CO2 and plants in Greenhouse

ROLE OF CARBON DIOXIDE IN GREENHOUSE

Co₂ Concentration and plants

Ø  Photosynthesis is the process of plants using light energy to convert absorbed carbon dioxide (CO₂) and water into sugars. Plants use these sugars for growth through the process of respiration.

Ø  Plants absorb CO₂ through their stomatal openings located mainly on the underside of leaves.  Although light, moisture, temperature and humidity level all affect the rate of CO₂ absorption, the concentration of CO₂ outside the leaves is a significant influence.

Ø  The concentration of CO₂ in ambient outside air commonly varies from 300 to 500 parts per million (ppm) or more by volume depending on the season, time of day and the proximity of CO₂ producers such as combustion or composting, or CO₂ absorbers such as plants or bodies of water. 

Ø  Plants growing in greenhouses, particularly “tight” double-layer structures with a reduced air infiltration rate, can reduce CO₂ levels to well below ambient levels, greatly reducing the rate of photosynthesis. 

Ø  Conversely, enriching the concentration of CO₂ above ambient levels will significantly increase the rate of photosynthesis. In general, a drop in CO₂ levels below ambient has a stronger negative effect on plant growth than the positive effects of enriching CO₂ levels above ambient.

Ø  Daily CO₂ levels in un-enriched greenhouse environments will climb to several hundred ppm above outdoor ambient at night due to CO₂ produced by plant and microbial respiration. CO₂ levels drop quite rapidly after sunrise as the crop’s photo-synthetically driven consumption of CO₂ exceeds the basic rate of respiration.  In the absence of some other source, CO₂ levels remain low all day limiting plant growth.  At dusk, plant and microbial respiration once again begins to accumulate CO₂ in the greenhouse.

Ø  CO₂ is added in some greenhouses to increase growth and enhance crop yields.  The ideal concentration depends on the crop, light intensity, temperature and the stage of crop growth.         

How CO₂ concentration is monitored?

Ø  Most growers do not monitor CO₂ levels in the greenhouse because they have no intention of controlling it. As long as their crops are growing and developing to their satisfaction, this is a reasonable approach.

Ø  CO₂ levels in the greenhouse may be monitored using relatively low-cost dual beam infrared CO₂ gas monitors.  These monitors may be linked to climate control systems that integrate other factors such as indoor & outdoor air temperature, humidity & light intensity.  More expensive monitors with higher accuracies are available, but in most applications reliability and economical cost are the most important factors.

When Co₂ enrichment needed?

Ø  CO₂ enrichment is not required as long as the crops are growing and developing to the complete satisfaction of the grower, or if high ventilation rates make CO₂ enrichment uneconomical. 

Ø  CO₂ enrichment should be considered, however, if crop production and quality are below required levels. In general, crop production times from late fall through early spring increases the potential need for CO₂ enrichment as it coincides with reduced ventilation rates due to colder outdoor air temperatures.

Ø  As photosynthesis and CO₂ consumption happens only during daylight hours, CO₂ enrichment at night is not required.  In general, CO₂ enrichment systems should be turned on 1 or 2 hours after sunrise, and turned off several hours before sunset, however, additional CO₂ enrichment may be needed if supplemental grow-lighting is used.

How are CO₂ levels enriched?

1.    Maximize Natural (Free) CO₂ Supply:  Maximize ventilation rates whenever possible starting 1 or 2 hours after sunrise when the overnight build-up of CO₂ has been depleted.  Improve horizontal air flow to distribute available CO₂ evenly throughout the crop and to reduce the leaf boundary layer, which will improve the diffusion of CO₂ into the stomatal openings of each leaf.  Keep plants healthy and well-watered so they are not forced to close their stomatal openings due to stress.  Depending on the crop, consider using natural sources of CO₂ such as decomposing straw bales and/or organic soil mixes in your production system.

2.    Liquid or Bottled CO₂ Gas:  When outside air conditions are too extreme for ventilation, additional CO₂ is available in the form of liquid or bottled CO₂ gas.  Specific processes are required for the safe & proper handling as well as the effective use of CO₂ from these sources.  Liquid CO₂ must be fully vaporized before delivering into the greenhouse, and manufacturers’ instructions and local codes should be strictly adhered to.  

3.    CO₂ from Carbon-Based Fuels:  Gas-fired appliances generate CO₂ and water vapour as primary by-products of combustion.  These appliances include equipment that is specifically designed & certified as CO₂ generating appliances, un-vented forced-air primary space heaters, and hot water boiler heating systems with flue gas condensers specifically designed for CO₂ enrichment. 

ü  Achieving complete combustion is the key to success of CO₂ enrichment through appliances burning natural or propane gas.  Incomplete combustion may occur due to relatively common factors such as improper or fluctuating gas pressure, impurities in the gas supply, inadequate oxygen for combustion, wind disturbance in the burner and clogged gas orifices. Harmful by-products of incomplete combustion include Nitrogen Oxides, Carbon Monoxide and Ethylene.

ü  As water vapour is also a primary by-product of combustion, un-vented gas appliances have the potential to create difficulties in the naturally humid greenhouse environment.  Condensation due to high humidity promotes many plant diseases.  Condensation from combustion is also slightly acidic, which may prematurely corrode metal structures, equipment and wiring on contact. 

Is CO₂ enrichment safe?

ü  CO₂ is harmless to human at all reasonable dosing levels, and OSHA has established workplace standards for worker exposure.  While humans can work safely at these elevated CO₂ levels, many crops start to show undesirable growth responses at CO₂ concentrations above 1,200 to 2,000 ppm.

ü  For gas-fired CO₂ generators, adequate ventilation air should be introduced to provide enough oxygen for complete combustion, and to limit the build-up of water vapour and other potential contaminants in the greenhouse.

General tips for co₂ enrichment

Ø  CO₂ enrichment can be a useful tool for maximizing the quantity and quality of your greenhouse product. Healthier crops and higher yields helps to satisfy customers, command higher prices and reduce costs, all of which makes a greenhouse operation more competitive. 

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Milan Tomic

Hi. I’m Designer of Blog Magic. I’m CEO/Founder of ThemeXpose. I’m Creative Art Director, Web Designer, UI/UX Designer, Interaction Designer, Industrial Designer, Web Developer, Business Enthusiast, StartUp Enthusiast, Speaker, Writer and Photographer. Inspired to make things looks better.

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