When plants appeared and evolved on Earth, it is known for a fact that carbon dioxide (CO2) concentration was much higher than it is now. Then, the CO2 concentration was certainly above 1000 parts per million (ppm). Actually, the average CO2 concentration in outdoors air is about 400 ppm on the planet (not really true at your location). Thus, plants enjoy and are stimulated by breathing air with a higher CO2 concentration. That’s why so many indoor gardeners enrich their garden with CO2 during photosynthesis to supply the plants with this essential building material. Through photosynthesis, the carbon in CO2 is extracted and takes part in the building of leaves, stems, flowers and fruits. Proper CO2 concentration from early growing to fructification allows for faster maturation and larger yield.
The benefits of CO2 enrichment are to reduce the time from seedling to harvest, and generally accelerate growth and augment crop yield. Plants also better resist some pests like moulds. Rutgers University compared Romaine lettuce grown outdoors and in a climate controlled greenhouse with CO2 enrichment. The results were clearly to the advantage of the indoor greenhouse grown lettuce. Romaine lettuce grown outdoors reached ready-to-market maturity in 62 days. In the greenhouse under a well controlled climate and CO2 enhancement, lettuce heads were ready-to-market in 48 days: a clear gain of 14 days to get to harvest. Also the greenhouse yield weighted 33 % more than field grown lettuce heads. Yield quality was more uniform and greenhouse heads were paid a higher price.
Plants get the essential carbon atoms only from the air through breathing through their stomas (Figure 1). As soon as stomas close, the plant is deprived from the carbon essential material to build the vegetal cells. Most essential nutrients to plants are drawn through the roots. If one essential nutrient is not in the proper proportion relating to all others, it becomes the limiting factor that slows plant growth. Very often CO2 and carbon availability become the limiting factor. Proper air temperature and humidity are prerequisite to maintain the stomas opened at all times for the plant to breath in CO2 (photosynthesis) and breath out gaseous surplus and waste (in the dark). That is why a close to optimal climate control is required in an indoor garden to keep the stomas opened as much as possible to get maximum plant productivity.
Figure 1: Plants leaves are covered with hundreds to thousands stomas per square inch to let the plant breathe gases like CO2, oxygen and water vapour.
It is well known that a CO2 level in the garden's air between 700 and 900 ppm improves crop development and yield. Most plants grown for their beautiful flowers or foliage optimally develop at about 800 ppm. Roses are distinctive as they require about 1200 ppm in carbon dioxide concentration for best results. For many fruits and vegetables, the ideal CO2 level in the garden should be at least between 1000 and 1200 ppm.
Too much CO2 is bad to the plants. Too high CO2 level lowers plants' transpiration during photosynthesis: without or with less transpiration less nutritive solution is drawn thru the plant, thus less food enters the plant and growth slows down. Under too high CO2 level, necrosis spots (dead vegetal tissue, Figure 2) appear on leaves that may also roll into themselves. These dead tissue spots are a great food for bacteria and molds. Too much of a good thing, again, turns out bad results like a lower weighted yield per plant and a lower quality produce.
Figure 2: Necrosis spots a leave's surface and sometimes the leave's edges may change colour turning either yellowish or brownish or even black.
Generally, enriching the garden's air to raise the level between 1,000 and 1,500 ppm is recommended. There is apparently no benefit to augment the concentration higher than 1,500 ppm. Higher levels are a human health hazard. Plants do not benefit from higher levels either. That can also be a waste of money. But for exceptional species, most plants breathe CO2 only during photosynthesis which is when there is light.
Raising the CO2 level when the plants are in the dark is useless and harmful to plants. In the dark, plants reject all left over gaseous materials not integrated during photosynthesis. Some CO2 is also rejected. So, raising the CO2 level in the dark may cause a "jam" preventing the plant's cleanup in preparation for the next photoperiod.
Carbon dioxide (CO2) is not toxic in itself. Nevertheless, the higher the CO2 level, the more the human breathing and brain functioning are affected. Persons with respiratory problems like asthma may be affected at a low level like 1000 ppm where other people will not feel any discomfort. But for the hazardous high levels in excess of 6 000 ppm, the CO2 caused discomforts are totally reversible. The affected person just has to go breathe in a lower CO2 level place like outside until the discomfort disappears as the CO2 level in blood lowers.
The following table shows CO2 levels associated with various experienced discomforts.
|Concentration||Situation||Symptoms and feelings|
|600 - 800 ppm||Office or well vented dwelling
|1000 ppm||Acceptable level for closed room
||Possible symptoms for asthmatics and beginning of « intellectual fatigue » for sensitive persons|
|1200 - 2000 ppm||
||Yawning and drowsiness or dizziness
Asthma and previous symptoms increasing
|5000 ppm||High limit for a continuous exposure during 8 hours||Only for tolerant persons
Previous symptons reinforced
|6000 - 30 000 ppm||Short exposure only||Fainting possible prior to death*
|3000 - 8000||Out of control indoor garden CO2 enrichment||Breathing and cardiac rythms increase|
|10 000 ppm +||Nausea, vomitting, dizziness, fainting|
|20 000 ppm +||
Fainting rapidly and death* is likely if nobody acts rapidly to get the affected person to breath less CO2 concentrated air
|* CO2 being heavier than Oxygen, the latter is pushed up and replaced by CO2. Unconcious person lying down dies by suffocation.|