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In a cannabis grow, new and experienced cultivators alike usually focus on the big-picture items like strains, soil, lights, nutrients, containers, and water quality. Essential considerations like carbon dioxide (CO2) levels often take a backseat in importance or get ignored altogether.

This is unfortunate, because carbon dioxide is acrucial component of plant development and growth, and boosting CO2 levels in your grow room can profoundly impact your production and overall quality. With CO2 enrichment, your yield can increase by as much as 20 percent.

Why Do Cannabis Plants Need CO2?

As every botany student discovers, CO2 is critical for the rapid growth and sustainability of plants. For the human respiratory system, people inhale air into their lungs, where they utilize oxygen (O) and exhale CO2.

Plants operate in a similar but somewhat opposite manner, absorbing CO2 (as well as water and minerals) through their stomata (the pores through which plants “breathe”) under the leaves, where they use this carbon dioxide to convert it to sugars and oxygen.

They use sugar for energy (growth) and release oxygen into their environment. The process of photosynthesis relies on carbon dioxide for plant growth. Without photosynthesis and the release of oxygen from plants and trees, humans would perish, and planet earth would cease to function as we know it.

In plants, CO2 levels of 200 ppm or below can cause detrimental effects. While your plants may continue to grow for a short while, eventually, they will run out of sugars that they have stored. When this happens, their metabolism decreases and growth becomes stunted.

However, if they get too much CO2, their growth will also slow down andphotosynthesis will stop. Getting the CO2 levels just right can help optimize photosynthesis and lead to bigger yields and better buds.

Should You Use CO2 in Your Grow?

Whether or not you should use carbon dioxide supplementation in your grow is something that warrants serious consideration. Carbon dioxide systems range from simple and inexpensive to elaborate and costly.

For most grows, from a small personal grow to an elaborate commercial operation, there is usually a way to provide effective CO2 supplementation without breaking the bank.

Before taking steps to add carbon dioxide supplementation, make sure you’ve mastered the essentials of cannabis cultivation and have at least a few successful grows under your belt. Learn the basics before getting into this more technical aspect of growing cannabis.

One of the important things to consider is whether or not you have adequate lighting. Regardless of whether you use high-pressure sodium (HPS), light-emitting diode (LED), or even metal halide (MH) bulbs, the lights should be functioning at an optimum level—meaning you handle them with care and replace them regularlybefore they burn out completely.

Optimum lighting is essential to photosynthesis and the efficiency with which plants use carbon dioxide. Once you ascertain that your lighting is ideal, you can consider CO2 supplementation.

Keep in mind that, like water and nutrients, too much CO2 in your grow is a bad thing. One of the most common mistakes among beginner and novice growers is the notion that more is better. Providing too much water and fertilizers are the most common mistakes and will have a detrimental effect on your plants. Carbon dioxide is much the same.

Too much is bad, while supplementation at the appropriate level is good. Optimum carbon dioxide levels are one of the final steps indoor growers learn to take their cultivation to an advanced level—although quality indoor cannabis can be grown without it.

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Pros and Cons of How CO2 Affects Your Cannabis 

We recommend having a few grows under your belt before you attempt CO2 enrichment in your garden. Getting the hang of growing cannabis with a basic setup is important before you level up your equipment and alter your environment’s conditions.

Here are some pros and cons to consider before attempting CO2 enrichment in your garden:

Pros

  • Bigger yields: In the right grow environment, CO2 enrichment can provide a noticeable increase in yield.
  • Faster growth rate: Some growers have seen a 100% increase in their plant’s growth rate with CO2 enrichment at 1500 ppm.
  • Higher temperatures: If you use CO2, you can have higher temperatures in your garden (between 82º and 84º F), which is ideal for growers who use intense lights and want to maintain optimal temperatures for plants.

Cons

  • Costly: Injecting your room with carbon dioxide requires extra equipment (tanks and generators) and energy use (ventilation for extra heat) which will increase energy costs.
  • Grow lights: If you have the standard CFLs or T5s, you will need to invest in higher-powered (and more expensive) lights such as LED or HID light bulbs.

How Much CO2 is Too Much?

Carbon dioxide is measured in parts per million (ppm). The natural CO2 levels naturally occurring in our air hovers at around 400 ppm. Botanical research indicates that once CO2 levels surpass 3,000 ppm, it begins to have a detrimental effect on humans, and at 5,000 ppm, it can be lethal. Plants, on the other hand, can survive and grow in environments with carbon dioxide levels that surpass 10,000 ppm.

When used in combination with good light, CO2 levels at 1,200 to 1,600 ppm or so will significantly increase the growth of your cannabis plants. At these carbon dioxide levels, plants can also tolerate and still thrive at higher temperatures reaching 85 to 95º F.

Carbon dioxide is a heavy gas and will sink to the floor of your grow room, so when providing supplemental CO2, ideally, it should come from above the plants.

This is easier said than done in some grows, so adequate air circulation from fans is essential. Fans should be a part of every grow anyway, but when using supplemental carbon dioxide their importance is even more significant. Since several supplementation methods provide CO2 from the ground level, running fans at low levels to disperse the gas higher is particularly important.

When using carbon dioxide, you want your grow room to have air circulation and incoming fresh air, but not so much that it allows too much of the CO2 to escape. A sufficiently sealed room will allow your plants to benefit from it, so remember that if too much leakage occurs, the benefits will be minimal or lost altogether.

Most experienced cannabis growers agree that plants benefit from CO2 throughout most of their vegetative growth, from the time they are young seedlings to well into flower mode.

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Discontinue carbon dioxide supplementation two or three weeks before harvest. At this stage, the plants have received virtually all of the benefits that extra carbon dioxide provides. The use of CO2 can fall a week or two before you begin flushing your grow and preparing for harvest, drying, and curing.

CO2 and Lighting

In low light conditions (about 12,330 lux/1150 fc), the rate of photosynthesis can rise as you increase CO2 levels to 400 ppm. Going above these levels won't increase the rate of photosynthesis because of the low light intensity. Essentially, the rate of photosynthesis is limited by the intensity of light.

If you want to increase your rate of photosynthesis, you will need to increase your light intensity.

When you increase your light intensity to around 49,310 lux/4600 fc, your rate of photosynthesis will increase as the CO2 ppm rises to 400. While the rates of increase can slow down slightly after 400 ppm, you can still get up to 600 ppm of CO2. Above 600 ppm, the rate of photosynthesis will slow down even more and can taper off at about 1200 ppm.

Increasing the light intensity to 59,201 lux/5500 fc can allow your plants to use between 1200 to 1300 CO2 ppm. While rare, some gardens can supply their space with 80,400 lux/7500 fc or above to use up to 1500 CO2 ppm.

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How Much CO2 Is Needed In Your Grow Area?

Before you start investing in CO2 tanks and generators for your indoor garden, it is important to assess different variables of your grow space to determine how much CO2 your garden needs.

The right amount of CO2 required to increase your yields depends on a variety of factors, including the intensity of your lights and your grow space ventilation. If you don't have the right environment in place, your plants will not be able to take advantage of the CO2 enrichment.

We recommend using CO2 enrichment only during the vegetative stage. The optimal levels should be between 1250 to 1500 ppm. If you want to determine how much CO2 you need for your growing area, you need to start with your CO2 ppm.

For instance, if you will be using 1250 ppm, here's a quick formula to follow to determine how many cubic meters of CO2 you will need:

Multiply the cubic area of the grow room (length x width x height) by 0.00125

The total of this formula will be the number of cubic feet/meters of gas needed to get to your optimal CO2 range.

If you have a grow room that is 13 feet long, 15 feet wide, and 10 feet high, your cubic area would be 1950 cubic feet. Multiply your cubic area (1950 feet) by 0.00125 and the cubic feet of gas you will need is 2.44.

Methods of Supplying CO2 and Determining Levels

There are several ways of providing carbon dioxide to your plants. The various methods range from simple homemade systems to expensive manufactured equipment.

Some of the most basic carbon dioxide systems include the use of yeast and sugar for fermentation. As the sugar ferments and converts to alcohol (check your local laws to ensure this is allowed in your area), it releases carbon dioxide into the room, providing beneficial carbon dioxide.

Another method is to introduce dry ice to the room regularly. Dry ice is frozen carbon dioxide, and as it melts, it converts into CO2 gas. A block of dry ice will take approximately 24 hours to dissolve into gas, so regular replacement of the ice blocks is necessary.

A more efficient way of providing CO2 is by investing in tanks available at your local hydroponic and home brewing store. Although this method may seem more expensive, ultimately, it’s more cost-effective as well as efficient.

You can rent or buy a 20-pound carbon dioxide tank for approximately $140 to $160, and it will last you about two or three grows. 50-pounds tanks are also popular. When using CO2 tanks for your grow, use a regulator to release it.

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For large-scale commercial grows, CO2 generators are the best option. These commercial grade machines burn propane or natural gas to produce carbon dioxide for the entire grow.  Growers can program CO2 set points to automatically turn on and off the machine as needed.

While propane and natural gas are readily available, the burning gas can produce extra heat in the grow. For small operations, the extra heat can be hard to regulate, which is why they are mainly used in larger facilities.

When providing CO2, you will want to know its levels in your grow room. This is easily monitored with the use of a meter. A wide variety are available with a number of applications. Obviously, you will want to buy one that’s good for agricultural purposes.

Pro tip: Turn off your CO2 system about a half hour before you turn off the lights and turn it on a half hour after you turn them on. Your plants will not absorb CO2 when the lights are off.

Using carbon dioxide in your cannabis grow will help ensure you’re providing your plants with all the essential ingredients they need for maximum quality and production. Experienced growers often decide the investment in necessary equipment is well worth the cost and effort.

Once you’ve mastered the fundamentals of cannabis growing, adding CO2 may be the final step toward mastering your craft. Your plants will appreciate the boost in energy, and if they could, they’d thank you for it!

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