Nothing makes the indoor gardener happier than a large, healthy crop—beautiful green leaves, colorful fruits and vegetables, heavy and aromatic flowers. But no matter what you are growing, the limiting factors to crop health and overall yields are the same: nutrients, climate, lighting and pest control.
Every plant has different requirements for optimal health, but these four limiting factors can make or break a harvest in any gardening scenario—if a single factor is imperfect, crops can be negatively affected. Some imperfections can destroy a crop entirely.
One important, and often overlooked, aspect of the garden’s climate is the level of carbon dioxide in the garden. What many of us don’t understand completely is that CO2 is actually consumed by our plants. Carbon dioxide is made of two oxygen atoms and one carbon atom bonded together.
How Plants Use CO2
During photosynthesis, when the lights are on, plants absorb CO2 through the stomata (pores) found on the leaves of the plant. The plant turns the carbon atom into plant material such as leaves, flowers and fruit, and discards the two oxygen atoms during transpiration—usually when the lights are off, with few notable exceptions. Adequate CO2 levels are absolutely vital to the health and growth of plants, and elevated levels can contribute to explosive growth and production.
Normal levels of CO2 in the atmosphere are generally between 300 and 500 ppm, but this is a basic range and can vary somewhat based on location. Under all circumstances, your plants should be provided with 300 to 500 ppm of CO2 to ensure proper growth.
This can be tricky, especially in a sealed garden environment, because plants will consume the CO2, so even though the normal range of carbon dioxideis present at first, supplementation might still be required because the plants will drop the levels of carbon dioxide in a sealed garden down to zero. Once the CO2 levels drop below what is normally available in the atmosphere, plant growth will be negatively affected, often dramatically.
Studies have shown that plant growth and production increases with CO2 levels that are higher than atmospheric norms. Different species of plants will respond in different ways, but overall, all plants that consume CO2 will respond positively to elevated levels.
A 1979 study by the Australian National University found that when carbon dioxide levels were elevated to approximately double the atmospheric normal, 40-day-old cotton plants were double the weight of the same cotton plant grown at normal carbon dioxide levels. Maize plants that were part of the same study increased in weight by 20%.
While elevated levels are beneficial to the health and overall production of most plants, levels above 1,500 ppm are unnecessary. There is no benefit to providing more CO2 than your plants can consume and carbon dioxide levels above 2,000 ppm can harm your plants by limiting transpiration and reducing nutrient uptake.
Carbon dioxide is generally safe for humans at these levels, but levels above 6,000 ppm are harmful to humans after exposure for a period of time, especially for those with respiratory problems.
For this reason, if you are enriching your garden with CO2, it is vital that you use a monitor in conjunction with your enrichment devices to control the levels at all times. The generally accepted sweet spot in the indoor gardening world is maintaining levels between 1,000 and 1,800 ppm, with most gardeners agreeing on 1,500 ppm as the level of maximum benefit.
With few exceptions, it is unnecessary to provide the garden with CO2 enrichment when the lights are off—this is usually only necessary when the lights in the garden are on and photosynthesis is in full swing.
How To Supplement Your Grow Room's CO2 Levels
There are several different ways of delivering CO2 to your garden. The simplest way to replenish the CO2 levels is by venting, where a fan exhausts garden air out and replaces it with air from the outside. This replaces any air that has depleted CO2 levels.
This method is common, but it limits the garden to only the atmospheric normal levels of CO2, does nothing to enrich the air with higher levels and it is not without problems, primarily related to the other aspects of the garden’s climate.
For example, if it’s humid outside, you introduce humidity into the garden, which can lead to fungus growth on your crop, and since most gardens are already humid, no more moisture is needed. Introducing outside air can also introduce pests into the space.
Lastly, if the temperature outside is hotter than you want your garden to be, you must compensate for the introduction of outside air with your air-conditioning system. Most gardeners who start out with a venting system eventually move to a different type of enrichment system.
CO2 Bags and Pads
There are also systems available that boost CO2 levels with decomposing natural materials. These come in a package that you place in the garden, during which time the decomposition off-gasses CO2 into the garden, boosting the levels somewhat.
These products are natural and inexpensive and don’t require a CO2 monitor or other equipment to operate, as there is little chance of overdosing your garden unless you are using an extremelysmall space.
However, CO2 volumes will be variable, and it is difficult to determine exactly what ppms can be reached . Additionally, they can’t be turned on and off with your lighting, so will continually off-gas even during the night cycle when CO 2 isn’t being consumed. The manufacturer’s estimate for lifetime on this product is usually about two months. Depending on the size of the garden, multiple packages might be required.
Bottles of pure CO2 are commonly available at most hydroponics retail outlets, where they can also be refilled or exchanged for new bottles. With this method, you use a regulator connected to a CO2 monitor.
The monitor reads the CO2 level in the space, opens the regulator when CO2 is needed in the space, and closes it when the levels reach the presets on the monitor. This is a relatively inexpensive up-front cost, as the bottles aren’t particularly expensive. However, they will require frequent refilling or exchanging, which can become tedious for many gardeners.
Carbon dioxide generators are also commonly available from most hydroponics retailers. These generators create CO2 in the air by burning propane or natural gas—the primary byproduct of burning these gases is CO2. They should also be used in conjunction with a CO2 monitor that turns them on when carbon dioxide is needed and turns them off when it is not.
Using a carbon dioxide generator is an easy and efficient way to introduce CO2 into the garden, as gas creates about three times as much CO2 as an equivalently-sized bottle. If you are operating your generator using bottles of propane, this reduces the number of refills required by two-thirds.
Additionally, most generators can be plumbed into the gas supply of your home or commercial space, resulting in no need to replace or refill a bottle. The obvious downside to using a CO2 generator is that burning propane or natural gas produces a lot of heat.
This is undesirable in most gardens, as heat is a constant struggle for many of us. You can compensate for this with additional air conditioning. There is also a CO2 generator available that is water-cooled, resulting in optimal CO 2 levels with no heat being released into the garden environment.
The generator and the bottle used in conjunction with a CO2 monitor are the only two ways of maintaining constant, specific levels of carbon dioxide in the garden.
General CO2 Tips
Carbon dioxide is heavier than the rest of the air in the space, meaning that it will fall to the floor and levels 12-in. above the ground will be higher than levels 60-in. above the ground.
For this reason, monitors should be placed at plant canopy level, so they can accurately read the CO2 levels where they matter most. Adequate airflow inside the garden will ensure CO2 is homogenized into the air as much as possible.
No matter what you are growing or how you deliver it, adequate CO2 levels in the space are absolutely vital for the health of your crop. Since the carbon translates directly into plant material, CO2 levels elevated above normal levels found in the atmosphere will result in density and yields not otherwise achievable.