others grew to a height of almost ten feet (3 m).
Car fumes are not good for plants or humans. The exhaust from clean burning cars contain carbon dioxide (CO2); carbon monoxide (CO), which is toxic; carcinogenic alkenes and polycyclic hydrocarbons; various lung damaging acids, such as nitric oxide (which is what comprises smog); heavy metals, and other harmful gasses and particles.
Diesel vehicles produce even more poisons than gasoline engines. Every so often a broken, old or poorly maintained vehicle passes by, releasing considerably more toxins into the air. Plants absorb these particles through the stomata. When the exhaust fumes are in the air they absorb them, too. Some hydrocarbons and small particles wind up as a coating on top of the soil, or get washed into the ground and are drawn into the plant by the roots.
Plants growing beside a well-used highway should not be ingested or smoked. In the interest of harm reduction, I have some tips for people who insist on growing by the side of the road:
If there is a prevailing wind, plant upstream from the highway rather than downstream. That way, the pollution moves away from the plants.
Plant as far away from the highway as possible. Pollution at 25 ft (8 m) is much worse than at 50 feet (16 m) from the road.
Plant in an area with a physical barrier. A good place to plant would be an area that is protected from the road by some barrier such as trees, brush or a
wall.
Plant on a flat rather than hilly part of the road. Even accounting for the lack of gas used going down hill, cars use more gas in hilly terrain. When more gas is used, more pollution is generated. Also, engines under more stress as they navigate hills are more likely to emit pollutants.
Plant uphill from the road. Pollutants are more likely to travel downhill rather than uphill.
Still, your plants and you will both be happier and healthier if you choose to garden away from such toxic influence.
DRY ICE
Dry ice is CO2 that has been cooled to -109˚ F (43˚ C), the temperature at which it becomes a solid. Each pound (0.45 kg) of the ice evaporates to 8.7 cubic feet (0.25 cubic m) of gas. Dry ice usually comes in 30 pound (13k kg) blocks and costs about the same as CO2 in tanks. Dry ice evaporates CO2 at different rates, depending on the temperature of the surrounding air—about 7% a day when kept in a freezer and considerably faster at room temperatures. Since it is hard to control the rate of evaporation, and dry ice is inconvenient, even dangerous to handle, most growers conclude there are easier ways to provide a garden with CO2.
One cubic foot (0.028 cubic m) of gas increases the percentage of gas in a 1,000-cubic-foot (28.3 cubic m) room (10 x 10 x 10) by 1,000 ppm.
CO2 Boost uses organic matter as food for bacteria that digests it, and releases oxygen. Everything is contained in the kit. It produces a steady stream of CO2 for about two weeks. Then the organic matter is replaced.
SELTZER SPRAYS