CO2 Best practice guide: How much to use

This summary draws on existing information, which is currently unable to answer all the questions growers have following the recent rapid expansion of energy centres, including gas and biomass CHP, biomass boilers and anaerobic digesters, as well as inter-row lighting, which impact capacity to generate CO₂ and for crops to photosynthesise.

We therefore outline the information available to date and will update as more becomes available, e.g. from project PE 021 ‘Targeted CO₂ enrichment management for modern varieties in long-season tomato crop production in the UK’.   

Economics drives CO₂ enrichment strategies and accurate measurement is an important factor in order to avoid wasteful generation and, possibly, adverse plant reactions. Concentrations have traditionally been measured using infrared gas analysers (IRGAs). These can be very accurate and reliable, but regular calibration is important. This can be done using calibration gases of known concentration. The zero value can also be tested by removing CO₂ from the air with an appropriate absorbent, such as soda lime. Glasshouses often have a single analyser to measure several areas or compartments and, consequently, sample pipes taking air samples to the analyser may be of considerable length. Commonly, air will be drawn continuously from all of the pipes and samples will be introduced sequentially to the analyser via a multiplexer. Sufficient time therefore needs to be allowed for the system to purge between samples to ensure accurate recording of each new glasshouse area. If measurements are taken too soon, air from the first block may still be present, and the readings will be for a mixture of the two blocks. It is good practice to sample at a height that will reflect the CO₂ concentration in the upper crop canopy, since this is an important zone for photosynthesis. More recently, electronic CO₂ sensors have become more widely available. There is still a need to calibrate these regularly, but because each growing area can have its own sensor, they overcome some of the problems associated with a centralised IRGA and long runs of pipework. Recent commercial experience suggests that these are reliable and give accurate readings.

Carbon dioxide supplementation

Rates of carbon dioxide supplementation are dependent on the crop response and economics. Ornamental and edible growers may take somewhat different approaches. In general, carbon dioxide supplementation of 1,000 ppm during the day when vents are closed is widely used. In order to improve economic efficiency, CO₂ levels can be set depending on vent position to minimise losses; for example, at 10% vent opening, the CO₂ supplementation set point might be reduced, e.g. to 400–600 ppm as a typical setting where there may be some economic or physical constraints to CO₂ supply. The following is a recommended strategy for edible growers provided by a Canadian OMAFRA factsheet. On sunny days when the vents are closed, supplement with 1,000 ppm CO₂, while on cloudy days when the light level is below 40 W/m², supplement with only 400 ppm CO₂. However, most growers will supplement with 1,000 ppm regardless of level of cloud cover. The climate-control computer can be set to adjust the CO₂ level depending on the light measured, but once the vents open beyond 10%, or the second stage of exhaust fans becomes operational, the focus is to maintain a CO₂ level in the crop canopy at 400 ppm.

Cucumber growers had reported the development of phytotoxic growth symptoms, mainly leaf bleaching, at high concentrations of CO₂ in project PC 159. A factsheet with guidelines for CO₂ management was produced, with the key points being to keep enrichment set points at 600 ppm in the low light levels of February and March, then 800–1000 ppm in spring and summer. CO₂ toxicity in tomato may also result in thick, curling, discoloured leaves at concentrations of 1,000–1,500ppm, in conjunction with high light levels, and flower abortion may occur if levels are sustained for long periods of time (Benton-Jones, 2007).

CO₂ utilisation efficiency

Hand (1984) defined CO₂ utilisation efficiency as “the net CO₂ uptake by the crop expressed as a percentage of the CO₂ that is added to the greenhouse atmosphere”. According to Hand, 100% efficiency is obtained when the CO₂ that is added to the greenhouse atmosphere is just sufficient to maintain the ambient concentration present outside of the greenhouse. Under these conditions, there is no wastage of CO₂ and the amount that is added is exactly equal to the amount taken up and fixed in photosynthesis. With this definition, other regimens give different efficiencies and Hand estimated that enrichment to 1,000 vpm CO₂ in bright, calm weather in winter probably gave an efficiency of CO₂ utilisation as high as 69%, while it fell to 5% in dull, windy weather. This will obviously reduce in summer months with venting. If the efficiency of CO₂ utilisation is low because natural light levels are low, the efficiency can be boosted either by the use of supplementary lighting or by using an algorithm to help the grower control the CO₂ concentration at the level that gives the highest CO₂-utilisation efficiency. Taken to the ultimate, this could take the form of functionality built into a climate-control computer. This is the basis of work carried out by Chalabi et al. (2002a, b) that resulted in a set of computer programs for use by growers (Bailey, 2002). Unfortunately, these are now outdated. 

One alternative to the continual addition of CO₂ is to raise the ventilation temperature and enrich only when the vents are closed (or slightly open), while maintaining the external ambient concentration when the vents are more open. However, when this approach was tried with a tomato crop in the UK, fruit quality suffered badly from the elevated temperature (Slack et al., 1988). Another option is to raise the ventilation temperature for only part of the day and to enrich with CO₂ during that period. When the vent temperature was raised from 21°C to 27°C for the first four hours in the morning on spray chrysanthemum, and enrichment to 1,000 vpm CO₂ was maintained while the vents were less than 10% open, the weight of individual flower sprays was increased. However, they also produced slightly longer pedicels, due to the higher day temperature (Cockshull & Fuller, 2001). A similar strategy might merit being tested on other crops.

Temperature effects

Temperature has relatively little effect on net photosynthesis at ambient levels of CO₂ but can markedly boost growth when CO₂ is at an enriched level. Langton and Hamer found, for example, that on a single-leaf basis, raising the CO₂ level from 350 vpm to 1,000 vpm increased net photosynthesis in impatiens by around 30% at 12°C, by around 70% at 18°C, and by around 103% at 24°C in Defra-funded project HH1330SPC (2003). An implication of this is that enriching with CO₂ should (quality and timing permitting) be accompanied by appropriate increases in temperature, or, alternatively, CO₂ levels should be manipulated actively in relation to greenhouse temperature. This approach was adopted by Heins et al. (1986), who developed photosynthetic optimisation equations for chrysanthemum based on CO₂, temperature and PPFD (photosynthetic photon flux density) and used these in a computer-controlled greenhouse to optimise temperature and CO₂ in relation to prevailing PPFD every 15 minutes. This gave significantly greater leaf, stem and total dry weight at flowering. This approach was not taken up commercially at the time, but the concept has recently been extended and promoted by Danish researchers as ‘IntelliGrow’ and has attracted commercial interest (Rosenquist & Aaslyng, 2000).  

A cause of low CO₂ utilisation efficiency may be the leakage of CO₂ from the greenhouse. One reason for this is wind passing over the structure. As a result, measures to reduce external wind speeds are worth considering. All unintentional leaks from the greenhouse should be minimised or blocked, with the most likely sites being poorly sealing ventilators. Transparent screens within glasshouses are another means of reducing leakage, but they must be removed or withdrawn when the quantity of solar radiation that they are blocking becomes significant.

For more information on screening, visit the GrowSave website.