• Co2 Concentration and plants
      • 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. 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.


      • 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.  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.  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.


      • Daily CO2 levels in un-enriched greenhouse environments will climb to several hundred ppm above outdoor ambient at night due to COproduced 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.


      • 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 COconcentration is monitored? 
      • 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.


      • 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.


      • Although basic CO2 dosing may be applied without monitoring COlevels, the relatively low cost of a good CO2 metering system pays for itself in the form of cost savings from supplemental CO2 sources. 

    • When Co2 enrichment needed?     
      • 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.  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.  As ventilation rates are increased for cooling and dehumidification from late spring to early fall, the cost of CO2 enrichment escalates while the benefit to the crop may be minimal or reduced. As photosynthesis and CO2 consumption happens only during daylight hours, COenrichment at night is not required.  In general, COenrichment 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?
      • Maximize Natural (Free) CO2 Supply:  Maximize ventilation rates whenever possible starting 1 or 2 hours after sunrise when the overnight build-up of COhas 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.

      • Liquid or Bottled COGas:  When outside air conditions are too extreme for ventilation, additional COis 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.  

      • 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. 

      • 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.


      • To increase the likelihood of complete combustion, it is recommended to use only gas-fired appliances that are certified by 3rd -party testing agencies to meet nationally recognized safety standards.  Agency-certified appliances should only be used for the applications that they are certified for, and the appliances should include installation, operating & maintenance instructions with the product. These instructions should be strictly adhered to and saved in a convenient place.


      • 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. 


      • Building codes and manufacturers of un-vented gas appliances typically require minimum rates of air changes in the greenhouse per volume of fuel burned.  Although introducing fresh outside air will increase greenhouse heating costs in colder weather, these ventilation rates are necessary to ensure adequate supplies of oxygen for complete combustion, and to prevent the build-up of unwanted water vapour and/or contaminants due to incomplete combustion.

    • Is CO2 enrichment safe?
      • 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.


      • 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
      • 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.  The decision to proceed with CO2 enrichment should follow a thorough cost/benefit analysis, and success depends on each grower developing a strategy based on their unique combination of greenhouse structure, crop type, local weather, stage of production and capital/operating budgets.  Once a CO2 enrichment strategy is selected, always follow the instructions and installation & service manuals of equipment and/or chemical manufacturers. Make sure national and local codes covering greenhouse operations are adhered to, and use qualified & experienced service agencies and technicians for installing and maintaining CO2 enrichment systems.
    • Light regulation is practised in a greenhouse for the following reasons: (i) to alter the length of daylight hours (ii) to interrupt the darkness at night (iii) to extend or reduce the dark period of the night using artificial light or darkening screens; (iv) to increase photosynthesis and (v) to decrease the light intensity.


    • The objective is to maximize photosynthesis by maximizing the light interception (PAR) by the greenhouse, which involves optimizing its design and orientation. In order to make the radiation useful for photosynthesis it must be intercepted by the crop, which will require the crop rows to be appropriately orientated (north–south) and a proper arrangement and density of the plants (lower in winter than in high radiation seasons), depending on the species, cultivar and crop conditions.


    • Under normal conditions, the LAI (leaf area index) is an indicator of the light interception. During the first stages of the crop a high plant density allows for better light interception, so early production will increase (in relation to a normal density). Once the crop covers all the available space the plant density is less relevant. A high planting density involves a decrease in the quality of the product in most species, and beyond a certain threshold a decrease in yield, when expressed on a per unit area basis.


    • When the solar radiation is insufficient, it may be complemented with artificial light, to increase the PAR level above the radiation compensation point and maintain an active growth. The positive effect of an increase of the PAR on the growth is more relevant at low PAR levels.  Artificial light may also be used to extend the period of photosynthetic activity in the winter season.

    • Light increase


      • Inside the greenhouse, various techniques have been used to improve the availability of light to the crop, such as: (i) painting the greenhouse structural components white; (ii) applying a white plastic film as soil and (iii) in general making extensive use of other light reflecting materials. A usual practice is to use reflecting walls.  Several reflection devices have been proposed to increase radiation, but they are usually uneconomic. However, the reflectors perform well with direct light and not with diffuse light, and unfortunately the highest interest for increasing the light availability is in the winter months when diffuse radiation prevails. Artificial light is the most reliable and effective method to increase the light availability.

    • Artificial light to increase the illumination
      • In commercial production, artificial light sources are used in a variety of ways:
      • Replacement lighting – complete replacement of solar radiation for indoor growth rooms and growth chambers

      • Supplemental or production lighting – used in greenhouses to supplement periods of low natural light.

      • Photoperiod lighting – used to stimulate or influence photoperiod dependant plant responses such as flowering or vegetative growth

      • The need for and quality of artificial illumination required is determined by a number of factors including:

        • The light requirements of the species being grown

        • The natural day length

        • The average hours of sunlight

        • The sun angle and intensity (latitude and weather)

        • The amount of structure-induced shading

      • Supplemental Lighting


        • For commercial greenhouse production, supplemental lighting is most beneficial in areas that receive less than 4.5 hours average daily sunshine. In many greenhouse growing regions this occurs in winter as a result of the combination of high northern or southern latitudes and overcast weather.

      • Types of lamps
        • Conventional horticultural light sources can be grouped into three categories:

          • Incandescent

          • Fluorescent

          • Discharge


  • Incandescent lamps typically emit light as a result of the heating of a tungsten filament to about 2500°C. At this temperature, the emission spectrum from the filament includes a substantial amount of visible radiation. Only about 15% of the energy (watts) applied to an incandescent lamp is radiated in the PAR (photosynthetically active radiation) range of 400-700 nm. 75% is emitted as infrared (850-2700) nm, and the remaining 10% is emitted as thermal energy (> 2700 nm).
  • Since they are not very light efficient and they have a relatively short lamp life, incandescent lamps are usually not the most effective radiation sources for providing supplementary light for photosynthesis. They are, however, useful for phytochrome-dependent photoperiod control since they are relatively inexpensive to install and operate, they can be cycled on and off frequently, and they produce large amounts of red and infrared radiation. This is why incandescent sources are often the lamp of choice for night break, and long day lighting applications, particularly when other supplementary lighting sources are not installed.


  • Typically, incandescent lights are used to break the night into two or more short dark periods thereby stimulating a long day growth and development response in the crop. This may be used to promote flowering in long day species such as asters, azaleas, and fuchsias, or to delay flowering in short day species such as chrysanthemums, begonias, and poinsettias.


  • Since plant photoperiod response occurs under relatively low light intensities, less power is needed for photoperiod lighting than for supplemental lighting. The long standing recommendation for maintaining vegetative growth in chrysanthemum crops has been to place strings of 60 watt bulbs spaced 1.2 meters apart and suspend them 1.5 meters above the crop. This will provide sufficient photoperiod lighting for a 1.2 meter bed or bench. Similarly, any combination of incandescent lamp wattage, spacing, and mounting height that can produce an output of at least 10 foot-candles evenly on the crop will work. This corresponds to about 16 electrical input watts per square meter (rated bulb wattage divided by the area illuminated). Special reflector bulbs are available to focus most of the radiation downwards or do-it-yourself reflectors are often fashioned from aluminium foil pie plates.


  • Unlike incandescent lamps, which emit light from the heating of a metal filament, fluorescent lamps produce light from the excitation of low pressure mercury vapour in a mixture of inert gases. A high voltage differential at the electrodes on opposite ends of the lamp tube produces an arc through the gas mixture exciting the mercury ions, which in turn emit short wavelength (primarily UV) radiation as they drop back to a ground state. Special fluorescent coatings on the glass tube walls are activated by this short wavelength radiation producing a discharge of visible spectrum radiation from the lamp. By altering the composition of the fluorescent coatings, variations in spectral output are accomplished.
  • Florescent lamps are more light efficient than incandescent lamps and they have a much longer life span. They also run cooler and produce a fairly balanced spectrum in the PAR range. They operate best in warm temperatures with peak light output occurring when the lamp wall reaches about 38°C. As the temperature decreases, light output falls dramatically to only 50% when the lamp wall temperature is 16°C. Light output also declines as fluorescent lamps age, falling to about 60% after 10,000 hours.
  • Fluorescent lamps are available in three load types: normal output 400 mA (normal output), 800 mA (high output), and 1500 mA (very high output).
  • One disadvantage of fluorescent lamps is their relative bulk in relation to output. Even the very high output fixtures and the new slimmer T8 tubes, when configured in sufficient densities for supplemental lighting, can cast considerable shadows that can interfere with ambient lighting. They are however, useful in growth chambers and particularly in multiple tier applications since their relatively cool operating temperatures allow them to be mounted in close proximity to plant surfaces.
  • Fluorescent lamps are available in a range of spectral qualities. Relatively inexpensive cool white lamps are fine for supplementary lighting, and ‘full spectrum’ lamps are available for replacement lighting applications.


  • Modern high intensity discharge lamps are similar to fluorescent lamps in that they introduce an electrical arc into an elemental gas mixture. This produces a spectral discharge that is characteristic of the elements in the arc. However, they differ from fluorescent lamps in that no fluorescing powders are used on the lamp glass, and the elemental gases are heated under much higher vapour pressures and temperatures. The light intensities and efficiencies obtained by high intensity discharge are higher than either incandescent of fluorescent lamps. The two most common discharge lamps used in modern horticulture are metal halide and high pressure sodium lamps.

Metal Halide (MH)

  • Metal halide lamps use mercury vapour in a quartz arc tube and various iodide mixtures of sodium, thorium, or thallium. The electrical arc vaporizes the halides, heating them to a plasma state, whereupon they emit line spectra characteristic of the elements in the plasma. Metal halide lamps produce a relatively full spectrum of white light that is often preferable to the yellowish light of high pressure sodium when used in public or retail horticultural environments. They provide the best overall spectral distribution of all horticultural lamps, but are not quite as efficient in energy conversion as high-pressure sodium lamps in the PAR range, particularly in the yellow-red spectra

High Pressure Sodium (HPS)

  • High pressure sodium has become the most popular lamp type for commercial supplemental lighting in horticulture. They are the most efficient in the PAR range with the exception of low pressure sodium lamps which, although more efficient in their conversion of watts to lumens, produce a spectral distribution so narrow that they are of little horticultural use. High pressure sodium lamps produce light from an arc-induced discharge in a mixture of sodium vapour and mercury vapour. The emission spectrum is highly concentrated in the yellow-orange-red range (500-650 nm) but is fairly low in the blue range. Used as a replacement light source, HPS lamps may require supplementation with fluorescent, mercury vapour, metal halide, or other light sources high in blue light. However, they are fine as a supplemental source since adequate amounts of blue light are usually available from ambient light to sustain blue-light-specific plant morphogenic responses. HPS lamps have a long life, and are available in a range of wattage sizes as well as ballast/reflector configurations optimized for horticultural production.
  • Radiant efficiency for supplementary irradiation

    The following table summarizes the relative radiant efficiencies for the standard illumination sources used in horticulture.

    Radiation Source


    Lumens per Watt

    Average Life





    Metal Halide



    High Pressure Sodium






    Mercury Vapour



  • Luminaire Placement and Light Distribution Uniformity
    • The degree of growth uniformity in a crop is influenced directly by the uniformity of light falling onto the crop canopy. The manufacturers of horticulture luminaires often recommend specific grid and spacing patterns for various intensities and lighting configurations. These are determined by the specific lamp output, crop requirements, and luminaire reflector designs. Often, an overlapping pattern is designed with some additional modifications to lamp placement and density at the crop margins to produce the most uniform lighting over the entire cropping surface.
  • Supplementary illumination levels and duration


      It has long been accepted that it is more efficient to provide a lower amount of irradiation over a longer period than a high amount over a short period. For example, it is usually better to light a crop at 5 Wm-2 for 18 hours, than at 10 Wm-2 for 9 hours, provided there are no photoperiod requirement conflicts. Not only do the plants use the light more efficiently, but the total number of luminaires and electrical service loading can be reduced, thereby reducing capital investment costs.


    • It has also been shown that the maximum incremental benefit of supplementary illumination occurs when the plants are lit beyond the daylight period, so lighting at night is generally more effective than lighting during the day period. During periods of low ambient light levels, it is a common strategy to light during the day wherever levels fall below a predetermined set point, and to extend the lighting duration period to the maximum recommended for the crop. For example, cucumbers and roses can be lighted for 24 hours per day, while tomatoes and most bedding plants should only be lit for 16 – 18 hours to avoid problems with flower delay.


    • In greenhouses, supplementary light levels have been suggested ranging from 3 W m-2 for ferns and other low light crops, to 20 W m-2 for vegetable crops and propagation areas.

  • Supplemental illumination control


      Lighting systems can be operated automatically using simple time clocks or sophisticated integrated controllers. Some greenhouses with large installations may not have sufficient electrical service to operate all of their lights at the same time, so they may need to be staged in accordance with available electrical power. When integrated controllers are used it is possible to control the operation of supplemental light systems by a number of parameters including:

      • Time – cyclical lighting (for photoperiod control) – supplemental lighting duration control

      • Light – integrated daily light levels – instantaneous radiation set points

      • CO2 synchronization


      • Cyclical lighting is normally only used with incandescent lamps only to provide photoperiod control by cycling a series of relatively short duration lighting periods in the night. By using this method, it is possible to use less overall illumination time and electricity consumption than with conventional long day illumination. It is not recommended for use with HID lighting, since these luminaires are not designed for frequent cycling.


      • For supplemental lighting, regardless of the control method, it is best to operate the lights for extended periods, since short cycling of these luminaires will greatly reduce the lamp and ballast life. Therefore, when setting up programs based on available instantaneous or accumulated light energy it is best to set up some conditions that prevent cycling.


      • These can include a proving time, where the need for either turning the lights on or off must be sustained for a desired period. This prevents the lights from cycling on and off in partially cloudy weather. Another method of preventing cycling when using light based control is to provide for a minimum on and off time override.

    • A typical integrated control strategy for HID lighting operation might include the following:
      • Lighting window – allow the light to be turned on between 5:00 am and 10:00 pm.

      • Lighting set point – allow lights to be turned on during the lighting window period if light levels are below 200 Wm-2.

      • Light accumulation – turn off lights (or don’t allow them to be turned on) if the daily accumulated light exceeds 5.0 kWh.

      • Proving time – light levels must be below the lighting set point for 30 minutes.

      • Minimum on time – once the lights are turned on, to prevent cycling, they must remain on for 2 hours, regardless of other conditions.

      • Other additional strategies could be used alone or in conjunction with the above illustration. For instance, to get the maximum value from CO2 supplementation it is necessary to have adequate light levels. A separate program could be set up to ensure that crops always receive a minimum light level during COsupplementation periods.