Gardening in a greenhouse or indoor garden offers some unique advantages. One advantage is heightened control over environmental conditions. The heightened control over a garden’s environmental conditions opens up many opportunities for horticulturists to maximize plant growth. One opportunity is CO2 enrichment. Enriching a greenhouse or indoor garden with CO2 has been shown to increase growth rates in various species of plants. Supplementing CO2 was once viewed as something a gardener implemented after he or she had dialed in lighting, nutrition and ventilation, but now many novice growers have found inexpensive, yet effective, ways to experiment with CO2 in the garden. Before jumping on board the CO2 bandwagon, it is important to have a basic understanding of why, how, and when CO2 enrichment is beneficial to plants.
Increased CO2 levels are beneficial to plants when the sun is out or when the garden’s lights are on. In other words, plants use CO2 during photosynthesis or when the plants are receiving light energy. Because of this, growers should only use CO2 during the lights on cycle of an indoor garden. It is important to note that photosynthesis is a chemical reaction. Photosynthesis can be represented using the chemical equation: 6CO2 + 6H2O + light energy = C6H12O6 + 6O2. The first part of the equation represents CO2 (carbon dioxide) and H2O (water). When plants have access to both carbon dioxide and water in addition to light energy, they are able to produce the second part of the equation: C6H12O6 (glucose) and O2 (oxygen). Like other chemical reactions, photosynthesis can be affected by access to the proper chemical compounds and other contributing factors, such as temperature. One of the crucial chemical compounds in the photosynthesis chemical equation is CO2.
As they receive light energy, plants absorb carbon dioxide through the stomata on their leaves. When the stomata are open, the plant can absorb CO2. Transpiration also occurs when the stomata are open. Plants will transpire water vapor to help keep an even temperature and this leads to water loss for the plant. Water is another significant chemical compound for photosynthesis. To conserve water, plants will regulate the amount of time the stomata are open. Because CO2 can only be absorbed when the stomata are open, temperature ends up playing a key role in a plant’s rate of photosynthesis. However, if an indoor horticulturist enriches his or her environment with CO2, the amount of CO2 available for absorption during the time the stomata are open increases. Put another way, the plant is able to absorb more CO2 while limiting the amount of water lost through transpiration. Some plant species will not open the stomata as wide when grown in a CO2 enriched environment. This will also reduce the amount of water lost through transpiration.
After a gardener has decided to enrich his or her garden with CO2, he or she must decide how to administer the CO2. For greenhouses and large indoor gardens, the preferred method is usually a CO2 burner. CO2 burners are typically fueled by propane or natural gas. As the fuel burns, CO2 is created as a by-product of the combustion and is released into the garden’s atmosphere. The longer, or more aggressively, the fuel is burned, the higher the concentration of CO2 will become within the growing space. The amount of fuel burned is determined by the type and/or size of the CO2 burner. The CO2 burner required is determined by the size of the area the gardener is trying to enrich with CO2. The biggest disadvantage of CO2 burners is the excess heat they add to the garden space. For many indoor gardeners, the additional heat created by CO2 burners is reason enough to seek an alternative route. However, in greenhouses or in very large indoor gardening facilities, CO2 burners are the most effective and efficient way to produce the large amount of CO2 needed to enrich the environment.
Another common method used by indoor gardeners to increase CO2 levels is to use tanks or cylinders of compressed CO2 teamed with a CO2 emitter. The CO2 emitter is the device that regulates the rate at which the CO2 is released from the tank. The emitter is controlled by a timer or a more sophisticated CO2 controller which determines when and how much CO2 should be released. A compressed CO2 tank system is a great choice for most indoor horticulturists because it is not only effective at increasing CO2 levels, but also will not create additional heat in the garden’s environment.
Another method for releasing CO2 without creating additional heat is using CO2 pads. The CO2 pads are made from natural chemicals, which, when exposed to humidity, begin to release CO2. CO2 pads come in different sizes and multiple pads can be used to enrich larger garden spaces. CO2 pads are handy because they are lightweight and can be easily hung or placed directly above the plants. CO2 pads are probably the most cost effective way for a grower to experiment with enriched CO2 levels throughout all stages of growth.
A method of CO2 enrichment commonly used by indoor growers with smaller spaces is mycelium-based CO2 systems. These CO2 devices can be purchased in buckets, bags, or boxes and all contain specific strains of fungi along with a food source for the fungi. As the fungi eat and grow, they release CO2 as a by-product. This type of CO2 enrichment is perfect for small areas, such as a closet or a small bedroom. They are also relatively inexpensive, which allows any grower to experiment with CO2 enrichment without making a huge investment.
Ideally a gardener should use a CO2 monitor/controller to regulate the concentration of CO2 in the garden’s atmosphere. Many atmospheric controllers already come equipped with built-in CO2 control systems. There are also stand-alone CO2 control systems available. In order to automate the CO2 system, the gardener must control both the concentration of CO2 (usually expressed in PPM) and the appropriate times for CO2 enrichment. For example, it is counterproductive to add CO2 to a garden’s atmosphere when exhaust fans are operating because the CO2 enriched air would soon be exhausted from the growing area. The best CO2 controllers will bypass the CO2 unit when the exhaust fans are in operation or when the lights are off so the CO2 is not being wasted.
CO2 controllers are also the devices responsible for consistent CO2 levels. Plants love consistency throughout their environment and CO2 levels are no exception. A CO2 controller will detect the amount of CO2 in the environment by periodically sampling the atmosphere’s concentration of CO2. Newer, software-based control systems offer additional control features, such as remote access to the devices and data logging. Remote monitoring and control means growers have real time access to their CO2 equipment via their computers or mobile devices. Logging data of CO2 levels can provide valuable information over the course of a few garden cycles.
As previously mentioned, temperature plays a key role in the way CO2 is absorbed through the leave’s stomata. As the temperature increases, the amount of time the stomata are open for transpiration also increases. The longer the stomata are open, the more CO2 they can absorb. This is why the optimal operating temperature for an indoor garden enriched with CO2 is slightly higher (75-85 degrees F) than a room without CO2 enrichment (70-80 degrees F).
Although most commonly used in the fruiting and flowering stages of growth, CO2 can be beneficial in the vegetative and cloning stages as well. CO2 enrichment is a great tool for increasing growth rates in the vegetative room, which, in turn, reduces the amount of time it takes for the plants to reach the desired height for flowering. In other words, CO2 enrichment can reduce the time it takes to get vegetative plants ready for flowering. For some gardens, this reduces the total time it takes to finish a grow cycle. The quicker a garden can be cycled the more harvests a gardener can get per year.
Enriching the cloning environment with CO2 offers a couple of significant advantages. First, as with other stages of growth, CO2 in the cloning stage speeds up the rate of growth or, in this case, the amount of time it takes to produce roots. The faster a grower can get his or her cuttings to root, the better. Not only does faster rooting equate to a shorter overall grow cycle, it also usually means higher cloning success rates. However, the most significant advantage of using CO2 in the cloning stage is increased pathogen protection. This benefit is huge, especially when you consider how the environmental conditions of the cloning stage are conducive to powdery mildew, grey mold, and other pathogens. It is believed that CO2 is an effective anti-fungal due to its ability to alter intracellular pH levels. Put another way, CO2 will change the pH of a clone’s leaf surface, making it impossible for particular fungi or molds to take hold.
With so many different CO2 techniques available, there is really no reason a gardener should avoid experimenting with CO2 enrichment in his or her garden space. CO2 pads or mycelium-based products offer even hobby growers the ability to affordably enter the world of CO2 enrichment. For large scale operations, compressed CO2 tanks or CO2 burners can be the key to unlocking a garden’s full potential. When we view photosynthesis as a chemical equation, we are able to see how increasing CO2 levels can help the plants achieve accelerated growth rates. The benefits of CO2 enrichment can be realized in all stages of growth, including the cloning stage. The benefits of CO2 enrichment are multifaceted and include pathogen protection, increased rate of growth, and larger yields.
Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse senior editor and can be contacted at Ehop@GardenAndGreenhouse.net.