Simple CO2 Sources for Plants in Grow Tents and Grow Rooms

Everyone has learned in school that plants prepare their own food by a process called photosynthesis in which sunlight, water, CO2 (carbon dioxide), and chlorophyll play an important part.

This means that along with water, fertilizers, and sunlight, plants need the right amount of CO2 in their surrounding environment for their healthy growth. But what if CO2 is not in adequate amount in the environment surrounding the plants?

Published: February 11, 2021.

co2 sources 1

This can happen in the indoor atmosphere, grow tents and grow rooms, although, in the outdoor environment, there is usually enough CO2. If a plant doesn’t get enough CO2, obviously its growth will be affected. It will be stunted, its leaves will be yellowed and it won’t bloom or produce fruits.

Therefore it’s important for gardeners who maintain grow tents or grow rooms to ensure their plants inside the grow area are getting enough CO2 to carry on photosynthesis and if they aren’t getting it, they should provide it via outside sources.

How Much CO2 is Needed by Plants?

Humans and other animals exhale CO2 during respiration which plants use up for photosynthesis. In return, they give oxygen to the atmosphere which humans and other animals inhale and survive. Thus, plants are living beings due to which the right amount of oxygen is maintained in the atmosphere.

It should also be remembered that at night, plants don’t carry out photosynthesis, since the sun is not there, and they too need to breathe. Thus at night plants inhale oxygen and give out CO2 just like other living beings.

The amount of CO2 is measured in PPM (parts per million).

Typically in a room, there is 300-400ppm of CO2. If one’s grow room has adequate circulation, they should be able to achieve the standard of 390ppm. At that level, they should find plant growth similar to what they see in their yard.

If the amount of CO2 drops below 200ppm, plants will stop growing. Understanding this can benefit one’s grow room because then the gardener can do everything correctly and have a fine-tuned system. Adding enriched CO2 levels to one’s grow room and grow tents can remarkably boost plant growth without having to change the way they grow.

If one is looking for accelerating the growth cycles of their crops, they should consider adding extra CO2 to their grow room and grow tent. As one increases CO2 levels up to around 1,500ppm, they can find plant growth accelerated. Growers have even found an increase of up to 100% in grow speed at 1,500ppm.

However, the gardener should avoid the temptation of increase the CO2 content up to 2,000ppm or above because then it becomes toxic to plants. Above 4,000ppm it becomes toxic even to humans.

Simple CO2 Sources for Plants in Grow Tents and Grow Rooms

Mushroom Patch/Exhale CO2 Bag Method

The task of mushrooms is to decompose organic matter and release CO2 back into the environment. It’s the natural byproduct of their metabolism. Thus a mushroom patch can liberate up to 2.5 lbs. of CO2 or approximately half of its weight. This method of CO2 generation is natural, efficient, and environment-friendly.

If the gardener is interested in growing mushrooms for CO2 generation, Oyster and Shiitake mushrooms are good options. Gardeners should remember that these kits will need high humidity to grow mushrooms. Oyster Patches are aggressive and start producing mushrooms faster than other kits and release CO2 most quickly. Thus they are perfect for fast-growing or bigger, flowering plants. Reishi, Maitake, King Stropharia, or Turkey Tail decompose slowly and are ideal for slower-growing or smaller plants that don’t need as much CO2.

Each patch will enhance a 4x4 feet area and has the ability to produce a few pounds of mushrooms during its lifetime.

But if a gardener is not interested in growing mushrooms, they can use any ready-to-use Mushroom Patch to add CO2 to their garden. The kits available in the market are low-maintenance. These are grown in a self-sufficient bag and need little care.

This can also be done through exhaling bags or CO2 bags which contain fungal mycelium which is a vegetative growth in soils. This is a type of fungus and mushrooms are just their above-ground blooms, and they produce CO2 24 hours a day without requiring expensive production units or refill bottles. The mycelial mass inside the vented cultivator in the bag produces carbon dioxide and the one-way breather patch keeps releasing CO2 continuously for up to 6 months.

But it should be noted that as CO2 is heavier than air, the exhale bag should be hung directly on the top of the grow room to deliver a CO2 shower over the plants.

If a gardener maintains a greenhouse, they can fill it with slower-growing mushroom species (or if they have the required skills, they can even inoculate their own mushroom bags or beds) to get the added advantage of additional CO2. Mycelium Running Oyster Patch or the Garden Giant work well for beds or companion planting. But they are not ideal for indoor or hydroponic gardens, where it’s important to maintain sterility and cleanliness.

Dry Ice Method

The dry ice method is a perfect method for small areas and thus for home gardeners. It’s ideal especially when some cooling effect is also desired.

Dry ice or solid carbon dioxide is extremely cold i.e. around 109°F below zero. Hence one should handle it only by wearing gloves. One can get it at meat packaging outlets and is relatively cheap. In a standard 8’ x 8’ x 8’ grow room, one would need around 0.8 lbs. of dry ice per day to increase the amount of atmospheric CO2 to 1300ppm. If the grow room is pretty warm, 0.8 lbs. can melt much quicker than 18 hours. This can be regulated by two methods:

Firstly one can cut just small pieces, around 0.1 lb. and add a new piece every two hours to the grow room.

Secondly, one can place the needed amount in an insulated Styrofoam box with a few small holes cut in it. This will reduce the rate of melting remarkably but it should be “tuned in” to achieve it just the right way so that 0.8 lbs. melts in the 18 hours of light “on time”. The gardener should keep the remaining dry ice in a freezer to prevent loss due to evaporation.

As CO2 is heavier than air, a good technique of dispensing it to the plants is to attach the container of dry ice to the light reflectors which are often placed over the plants. This way the CO2 will flow down through or over the lights evenly bathing the plants. If the gardener has installed a circulating fan, they should place the dry ice or its container directly in front of or behind it for even dispersion. The gardener should try to seal up the grow room as best as possible, particularly around the bottoms of walls and doors.

This method will cost about 60 cents per day for the standard sized 512 cu. ft. grow room. This method has the benefit of the cooling effect it generates.

Burning Hydrocarbon Fuels

Burning hydrocarbon fuels has been the most common and popular CO2 enhancement method for several years.

The most common fuels are butane, natural gas, propane, and alcohol. Any of these fuels that burn giving a white, blue, or colorless flame will produce carbon dioxide which is beneficial.

However, if it gives a red, yellow, or orange flame, it means that carbon monoxide is being produced due to incomplete combustion. Carbon monoxide is toxic to plants as well as humans even in the smallest quantities. Also, fuels containing sulfur or sulfur compounds should be avoided because they generate harmful byproducts.

Most commercial growers and big greenhouses use CO2 generators that use the method of burning fuels, but they are too large for private small greenhouses, indoor grow rooms, or grow tents.

For such gardeners, Bunsen burner, Coleman lantern, or a small gas stove are perfect options. However, these CO2 generators also generate heat as a by-product of CO2 production, which is rarely required in the controlled environment of a grow room or grow tent. However, it may be useful during winter growing and for cool area greenhouses.

The rate of CO2 generation is controlled by the rate of burning of fuel. In a gas burning CO2 generator that uses butane, propane, or natural gas, 3 lbs. of CO2 gas and around 1.5 lbs. of water vapor are generated, along with around 22,000 BTUs of heat. In the case of other fuels, these values may differ.

If a gardener has a grow room of size 8’ x 8’ x 8’ and they use ethyl or methyl alcohol in a burner or gas lamp at the rate of 1.3 oz. per day, the atmospheric concentration of CO2 would be enhanced to 1300ppm if the room is completely sealed.

300ppm is the normal CO2 concentration of the normal atmosphere. A 100% air exchange (leakage) every 2 hours is supposed to be the average air exchange rate in most grow rooms and tight greenhouses. If the grow room has several cracks and leaks, this exchange rate will be considerably increased but the added CO2 (above 300ppm) will be lost too. If the gardener has installed a vent fan, CO2 enrichment will be nullified because the fan will blow it out as soon as it’s produced.

A circulation fan is useful since it moves the air around the grow room or greenhouse. However, if the air is still, it can cause a ‘depletion layer effect’ due to which the CO2 will be formed right near the plant leaf and will be quickly depleted. If in that case, fresh air containing additional CO2 is not brought to this surface, photosynthesis and growth will reduce and eventually stop altogether.

Several factors are involved in keeping the CO2 concentration at the desired level:

- If the grow room consists of a large or compactly spaced plant, 20% to 30% should be added to the CO2 volume produced.

- If the temperature is increased from 70° F to 90° F, 20% should be added to the volume produced and vice versa.

- If the grow room or greenhouse is not tightly sealed, up to 50% should be added to the CO2 generator production volume.

If the gardener uses more light, more CO2 can be used up and so, should be proportionately generated up to the practical limit of 5,000 foot-candles per sq. yard and 1500ppm CO2 atmospheric content. If more CO2 is produced, more plant nutrients and water should be used again to a practical limit of 2 x normal. If one’s plants will grow faster due to CO2 enhancement, they will need nutrients and water in higher amounts too.

If a gardener uses different hydrocarbon fuels, they should consider the heat content in terms of B.T.U. If the BTU per hour rate is half that of ethyl alcohol, twice as much should be burned to produce the same approximate concentration of CO2 desired. The amount of CO2 produced depends on the fuel’s carbon content. One can get the literature from the suppliers about the BTU per hour heat content.

Compressed Bottled CO2

The next popular CO2 enrichment method is the compressed, bottled CO2. It offers quite accurate, controlled results. Compressed CO2 is available in metal containers under high pressure. There are small cylinders and large tanks that contain various amounts of compressed CO2. Pressure ranges from 1600 pounds per sq. inch to 2200 PSI.

co2 sources 3A gardener will need the following equipment to enhance the available CO2 with compressed gas:

- Compressed CO2 tank

- Flowmeter

- Pressure regulator

- Short-interval 24-hour timer capable of having an ‘on-time’ variable from 1 to 20 minutes

- Solenoid valve (metal or plastic)

- Connecting tubing, adapters, and fittings

Arrangement: This method allows the gardener to inject a controlled amount of CO2 in a grow room at a specific interval of time. The compressed gas pressure is reduced by the pressure regulator from 2200 lbs./sq. inch to a more controllable amount (100-200 PSI) which can be handled by the flow meter. The flow meter will deliver so many cubic feet of CO2 per minute for the duration of time for which the solenoid valve is opened. The job of the timer is to control the time of the day and the length of time for which the solenoid valve is open.

If one wants to add an adequate amount of CO2 in the standard 8’ x 8’ x 8’ grow room to increase the near depleted level of 200ppm to 1500ppm, one should add 1300ppm of CO2 to a 512 cu. ft. of volume. It’s preferable to do this in intervals of time relative to the natural air exchange rate (leakage rate) to maintain the CO2 level near the 1500ppm range.

Each pound of compressed CO2 gas consists of around 8.7 cu. ft. of CO2 gas under atmospheric pressure. Compressed CO2 costs around 50 cents/lb. Thus at the rate of 0.66 cu. ft. every two hours for 18 hours per day, this method will cost about 30 cents per day.

The method of compressed CO2 gas enrichment has the benefit of quite accurate control and readily available equipment, and it doesn’t add additional heat to the grow room. It also works fine for small growing spaces and besides the initial equipment cost, it’s not costly to operate.

Fermentation Method

The underlying principle of this method is that sugar, when fermented using yeast, produces ethyl alcohol and CO2. One needs the following ingredients and equipment for this method:

- Suitably sized glass or plastic container

- Sugar – common or invert (corn)

- Yeast – brewers or bourgeois wine yeast

- Yeast nutrient

- ¼ plastic tubing

- Sealant, cellophane, tape, or lid

- ¼ shutoff valve

- Starter jar or bottle

- Balloon

One pound of sugar will create around half a pound of CO2 and half a pound of ethyl alcohol (C2H5OH). A pound of CO2 makes 8.7 cu. ft. of CO2 gas at normal atmospheric conditions.

In a standard 8’ x 8’ x 8’ grow room, one will have to produce 512 cu. ft. x 0.0013 (1300ppm CO2) = 0.66 cubic feet of CO2 every 4 hours. Yeast takes time to ferment sugar. Therefore the size of the container the gardener should use is determined by dividing the cubic feet of the grow room (512 cu. ft. for a standard grow room) by 32 = 16 gallons.

A plastic kitchen garbage can is a convenient container to use for this purpose as it is cheap and easily available.

One can use hot water to dissolve the sugar to start with as sugar dissolves in it faster than in cold water. However, before adding yeast to it, it should be cooled off to 80-90 degrees F, otherwise the yeast will be killed. The container should be filled only to half as the gardener will need to add an extra gallon per week. One can start with 8 gallons per week and 24 lbs. of sugar.

One should create a starter batch by adding a pinch of yeast and two pinches of yeast nutrient to a mixture of 10 oz. of warm water and ¼ lb. of sugar. Such a mixture can be started in a bottle and a balloon can be placed over the bottle. The bottle should be placed in a warm location at around 80°-90°F for 1-2 days or until the balloon swells and small bubbles are seen in the solution.

Once this solution starts fermenting vigorously, the gardener should add it to the main solution at the same temperature. After a couple of days, to check if the system is working well and CO2 is being produced, the gardener should close the valve to the supply tube and if the system is sealed well, the balloon should swell in a short while. To regulate the quantity of CO2 delivered to the plants, the valve should be opened until the balloon comes to half the size of full expansion.

To serve as an air-lock, there should be a 2” loop in the CO2 supply tube with an in-line valve. A tape should be applied on the side of the fermentation tank to hold this loop in place. The open end of this tube should be placed either in front of a circulating fan or should be run through “T” fittings to make extra tubes the ends of which can be placed above the plants.

An extra gallon of sugar solution and yeast nutrient should be added once per week by undoing a corner of the saran wrap and then the top should be resealed with tape. 3 lbs. of sugar and 1 teaspoon of nutrients should be added per gallon.

After 6 weeks of process, after the last gallon is added, the fermentation should be allowed to continue until the balloon goes down and bubbles stop being seen in the “U” tube. At this point, the gardener should taste the solution. If it’s sweet, it means that fermentation is not finished and a new starter batch should be added to the tank. If the solution is not sweet, it means that fermentation is complete and the alcohol has destroyed the yeast. This is the time when the gardener should clean the tank and begin a new batch.

This process is pretty good for producing CO2 and is relatively cheap. Regular or invert sugar is cheap and easily available. One can get it at a wine supply store. This method will cost around 50 to 60 cents per day.

However, this method also is not the most accurate one and requires some practice.

Decomposition Process

The principle behind this method is that with the decomposition of organic matter due to bacterial action, carbon dioxide is produced. Plants in a tropical jungle are lush and grow vigorously due to this natural decomposition of dead plant and animal matter on the jungle floor.

This decay can increase the naturally available CO2 content from the normal 300ppm to more than 1,000ppm. The same process can be carried out even indoors, for low cost.

However, it’s not recommended because it is unsanitary and odorous. Moreover, the sterile conditions of a well-set hydroponic grow room could be disturbed and harmful bacteria, bugs, and diseases can be introduced and cause detrimental effects to the plants.


strawberries on the plate

CO2 enrichment is a very beneficial process for one’s grow rooms and grow tents and it can be done effectively with the above methods.

One can choose among these methods, based on their convenience, budget, and practicality. Some of them are more elaborate than others, while some are easier. Some require more time and attention. Some are costlier than others, while some are inexpensive.

But with CO2 enrichment, more light, temperature, water, and nutrients are also needed for the photosynthesis reaction to take place and for plants to grow vigorously. And a gardener may even need a machete to control the extra growth which is startling at times with the CO2 enrichment. However, ultimately the extraordinary growth should only make the gardener happy.