Supplemental Carbon Dioxide

J. Raymond Kessler, Jr.

 

      Supplemental carbon dioxide (CO2) refers to the addition of concentrated CO2 to the greenhouse atmosphere to provide more raw material for photosynthesis. Light, water, and CO2 are used by green plants in photosynthesis to produce carbohydrates for growth and metabolism. The rate of plant growth depends on a balance between the manufacture of high-energy compounds (carbohydrates) from CO2 and water in photosynthesis and the utilization of those high-energy compounds by respiration. Growth is only possible when the balance is in favor of photosynthesis.

      Of the two raw materials needed by photosynthesis, water and CO2, numerous studies using a wide range of crop plants have shown that normal atmospheric levels of CO2 limit photosynthetic rate. Water probably is not a limiting factor for photosynthesis directly. When plants approach the wilting point, there is probably enough water in the tissues for photosynthesis. However, wilting causes the stomata to close, CO2 within the tissues of the leaf are quickly used up, and no new CO2 can diffuse into the leaf. So the effect of limited water on photosynthetic rate is probably indirect by restricting the CO2 supply.

      Carbon dioxide is present at a concentration of approximately 350 ppm in the atmosphere. However, this is an average and the actual concentration in a given location can vary. Climatic changes can cause a 4 to 8 percent variation in CO2 concentration daily or seasonally due to increases or decreases in solar radiation, temperature, humidity and the passage of storm fronts. Concentrations of CO2 are also influences by human activity such as burning fossil fuels. CO2 concentrations are usually much higher close to cities, manufacturing, and combustion activities.

      In a greenhouse filled with plants, CO2 concentration will closely follow ambient outside concentrations during the day as long as ventilation is needed. CO2 concentrations rise during the dark period because plants are not using CO2 for photosynthesis and respiration by plants and other organisms are generating CO2, e.g. fungi, bacteria. During light periods in which ventilation is not required, CO2 levels may fall below ambient.

      Ventilation, where possible, is an effective way to maintain a constant supply of CO2 to plants. However, carbon dioxide concentration can fall below ambient conditions in a greenhouse filled with plants when light intensities are high but cold outside temperatures prevents ventilation. In a tightly closed greenhouse with plants, the CO2 concentration can drop to 150 to 200 ppm. This concentration is at or close to the CO2 compensation point or where CO2 produced by respiration equals the amount utilized by photosynthesis. Plant growth is reduced when the CO2 compensation point is reached even for brief periods.

      Many studies have shown that CO2 concentrations well above ambient can benefit plant growth. Typically, a three- to four-fold increase in CO2 concentration yields a 10% to 25% increase in plant growth. Supplemental CO2 increases leaf area, dry weight, lateral branching, and in some cases decreases time to flower. In the greenhouse, supplementing CO2 to about 800-1000 ppm from an hour after sunrise to about an hour before sunset has been used to increase growth of many crops including pansy, geranium, impatiens, begonia, coleus, petunia, chrysanthemum, china asters, carnations, and roses.

      In the greenhouse, there may be times when CO2 levels fall below outdoor levels and limit growth. Carbon dioxide levels can fall quite low in airtight greenhouses when vents are kept closed for extended periods during the winter. With little air exchange to the outside on cold bright days, photosynthetic rates can be high and deplete indoor CO2 levels below outdoor ambient, thus limiting photosynthetic rates.

      However, CO2 is a gas, and like most gasses, is difficult to control. As soon as the greenhouse warms up and requires venting, supplemental CO2 is blown out the vent. For this reason, adding CO2 to the greenhouse atmosphere may be limited to cooler climates and times of the year.

      Supplemental CO2 can, therefore, be viewed as an additional crop input, no different from light or nitrogen. In fact, some authors refer to supplemental CO2 as “CO2 fertilization” or “CO2 enrichment.” However, it is also possible to get CO2 levels too high. High CO2 levels (>3-5000 ppm) will cause an initial growth increase followed by a decrease in growth.

 

CO2 - Plant Age

      The effectiveness of supplemental CO2 depends on timing, duration, and concentration. Applying supplemental CO2 to seedlings in plug flats generally results in reduced time to transplant, greater accumulation of dry matter, and larger leaf area than those under ambient conditions. Begonias given various CO2 levels and artificial light in growth rooms were transplantable four weeks from sowing when given 970 ppm CO2, moderate light, and warm temperature (80°F). This represents a 47% reduction in time to transplant compared to seedlings receiving no additional CO2. Recent work with geranium and pansy showed that 1000 ppm CO2 applied to seedlings at least two weeks old decreased time to transplant. Applying 1500 ppm was only slightly more effective than 1000 ppm, and four weeks were more effective than two weeks, but one week was insufficient. Two-week-old seedlings with one or two true leaves were as responsive as four-week-old seedlings with two to four true leaves. Therefore, plants are most responsive to supplemental CO2 when young and the timing can be important.

 

CO2 - Temperature

      Temperature can have an important influence on the extent of response to supplemental CO2. In the table below, chrysanthemums were given ambient or 1000 ppm CO2 at either 70°F Day / 60°F Night or 80°F Day / 60°F Night temperatures. An increase in day temperature and CO2 concentration increased cut flower stem length and fresh weigh more than increasing either factors alone.

 

Day-night temperature and CO2 effect on relative fresh weight and stem length of Chrysanthemum.

Day-Night Temp °F

CO2 (ppm)

70-60

Ambient

70-60

1000 ppm

80-60

Ambient

80-60

1000 ppm

 

Fresh Weight

‘Souvenir’

100

132

129

148

‘Pink Champagne’

100

122

118

133

 

Stem Length

‘Souvenir’

100

117

121

128

‘Pink Champagne’

100

114

118

126

 

A higher night temperature had very little effect on the plants response to supplemental CO2. Numerous studies have shown that the optimum day temperate for plant growth increases as CO2 increases. A good rule of thumb when using supplemental CO2 is to elevate the day temperature by 5-10°F. One consequence of raising the day temperature is that ventilation can be delayed and the CO2 enrichment period can be extended.

CO2 - Light

      In terms of its effect on photosynthesis, each plant has a unique maximum light intensity above which the rate of photosynthesis cannot increase called the light saturation point. As light increases from a very low level, photosynthesis increases up to the light saturation point. However, if additional CO2 is added to the atmosphere, the light saturation point is reached at a higher light intensity and at a higher photosynthetic rate. In fact, studies have shown that enriching the greenhouse atmosphere with additional CO2 increases growth at all but the lowest light levels. This implies that even under low light conditions that may limit growth, the addition of CO2 can improve photosynthesis and growth. In the winter under very low light conditions, the effectiveness of supplemental CO2 may be limited by low solar radiation. The addition of supplemental light with supplemental CO2 can be used together to improve growth.

 

Effect of supplemental light and CO2 on vegetative growth of tomato.

 

Height (cm)

% Increase

Dry Weight (g)

% Increase

Control + Ambient CO2

28.1

3.8

Light + Ambient CO2

51.3

82.6

16.8

342.1

Control + 1500 ppm CO2

37.2

32.4

5.0

31.6

Light + 1500 ppm CO2

55.7

98.2

17.2

352.6

 

The addition of supplemental light had a larger effect on increasing growth than CO2 but the two together gave the largest increase in growth.

 

CO2 - Nutrition

      Rapid plant growth under supplemental CO2 and bright condition also means an increase in the rate of nutrient uptake and utilization by plants. Low concentrations of nutrients in the medium have been shown to reduce photosynthesis and growth and nutrient deficiencies can occur quickly under CO2 enrichment, especially when combined with supplemental light.

      At present there are few recommendations to guide growers in adjusting fertilizer programs under supplemental CO2. It is generally believed that fertility should be increased under supplemental CO2. However, several studies indicate that some nutrients are depleted quickly while others change very little. The best recommendation to date is for growers to monitor nutrition closely using soil and tissue testing, then adjust fertility programs accordingly.

 

Carbon Dioxide Sources

      Practical application of supplemental CO2 to the greenhouse usually requires a relatively pure source of CO2, a distribution system, and a monitoring and control system capable of maintaining set levels. The more practical sources of CO2 include pure tank CO2 or the clean combustion of fuels, usually propane or natural gas. The economics of equipment and fuel costs often dictate choices for a greenhouse size and location. Carbon dioxide is heavier than air. Therefore, distribution systems should maintain enough turbulence to keep added CO2 evenly mixed with the greenhouse air. Monitoring and control systems vary in sophistication but should be able to measure CO2 levels at several locations in the greenhouse, compare current concentrations to a set point, and adjust the concentration by adding CO2 when required. Many greenhouse environmental control computers have this capability already programed in.

CO2 Burners: These units burn propane or natural gas very cleanly so the products are CO2 and water. They consist of a housing, a precision calibrated gas burner, and a 24-volt gas solenoid valve. One unit cost $450 and can provide 1500 ppm over a maximum of 5000 ft2. The unit also generates about 60,00 Btu/hr and consumes about 60 ft3/hr of natural gas. These generators are hung above head height along the center of the greenhouse. Internal air mixing is needed to distribute the CO2 evenly. The gas supplied to these units must be of a high purity since sulfur contamination will be burned to sulfur dioxide. Calibration of the burners should be checked often because incomplete combustion cane lead to ethylene and carbon monoxide which are toxic to plants. The burner should be kept clear and adjusted to a clear blue flame. The plumbing should be checked carefully for any gas leaks since unburned fuel can harm plants. It is equally important to provide enough oxygen for complete combustion. In tight plastic houses or glass houses in areas prone to ice formation, provide one square inch of opening to the outside for every 2500 Btu/hr burner capacity.

Liquid CO2: Large greenhouse ranges that use large quantities of CO2 often find it more cost effective to install large pressurized tanks of CO2. The CO2 is then piped into the greenhouse area. Service companies replenish the liquid CO2 using tanker-type delivery trucks. One advantage of the method is that the gas is usually very clean and free of contamination.

Solid CO2: In this case, dry ice is placed in special cylinders and CO2 is produced as the ice sublimes. The amount of CO2 going into the greenhouse is regulated by gas flow meters. This method is most often used in small greenhouses.

      It should be kept in mind that CO2 is heaver than air and, in the absence of air movement, it will sink to the lowest level and stratify in the greenhouse. Concentrations may also be higher close to the source of CO2. Therefore, it is important to mechanically mix the air in the greenhouse during enrichment. Heating systems, when operating, usually create enough convective air movement. Internal turbulators may be used at other times.

 

Control Systems

      Controls for a supplemental CO2 system usually consists of a CO2 generator, a control system, and a “feedback” monitoring system. The monitoring device is usually an infra-red gas analyzer (IRGA). A sample of greenhouse air is pumped into the IRGA which determines the current greenhouse CO2 concentration. This information is sent to the control system that compares the current concentration to a set-point which is pre-set by the grower. If the current concentration is lower than the set-point, the control system activates the CO2 generator.

      The introduction and wide-spread application of greenhouse environmental control computers has made automatic and continuous regulation of supplemental CO2 possible. Computers have been programed to modulate the CO2 levels in the greenhouse based on changing light condition. Plants make more efficient use of supplemental CO2 under bright condition and less efficient use as light levels decline. These programs provide more supplemental CO2 under bright conditions and a reduced rate as light levels fall. Supplemental CO2 is usually ceased at light levels below a minimum, (depending on the crop), often 500 foot-candles.

 

Economics

      The cost of installing and operating CO2 generators is usually low compared to the potential gains in growth and plant quality. Investment and installation of the generators usually amounts to about $0.10 to $0.12 per square foot. The cost for fuel may be $0.10 to $0.15 per square foot per year. However, most of this cost will come during the winter months. It should be kept in mind that each CO2 burner contributes about 60,000 Btu/hr that the heating system will not have to generate.

      The demand for quality crops, tight production schedules, and increases in the cost of production should encourage growers to take a careful look at supplemental light and CO2. Though little work has been done on plug-grown seedling, some evidence indicates the supplemental light combined with CO2 may have a synergistic effect on plant growth. The two combined might be a way to dramatically reduce time to transplant and increase quality of plug-grown seedlings.