CARBON DIOXIDE (CO₂)

Carbon dioxide (CO₂) and water are the two raw materials required for plant photosynthesis. CO₂ constitutes roughly around 0.042% or 420 parts per million (ppm), and rising, of the Earth’s atmosphere at the time of writing.

Cannabis uses CO₂ only in the presence of light. CO₂ supplementation during the dark period has no benefit to the plant. Photosynthesis and CO₂ absorption occur immediately after the plant receives light, and the plant’s stomatal pores open.

The plant mines CO₂ from the air by opening its stomata, tiny organs found primarily on the underside of the leaf. The stomata are responsible for the gas exchange of oxygen (O₂) and CO₂ into and out of the plant, and for transpiration, or regulating how much water evaporates from the leaf through the pores. While human skin pores sweat to regulate temperature and moisture, plants use their stomata to regulate how much water evaporates from the leaf to cool off the plant.

Once CO₂ is absorbed into the plant, it is directed to the chloroplasts—the plant organelles that contain light-absorbing pigments—where photosynthesis takes place.

Photosynthesis consists of a complex series of reactions in which light energy is used to convert CO₂ and water to sugar, releasing O₂ as a by-product.

The Process

Chlorophyll converts light energy into an electrical charge, a free electron that is used to cleave water into its constituent elements hydrogen (H) and oxygen (O). The O is released into the air and the H is combined with CO₂ to make sugar.

The amount of CO₂ in the air has a profound effect on the rate of photosynthesis and plant growth. As long as there is enough light to power the process, photosynthesis speeds up as the amount of CO₂ in the air increases. Conversely, as the CO₂ content of the air falls, photosynthesis slows to a crawl and virtually stops at a concentration of around 200 ppm, no matter the other conditions.

Plants that lack CO₂ continue respiration and growth for a short time, until their sugars are used up, before slowing down their metabolism to conserve energy. This occurs on evenings following brightly lit days. The plant creates excess sugar, some of which is used to continue plant growth and maintain cellular function during the dark period. Only when light and more CO₂ is available can the plant processes continue.

Cannabis Is a C3 Plant

Cannabis is a C3 plant: CO₂ is fixed to a 3-carbon sugar molecule during photosynthesis. The plant gathers carbon dioxide only when it is photosynthesizing. With adequate light and water, the plant absorbs more CO₂ when the concentration in the air increases. This increases the plant’s photosynthetic efficiency, producing more sugars and resulting in faster growth.

Plants absorb CO₂ into their intracellular spaces through pores called stomata found on the underside of the leaf. Every time a stomatal pore opens up to allow CO₂ to diffuse inside, it transpires water.

Outdoors, breezes and the exchange of gasses in the air constantly replace CO₂ that plants consume. This provides enough atmospheric CO₂ for vigorous growth. Until recently outdoor growers rarely thought of the gas as a limiting factor. Now, some farmers are supplementing outdoor gardens and fields.

In fact, the 420 ppm of CO₂ found in the Earth’s atmosphere is on the low end of the continuum of C3 plants’ ability to use it as an ingredient 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 less than four times the amount usually found in the atmosphere, increases plant growth rate significantly.

The orange line on this chart represents relative net photosynthetic rate at different CO₂ concentrations. The rate of photosynthesis continues to increase quickly and then flattens out as the continued addition of CO₂ no longer improves the photosynthetic rate.

Enhancing growth outdoors using increased CO₂ is discussed at the end of the chapter. 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 photosynthetic rate slows to a virtual stop at 200 ppm. Only when more CO₂ is added to the air 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 O₂ and CO₂ inside and outside the growing space, exchanging the higher O₂ levels from the plants with CO₂ from outside. Adding a small fan expedites the air exchange.

The rate of photosynthesis has the greatest increase as the CO₂ level climbs from 0-200 ppm. Under low-light conditions the rate of photosynthesis continues to increase as CO₂ rises to atmospheric levels. 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.

At high light intensity the photosynthetic rate increases as CO₂ concentrations increase to atmospheric levels. The rate of increase declines a bit after that, but the photosynthetic rate continues to increase as CO₂ levels reach 800 ppm. Increasing the light intensity encourages the plants to absorb even more CO₂, which increases growth and yield. Above 800 ppm of CO₂, photosynthesis continues to climb but at a slower rate, until it levels off at about 1,500 ppm.

CO₂ is a nonflammable gas. It is nontoxic at the low levels growers employ. According to OSHA, CO₂ can pose health risks in extreme concentrations (above 5,000 ppm), but this level is more than three times the maximum plants find useful.

CO₂ Enrichment

Augmenting CO₂ in the garden increases photosynthesis, resulting in faster growth and bigger yields. Optimal levels range between 800 and 1,500 ppm. These concentrations are much higher than atmospheric carbon dioxide levels and can be achieved only through supplementation. Plants do not absorb CO₂ during the dark period, so CO₂ enrichment is only implemented when the lights are on. Achieving optimized results also requires increased lighting and temperature.

There are various ways to augment carbon dioxide in closed environment grow rooms. The most convenient way to do this is with a sensor, regulator, and tank kit. Other methods include using a sensor-regulated CO₂ generator that burns propane or natural gas. Metabolic and chemical processes such as fermentation also produce CO₂ and can be used to supplement a garden.

Common Myths about CO₂

The following assertions are completely false:

• Plants can overdose on CO₂
• All that is needed 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
• Plants need CO₂ supplemented both day and night

The use of CO₂ involves serious life-safety issues; CO₂ is regulated by OSHA as an asphyxiant. Any system used to apply it will need to include appropriate monitoring and alarm protocols, which are generally enforced by local authorities.

For both human and plant safety, food-grade bulk CO₂ delivered in liquefied form is the safest source, is certified as an organic practice by the USDA, and will not lead to any product contamination during testing. The use of propane burners to generate CO₂ gas is banned by some state and local fire codes. Note that incomplete combustion can produce deadly carbon monoxide, as well as ethylene, which is highly toxic to plants and can result in crop loss.

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, flow meter, and a solenoid valve. For most gardeners, 20 or 50 pound (9-22 kg) 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 pounds (9 kg) of CO₂ has a gross weight of 50 pounds (22 kg), and an aluminum tank weighs about 40 pounds (18 kg) filled. The 50 pound (22 kg) tanks weigh, respectively, 170 and 110 pounds (77-50 kg) when filled.

Although CO₂ tanks are a bit cumbersome to lug around and are more expensive than burning propane or natural gas, they are still the best solution for most growers. They don’t run the risk of degrading the garden environment, as compared with gas burners. Tanks release nothing but cool CO₂, while CO₂ generators release heat and water vapor, neither of which is helpful in most gardens.

Using burners to generate CO₂ runs the risk of producing a small amount of ethylene gas. Ethylene (C₂H₄) is also a plant hormone that is bioactive at concentrations as low as one part per billion. Ethylene can have many effects on the plant depending on the stage of growth the plant is in. It causes faster fruit ripening.

CO₂ tank systems use a regulator and emitter to control the amount of gas being released and, consequently, the concentration of CO₂ in the garden. These are the parts for a complete CO₂ supplementation system:

1. A regulator that standardizes the pressure.
2. An adjustable CO₂ flow meter that controls the amount of gas released over a given time period.
3. A solenoid valve that shuts the gas flow on or off.

These systems are usually sold as a single unit. Most systems include a CO₂ sensor that constantly measures the ppm of CO₂in the air and turns the flow on or off to maintain the ppm the gardner sets. These systems keep an accurate gauge of the CO₂ in the air, eliminating guesswork and unwanted fluctuations.

CO₂ System Setup

In most tank systems CO₂ should be released just above the plants or circulation fans. The gas is heavier and cooler than the air, so it sinks. As it flows down, 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 systems circulate air through the plant canopy, drawing it up toward 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 setup is chosen, tanks should be placed in a space where they can easily be moved so that they can be refilled.

The three lines represent relative photosynthetic rates at various light intensities. The three curves represent the rates of photosynthesis when the ambient CO₂ concentrations are low, medium, and high. The increase of relative photosynthetic rate is largest when the CO₂ is highest and lowest when the CO₂ concentration becomes a limiting factor. SOURCE: Erik Runkle, Michigan State University

Use of CO₂ in Commercial Gardens

In many jurisdictions the application of CO₂ to gardens is regulated for safety reasons. In some areas the use of portable 20 and 50 pound (9 and 23 kg) tanks are regulated, and safety precautions are in effect. They might include handling procedures as well as CO₂ detectors, which alarm when the levels exceed 2,500 ppm, which is considered a precautionary health hazard. Sometimes small tanks must be held in a safety zone outside the structure. The gas is transported to the growing area via tubing.

The lines represent the relative photosynthetic rate at high and low CO₂ concentrations across various temperatures. The optimal (highest) rates are represented by the peaks of the curves. At low/ambient CO₂ concentrations the optimal rate is closer to 78°F (26°C). At high CO₂ concentrations the optimal rate is closer to 85°F (30°C). SOURCE: Erik Runkle, Michigan State University

Commercial cultivators often use micro-bulk CO₂ tanks with capacities of thousands of liters of compressed, liquid CO₂. These tanks are mounted on a pad on the exterior of the facility. The liquid carbon dioxide is converted to a gas and is either piped directly to sensor-controlled valves in each grow space or supplied via specialized HVAC systems that can simultaneously provide air conditioning, dehumidification, and supplemental carbon dioxide to each cultivation zone. These large-capacity tanks can last for many weeks, depending on the size and needs of the facility. Tanks can also be outfitted with a communications module that alerts the gas supplier when it is running low, so they can schedule on-demand, CO₂ deliveries to the site.

CO₂ Generators

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 without leaving toxic residues or creating 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 (Btu’s) of heat. Other gasses and fuels produce different amounts of energy per unit burned.

One pound of CO₂ (~0.5 kg) = 8.7 feet³ (0.246 m³)

If the 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. Commercial use of CO₂ generators in some areas has been banned, so check local and state regulations.

The orange line represents average CO₂ concentrations throughout a 24-hour period in a closed-loop space with minimal ventilation. As lights turn on and photosynthesis starts, the CO₂ concentration dips as the plants absorb CO₂. Many plants close their stomata between 150 and 200 PPM CO₂. The curve bottoms out when the plants stop photosynthesizing. As photosynthesis slows, the CO₂ concentration increases. At night the plants are respiring and releasing CO₂, which is illustrated by the higher than ambient concentrations of 420 PPM CO₂ at night.

Alternative Methods to Enrich Air with CO₂

Other ways to bring CO₂ into the garden space include chemical reactions and biological processes such as composting, fermentation, and animal respiration.

Vinegar & Baking Soda

When vinegar and baking soda are combined, they make a salt and release CO₂.

In small gardens, a simple method for creating a controlled release of CO₂ is to drip vinegar into a solution made of baking soda and water. As the vinegar combines with the solution, CO₂ is released. Regulating the frequency of the drip controls the amount of CO₂ generated. This is not cost effective because baking soda and vinegar are expensive to purchase in the quantities needed to create significant amounts of CO₂.

To generate one cubic foot of CO₂, combine three quarts of 5% vinegar with 3.7 ounces of baking soda. Generating one cubic meter of CO₂ requires 106 liters of 5% vinegar and 3.7 kg of baking soda.

Fermentation

Fermentation is the process used to brew beer and make wine. All that is required to ferment alcohol and make CO₂ is a jug or container, malt or another source of sugar, and yeast. Yeast are single-cell microorganisms that have been used in baking and fermenting alcoholic beverages for thousands of years. In fermentation, yeast digest sugar and release alcohol and CO₂. A gross simplification of this process is:

(Sugar => Alcohol + Carbon Dioxide (CO₂)

C₆H₁₂O₆ => 2(CH₃CH₂OH) + 2(CO₂)

A single TNB Naturals CO₂ Enhancer can supplement 1,200 ppm of CO₂ in a 144 square-foot (13 m²) space for up to two weeks. The Enhancer is made from 100% organic ingredients and is activated by adding four cups (1 liter) of lukewarm water and shaking. Shake daily for optimal performance. Canisters should be placed or hung above the canopy, as the CO₂ will sink to the floor.

As yeast feast on the sugars, they release about half the weight of the sugars’ CO₂, so one pound (~0.5 kg) of sugar yields about half a pound (0.25 kg), or 4.35 cubic feet (0.12 m³), of gas. The yeast complete their meal in about four days. Then the solution must be replaced.

Beer-Making Kit

Yeast have a hard time processing sugar as the alcohol content climbs. Most beer yeast slow down as the liquid approaches about 5-8% alcohol. The maximum amount of sugar that yeast can process well is about one-tenth of the weight of the water. A gallon of water weighs 8 pounds (3.6 kg), so about 13 ounces of sugar is used. A liter of water weighs 1,000 grams, so use 80 grams of sugar.

Human Respiration

Each breath taken by humans releases CO₂. A 150-pound (68 kg) human produces an average of about 1 kilogram (2.2 lbs) CO₂ per day. In an hour a human releases 0.326 cubic feet of the gas (0.0092 m³).

Outdoor CO₂ Supplementation

Although carbon dioxide supplementation is a common practice when growing indoors, it is not as well known that outdoor supplementation is possible as well. Plants grown outdoors can benefit from CO₂ supplementation. They will grow sturdier and produce higher yields. Plants grown in tunnels or in greenhouses also benefit from CO₂ supplementation because they rapidly deplete the air of CO₂ as they photosynthesize, so air enrichment is very effective at increasing their growth.

Because CO₂ is expensive, it takes an automated prescriptive delivery system for it to be economical in ventilated environments. This requires “in-grow” sensors that measure light, temperature, humidity, and CO₂ levels, so that the gas is only applied when it is needed. It’s also important to target the application of the CO₂ directly into the foliar canopy, which is where plants take it in through stomatal pores in their leaves.

If done right, CO₂ enrichment can generate yield improvements of 50% or more. This is because it does the following:

• Helps create larger root structures, healthier stems, and more leaves during the vegetative state
• Improves a plant’s heat-tolerance by up to 10°F (~5°C), allowing photosynthesis to continue at high summer temperatures when plants would normally “shut down”
• Helps reduce plant transpiration and therefore foliar canopy humidity (think mold)
• Supplies added carbon during the flowering stage, leading to bigger and denser buds

Large Outdoor Spaces

CO₂ release is most efficient in large fields, where it is less likely to leave the cultivation site, even if it is moved by a breeze. Because CO₂ is heavier than air, it tends to move more horizontally rather than vertically. If it’s released in a field, it will spread to other plants rather than be lost to the air.

Releasing CO₂ to rows of plants in a field during calm, sunny weather can increase yields by 50% or more. Coordinating CO₂ release to fields from CO₂-generating facilities will remove it from the atmosphere and may result in carbon credits.

ExHale by Garden City Fungi is a natural source of CO₂ that creates a steady sustainable supply that can meet the enrichment needs of enclosed gardens of all sizes, from small personal to large-scale commercial gardens. Inside each ExHale cultivator bag is a non-fruiting mycelial mass that is growing on organic matter. This mycelial mass cultivates CO₂, and the micro-porous breather patch releases it continually into the garden. No manure is used, so it is odorless.

Outdoor CO₂ Tanks

CO₂ from tanks can be applied to plants growing outdoors, in small and large gardens as well as in fields. To enrich the air around a plant outdoors, run a CO₂ line from a tank up the stem of the plant so that the gas disperses around the canopy. CO₂ is heavier than air and is cool coming out of the tank, so it will sink after it is released. It is best to set up the CO₂ line at the level of the canopy to be as close to where the leaves absorb CO₂.

There are several conditions that should be taken into consideration, particularly light, temperature, and wind velocity. CO₂ is most effective in warm conditions. It will help when the canopy leaf temperature is above 70°F (21°C) and becomes more effective as it climbs into the 80s. Ideal conditions for this method are sunny or very bright with low wind speed, especially in small gardens.

There are several commercial systems that regulate release based on three factors: light, temperature, and the amount of CO₂ in the space’s atmosphere. Sensors and switches can be installed that are sensitive to weather conditions, so CO₂ is turned on only when there is bright light, little wind, and warm temperatures. Very often there is a period daily during summer and fall when the sun is shining and there is little wind. This most often occurs between 11 a.m. and 4 p.m., when the sun’s rays are most intense.

These systems utilize appropriately sized CO₂ tanks that are refilled by gas companies in much the same way as propane is delivered.

In small gardens it works best when the emitters are placed around the top of the plant in a Christmas tree-like spiral draped down around the plant, or by placing the emitters along the central stem. Emitters can also be placed on the top of the canopy of rows of smaller plants.

Compost Piles

Compost piles produce considerable CO₂ and can be placed in the center of a densely planted outdoor garden. As the CO₂ is emitted by the pile, breezes will carry it through the plants. A small compost container can be placed under a large plant. Because compost piles generate heat, the emerging gas will be warmer than the surrounding air and will drift up through the plant’s canopy. Moist organic mulches or thin layers of compost can also be placed over the soil. When they are provided with a bit of nitrogen, the microorganisms they hold become very active and release extra CO₂. Their activity is regulated by temperature, so they produce more CO₂ during the warmer daylight hours, rather than at night.

Compost can be used to increase the CO₂ content of both outdoor and greenhouse gardens. It is not necessarily smelly and yields a large amount of CO₂. Using compost or vermicompost (compost with worms) to generate CO₂ is environmentally friendly. When it’s ripe, the compost can be used to enrich the soil or to make compost tea.

About a sixth to a quarter of a compost pile starting wet weight is converted to CO₂, so a 100 pound (45 kg) pile could contribute 33-50 pounds (15-22 kg) of carbon to the gas. Carbon makes up about 27% of the weight and volume of the gas, and oxygen makes up 73%, so that the total amount of CO₂ created is 61 to 93 pounds (27.6-42 kg), produced over a 60-day period, in warm conditions. That comes to two to three pounds (0.9-1.3 kg) or 16-25 cubic feet (0.45-0.7 m³) a day. The gas is supplied both day and night, so much of it is produced when the plants won’t utilize it.

Adding a heavy layer of organic compost or mulch on top of the soil provides warmth and protection from the elements while increasing the CO₂ content around the plants as microorganisms feed on it, releasing the gas.

CO₂ & Roots

Roots have no use for CO₂ because they don’t photosynthesize. In fact, CO₂ in the soil or water can pose a problem because it can drive out much-needed oxygen. (See Oxygen.)

Supplementing CO₂ Outdoors

Mendocino Grasslands ran a trial of outdoor CO₂ supplementation in its tunnel greenhouses. AG Gas systems were installed in three greenhouses. As a control, three additional greenhouses with no CO₂ supplementation were set to the same conditions with the same cultivars.

This technology was tested and validated at the Center of Irrigation Technology in conjunction with California State University at Fresno and the University of California at Davis. The technology was used to determine if CO₂ injection could improve yields for outdoor, field-grown tomatoes. The data from this experiment showed a 120% increase in the yield of tomatoes (Goorahoo, Cassel, and Carstensen 2003).

Mendocino Grasslands reported yield increases of 27 to 52%, with some seasonal variance in the greenhouses that were supplemented. The lowest increase was reported for a cooler season crop, while the highest yield increase was reported for a warm season crop.

Photo: AgGas

After two seasons it was decided they had used a control crop long enough and added CO₂ enrichment to all the greenhouses. Since then, the gas has been added to an open field.

The gas is delivered to the plants the same way in both the greenhouses and the open field. Pressure-compensating drip tubing with microholes facing down is held in place about 15 inches (38 cm) above the plants. The height of its support poles is adjustable, so it is raised as the plants grow.

The Carbogation “black box” control unit adjusts gas flow in real-time to deliver optimum results based on extant field conditions. The unique configuration allows for prescriptive delivery of CO₂ gas to the plants. Photo: AgGas

CO₂ is used by plants only when there is enough light for photosynthesis. The system controls when the gas is released, and its flow rate from the tank is controlled by a software algorithm with sensors to measure light intensity, temperature, vapor pressure deficit, wind speed, and ppm of CO₂ in real time and to make adjustments continuously.

At first there were sensors in each greenhouse, but their reports were averaged so that all greenhouses received the same amount of gas. This was changed, so now each greenhouse gets individualized treatment.

Based on environmental conditions, the algorithm figures the correct CO₂ and ppm, and sensors measure and adjust it much like a thermostat. The algorithms controlling greenhouse and open field differ a bit in their calculations. The gas is used from the nursery stage until harvest, although CO₂ enrichment in the last 10 days is not that crucial, since plant growth has slowed.

At the farm it is stored in a rented 14-ton (13 metric tons) vacuum-jacketed tank, which keeps the gas cool without refrigeration. A tank this size provides enough gas for most of the summer. It is resupplied by the local gas company, which delivers it.

In general the best time for the plants to experience the gas release is during the brightest time of the day, under clear skies, between 10:30 a.m. and 4:30 p.m. This unit uses sensors to control the release of gas by time, light, vapor pressure deficit (VPD), temperature, humidity, wind, and desired ppm.

In spaces where swamp coolers are used, there is a ventilated flow of air, but CO₂ still makes sense because it increases yield so much.

In greenhouses ventilated using roof vents or reticulated roofs, the application of CO₂ will not be greatly affected for three reasons: CO₂ is being directed into a downward flow, toward the canopy; CO₂ is heavier than air so it naturally sinks; and CO₂ is being released from a tank where it is compressed. As it leaves the container, the CO₂ decompresses and cools. With horizontal air circulation in the greenhouse, the CO₂ will stay at plant level until it is used or vacated by fans.

Not all cultivars react the same way to CO₂ enrichment. Increased CO₂ does not affect quality.

Aside from the increased yield, Chance Franck, vice president at Mendocino Grasslands, made some interesting observations after using it for three years:

• The plants have less mold and fewer insect infestations.
• The plants seem to be healthier and less prone to stress.
• The buds are denser.

Notice the CO₂ tubing placed over the canopy. Photo: AgGas