What CO2 Generators Can You Use To Boost Plant Growth?

Boosting CO2 levels for better cannabis plant growth
December 07, 2020

Not enough carbon dioxide (CO2) is harmful to marijuana plants. On the other hand, too much can also cause harm. Conventional thinking has it that there should be at least an adequate level of CO2 in the grow room. Granted that even if you provided the optimum level, there is no assurance that it would benefit the plants. That is because of other factors affecting its absorption and use. In this article, we will discuss the benefits of increasing CO2 levels, how to increase CO2 levels and also provide some examples of the different types of equipment you could use. We have also included some more DIY methods for those looking to set up a low-budget grow room.

How Does Carbon Dioxide Affect Marijuana Growth?

The gist is this. Your plants should have access to at least 250 ppm of CO2. Under normal circumstances, the plants will use it along with other elements in photosynthesis. Boosting the CO2 level is intended to enhance this activity, which results in quicker growth, fatter buds, and bigger yields. It is believed by many that adding CO2 can increase the harvest by up to 20%.

Did you know that marijuana plants need 17 essential minerals to grow and reproduce? Even so, hydrogen, carbon, and oxygen constitute 95% of the plant’s dry weight.

Before you think about the different ways of introducing more CO2 to the grow room, there are a few things you need to understand first. So, read on. Once you are armed with the rudimentary knowledge, you can better decide on a preferred method and go on boosting plant growth.

CO2 and Photosynthesis

As you know, marijuana plants need light to fuel the process of photosynthesis. They also need water and carbon dioxide to produce sugar and oxygen. Outdoors, these elements required for photosynthetic activity are in harmony. It means that each one is adequate and most definitely not excessive.

Indoors, the situation changes – depending on the equipment used and the growing conditions.

Here is a typical scenario that is prevalent among many home growers. After hearing about how more light equates to more buds, you decided to invest in 1000 watts HID lamps. Let’s assume, too, that you kept the temperature well within comfortable levels, and the plants adequately hydrated. Does it necessarily follow that you should see more explosive growth and vigorous bud production?

The answer, unfortunately, is not necessarily the one you want to hear.

There should be photosynthetic activity in a well-ventilated growing space as long as there is light. Because the plants are taking CO2 from the surrounding air, there should be a lower level than outside the grow room.

A powerful lamp, indeed, enhances photosynthesis. The availability of more light triggers the plants to try absorbing more CO2. At a certain point, it would not be able to absorb more and at an even higher rate simply because there is not enough. Quite possibly, your lamp is supplying more light than is needed. As a result, you may have a slight increase in the final harvest and still waste light energy.

For you to truly optimize the growing condition and to take advantage of using powerful lamps, the solution is to introduce more CO2. You want the best bang for your buck, and that is to make sure there is enough CO2 for every watt cranked out by your lightbulbs.

Optimum CO2 Levels

So, boosting the CO2 level is beneficial. How does a faster growth by some 20-30% sound to you? Some estimates even place the increased growth rate up to 40%. The question is, “how much CO2 do you pump into the grow room?”

Photosynthesis cannot occur when there is less than 100 ppm of CO2. At 100 ppm, though, the rate of respiration and photosynthesis is equal, and there is no net gain or loss.

Airtight grow room

Before continuing, here’s a little piece of trivia – a fascinating one at that. Human activities have led to an increase in the atmosphere’s CO2 concentration. The global average in 2019 was 409.8 ppm, the highest ever in the last 800,000 years, and a 30% increase from the 18th century. At present levels, this colorless, odorless gas accounts for around 0.03% to 0.04% of the air we breathe.

It’s not all doom and gloom, though, because the increased concentration of CO2 benefited plants, blessing them with faster growth. Think of it like this, by keeping the grow room well-ventilated, you are also replenishing CO2 for the plants to use, keeping the level close to 400 ppm.

Raising the CO2 level to 545 ppm does not result in a significant improvement. But if you were to provide 700 ppm, the net increase of photosynthesis can range from 38% to 48%.

Today, marijuana horticulture experts believe that you can raise the CO2 level in the grow room to 800 ppm, maxing at 1200 ppm. Going beyond is too much, and may even be detrimental.

Light, Heat, and Water Use on Elevated CO2 Levels

We have established that raising the CO2 concentration can boost your marijuana plants’ growth. But there are other factors that you should know about to make it happen.

Light Intensity

Remember, when you use high-wattage lamps, the photosynthetic activity increases, CO2 levels drop inside a grow room. At best, you can only hope that fresh air coming from the outside as warm air is expelled would be enough to keep replenishing the much-needed CO2. At this point, you may be providing more light than is needed. The limiting factor, in this case, is the availability of CO2.

The flip side is by raising CO2 concentration, you may be providing too much CO2 when using less powerful grow lights. There is plenty of CO2 for the plants in this scenario, but not enough light to utilize them, converting them to sugars.

Under normal conditions, without boosting the CO2 level, you should provide a light intensity of 49,310 lux. That is on the assumption that there is around 400 ppm of CO2. It also disregards the fact that it gets depleted as photosynthesis occurs. But 49,310 lux is a must if you are augmenting and keeping the CO2 level in the grow room at 400 ppm.

Once you raise the CO2 level to 1200-1300 ppm, the light intensity should be around 48,240 lux. Some people try to go beyond by upping the light intensity to 80,240 lux while providing around 1500 ppm CO2. The gain, though, does not appear to be significant, and not worth the extra costs.

Temperature and Water Use

Monitor temperature in the grow room

One new research sheds more light on the effect of CO2 levels and temperature on photosynthesis. Global warming is a concern with elevated CO2 levels in the atmosphere contributing to the greenhouse effect and increasing temperature.

Table 1: Effect of different levels of CO2 on net photosynthesis, and water use efficiency on the leaves of Cannabis sativa.

CO2 levels (μmol mol-1) PN (μmol CO2 m-2s-1 WUE x 100
250 12.48 ± 1.76 2.19
350 24.64 ± 2.24 4.64
450 24.76 ± 1.89 4.30
550 26.54 ± 2.12 5.46
650 30.48 ± 2.76 6.56
750 36.80 ± 3.18 9.81

Table 2: Effect of different photosynthetic photon flux density and temperature conditions on Intercellular CO2 concentration (Ci)/Ambient Air CO2 concentration (Ca) ratio in the leaves of Cannabis sativa.

(μmol m-2s-1) 20°C (68°F) 25°C (77°F) 30°C (86°F)  35°C (95°F) 40°C (104°F)
0 1.04 ± 0.12 1.04 ± 0.14 1.02 ± 0.11 1.01 ± 0.09 1.01 ± 0.07
500 0.82 ± 0.05 0.79 ± 0.06 0.74 ± 0.06 0.71 ± 0.06 0.68 ± 0.05
1000 0.80 ± 0.06 0.75 ± 0.04 0.66 ± 0.06 0.59 ± 0.04 0.57 ± 0.06
1500 0.71 ± 0.04 0.62 ± 0.06 0.58 ± 0.05 0.51 ± 0.05 0.45 ± 0.04
2000 0.70 ± 0.06 0.61 ± 0.05 0.57 ± 0.05 0.50 ± 0.04 0.43 ± 0.03

The above table shows you the concentration of CO2 stored in the plants. A higher concentration leads to the stomata semi-closing. In turn, less moisture escapes into the atmosphere. In other words, the plants are able to conserve water. However, it also presents another potential issue – heat stress if the temperature is out of control and rose to excessively high levels.

Evidently, there is a relationship between temperature and water use efficiency. If PPFD increased to 2000 μmol m-2s-1, for example, you’d see an increase in WUE at 68°F (20°C) to 77°F (25°C). But at 86°F (30°C) or more, the WUE increase is only up to 2000 μmol m-2s-1. In other words, a high amount of CO2 in the atmosphere and temperature in excess of 86°F (30°C) is not only insignificant but possibly detrimental.

So, concerning the temperature, what does this mean?

The ideal temperature for growing marijuana is around 70°F (20°C) to 85°F (30°C) during the vegetative stage. If you introduce 800 to 1200 ppm of CO2, stay on the high side. Not only can your plants handle the heat, but they could benefit from optimum photosynthesis. Should you opt to provide 1500 ppm CO2, then raise the temperature to 85°F (30°C) but no more than 95°F (35°C).

During the flowering stage, though, you should strive to stay within 65°F (18°C) to 80°F (26°C). In this case, stay on the warm side when boosting CO2 concentration.

How Do You Add CO2 to Your Marijuana Grow Room?

There are numerous ways of introducing CO2 into the grow room. You can use compressed CO2 or a CO2 generator. Of course, cheaper methods such as CO2 bags are also options. Before deciding, you also need to know how much CO2 you are introducing into the grow room.

And how do you do that?

It is easy.

First, determine the grow room’s size in cubic feet – that would be length x width x height.

Let’s say you have a large space measuring 12′ x 12′ x 10′.

12 x 12 x 10 = 1,440 cubic feet

Next, you need to know your desired CO2 concentration. For example, you prefer to raise it to 1,000 ppm. All you have to do is to divide that by one million.

1,000 / 1,000,000 = 0.001

Finally, multiply the size of the growing space in cubic feet by 0.001.

1,440 x 0.001 = 1.44 cubic feet of CO2

So, to raise CO2 concentration up to 1,000 ppm in a grow room that measures 12′ x 12′ x 10′, you would need to provide 1.44 cubic feet of CO2.

Why is it important to know how much CO2 is needed? Because that helps determine how you can provide CO2.

Before contemplating DIY measures, consider also using specialized equipment – compressed CO2 or a CO2 generator. They are highly efficient and allow you a more precise way of pumping CO2 into the grow room than any makeshift solutions.

Compressed CO2

Compressed CO2 tanks do not produce heat nor alter the moisture level in the grow room. It is not difficult to purchase as they are widely available in horticulture and specialized stores. You can even buy them from home brewing stores because they are used in pressurizing beer kegs.

The simplest way to use a tank is to flood the grow room by releasing CO2 for 20 seconds and repeat after a few hours. Growers who practice this also think that saturating the growing space with CO2 kills any pest that might be lurking. As you can imagine, it is most definitely not efficient.

A better way to do that is to have the means to release CO2 at a controlled rate. This way, it is possible to maintain CO2 concentration at your preferred level. For you to do that, you would also need a tank regulator. It comes with CO2 flow control that lets you set how much CO2 is released.

Going back to the example provided earlier, a 12′ x 12′ x 10′ grow room needs 1.44 cubic feet of CO2 to raise the CO2 level to 1000 ppm. You can set the flow control to 5 SCFH (standard cubic feet per hour) in this scenario.

Compressed CO2 Cylinder

5 cubic feet per hour divided by 60 minutes equates to 0.08 cubic feet per minute.

So, to fill the entire growing space, divide 1.44 cubic feet by 0.8 cubic feet per minute, that would be 18 – which means you release CO2 at 5 SCFH for 18 minutes.

Once you shut down the valve, you would have around 1,000 ppm of CO2 for the plants to absorb and utilize. However, you are soon presented with another challenge.

When do you Begin Replenishing, and How Much Should you Provide?

In general, one pound of liquid CO2 stored in a tank is equivalent to 8.741 cubic feet. A 5-pound tank would have 43.7 cubic feet, while the larger 20-pound tank would have 174.8 cubic feet.

In a typical grow room (and even in a greenhouse), it is normal to have a 100% air exchange every two hours. As you might imagine, air leakage not only occurs on the vents but also through the door and cracks).

Based on the example we have been using, the grow room’s size is 1.44 cubic feet. 100% air exchange every two hours is the same as 0.72 cubic feet per hour (1.44 divided by 2).

During the vegetative stage, the plants would have 18 hours of light. That means that you would be using 12.96 cubic feet per day (0.72 x 18). In this case, a 5-pound tank should last a little over 3 days (43.7 divided by 12.96). The 20-pounder would be able to provide CO2 for 13.5 days (174.8 divided by 12.96).

Can you Replenish Every Two Hours Over 18 hours Daily?

It is a challenge, which is when a CO2 detector and analyzer “might” come in handy. It is a widely available meter (somewhat similar to temperature, pH, and TDS/EC meters). You can monitor the CO2 level and replenish once it goes below the average level.

For sure, that can be a tedious process. For simplicity and convenience, therefore, you can opt for automation. A CO2 monitor and controller can help reduce your workload and prevent mistakes.

Once you have this rigged with the CO2 tank and regulator, all you have to do is set the target high and low CO2 levels, and it does pretty much everything for you. When CO2 concentration drops below the threshold, it starts pumping CO2 into the grow room and stops after reaching the desired level. A photosensor shuts down the controller when it detects darkness. It is a highly useful feature because photosynthesis does not occur in the absence of light. CO2 is pretty much useless, and so there is no need to raise the level.

CO2 Generator

Compressed CO2 tanks are perfectly suitable for home use. If you want to level up, there is no better way to boost the grow room’s CO2 level than using a CO2 generator. There are plenty of options here, but essentially, these systems burn natural gas (NG) or propane (LP) to produce CO2 for your plants. Keep in mind that burning fuel produces heat, moisture, and CO2. Due to its size and capacity, it is more suitable for large growing operations.

Buy a unit that has tip-over shut-off functions. You see, gas may not burn properly for some reason, and is characterized by red flame during combustion. It should be blue if the gas burns evenly. At any rate, having such a safety feature automatically shuts off the system. Besides preventing accidental fire, it also stops carbon monoxide – a gas lethal to plants and humans – from being produced.

A typical CO2 generator generates 12,000 to 28,000 BTU (British Thermal Units). That is enough heat to warm up an entire house. If you anticipate issues with high temperatures, then buy a unit that cools off on one side while expelling warm air on the other side. Moreover, you have to ensure that there is an adequate ventilation system in place to keep the temperature under control. In a large growing area, good air circulation is critical to the even distribution of CO2, so that all the plants can grow at the same pace.

Compared to bottled CO2 emitter systems (compressed CO2 tanks + regular + controller), CO2 generators are up to four times cheaper. Fossil fuels are also affordable and widely available. Kerosene, in particular, is a popular choice but be sure to use only high-grade quality. You do not want to skimp and risk kerosene that contains sulfur, which is harmful to marijuana plants.

Remember also that when refilling a tank, it should never be up to full capacity. Gas contracts or expands as the temperature turns cold or hot, respectively. The idea here is to avoid highly flammable gas being released through the pressure vent, which may cause an accidental fire.

The lower cost of a CO2 generator and kerosene (or other gas) is only one reason that makes it enticing. It is also a much more efficient system. A gallon of propane, for example, contains 36 cubic feet of gas. Each cubic feet of propane can produce 3 cubic feet of CO2. Therefore, a gallon can provide up to 108 cubic feet of CO2 (36 x 3).

And how much does a gallon of propane cost?

Depending on where you live, it ranges from $1.20 to $6.00, with most places costing between $2.00 to $3.00. Yes, it is cheap, except that installation or setting up may cost quite a bit.

CO2 Bag

CO2 bags sit between the more expensive compressed CO2 tank and CO2 generator systems. They are also incredibly beginner-friendly. Its use is as simple as hanging them over the plants. Once exposed to light and heat, mycelium fungi begin producing CO2. Make sure, though, that they are not too close to exhaust vents so that the CO2 produced would reach the plants.

Are There Cheap Ways of Increasing CO2 Levels?

Both compressed CO2 cylinders and CO2 generators require a significant investment. But fret not, there are far cheaper methods of introducing CO2 into the grow room. Yes, they are not efficient but should help – to a degree. You have some choices here – candle and yeast. You can even use vinegar and baking soda. Besides modest benefits, these are an excellent way to get your feet wet, so to speak.

Burning Candles

Burning Candles for CO2 Cannabis

You may have candles in glass jars stored somewhere in the house. Put them to good use by lighting them up in the grow room. The importance of placing them strategically and making sure they are secure without the possibility of causing a fire cannot be overstated. For your trouble, they release a small amount of CO2 into the air.

Yeast Fermentation

Here’s another idea for you. Combine 1/4 to 1/2 teaspoon of yeast, 2 cups of sugar, and 1 to 1.5 liter of warm water and leave the container in the grow room. The chemical reaction will result in CO2 being made and released into the air for the next few weeks.

And if you are one to make your beer and other alcoholic beverages, you can hit two birds with one stone. CO2, as it turns out, is a by-product of fermentation.

Vinegar and Baking Soda

For this method, your patience is going to be tested. Vinegar is acidic, while baking soda is basic. When combined, the chemical reaction between them produces CO2, water, and sodium acetate.

Here’s what you can do.

You are to suspend a bottle of vinegar upside down over a bowl of baking soda. Now, you have to make a tiny hole in the center of the lid or bottle cap. Be careful not to rush. It should be small enough that only one drop of vinegar per minute falls into the baking soda. Once done, you have a makeshift CO2 generator.

What Else Do You Need to Know About Using CO2 in the Garden?

Bits and pieces of information concerning the use of CO2 in a marijuana grow room may seem inconsequential. On the contrary, they do make a difference. It is, of course, in your best interest to take these things to heart.

Leak-proof the Grow Room

Regardless if it is a grow tent or a spare room turned into an indoor garden, one thing that you should do is to check and make sure it is leak-proof. During the lights on period, it would be a shame to lose CO2 because of leakage at a time when you want its concentration to be high. Besides, it is also a way to keep the odor sealed in and prevent the tell-tale scent of marijuana from flooding the entire house.

Be Mindful of Temperature and Humidity

You understand that the optimum temperature of marijuana is within 70°F (20°C) to 85°F (30°C). When you boost CO2 level, adjust the temperature to the higher end – 85°F (30°C). Do this only during the vegetative stage. When the plants progress to the flowering stage, it should stay between 65°F (18°C) to 80°F (26°C), and boost CO2 only during the first two or three weeks, before stopping.

Now, at higher temperatures, the capacity of air to hold moisture increases. Make sure that the humidity level is according to the current growth stage of the plants. Be sure to monitor temperature and humidity levels frequently and accurately in this period.

Lastly, make sure the plants are not dehydrated. Under normal circumstances, they release moisture into the air to cool down when the temperature is warm. However, the high levels of CO2 absorbed into their system also slightly closes cannabis plants’ stomata, which lessens the loss of moisture. It could be a problem if the heat is uncontrolled, leading to heat stress.

Release CO2 from Above

CO2 is heavier than air, which means the best way to release them is from above. The most common setup is to have a hose running from the tank (or whichever source) to the upper portion of the grow room, parallel to the upper plant canopy. It should have tiny holes facing downward for CO2 to escape and enter the environment.

Try to be ingenious if you have enough space. For example, you can position a compressed CO2 tank above. Or, you can use a small fan to blow and spread CO2 all over the room.

DIY or CO2 Generators?

Some growers swear by CO2 bags as a cheap solution that works. As for other DIY methods, that is subjective and does not usually work. If it did, the gain is too insignificant to go through the trouble.

For a meaningful increase in plant growth, you are better off raising the CO2 ppm level to 800. It is up to you if you want to go as high as 1200 or 1500. At these desired levels, compressed CO2 tanks make the most sense for most home growers.

CO2 generators that burn gas are usually reserved for commercial farming. However, that should not stop you from installing it in your house – provided you have space and the technical knowhow to operate such systems.

Whichever method you choose, as long as you are boosting CO2 concentration to high levels (along with providing powerful grow lamps), you should brace yourself for the plants’ explosive growth.

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