C02, or carbon dioxide, is absolutely necessary for plants to perform photosynthesis. They consume carbon dioxide and emit oxygen into the atmosphere. In a greenhouse, where your goal is to increase plant yields, increasing the carbon dioxide levels will cause your plants to grow more quickly and may produce higher yields (depending on the plant).
Introducing high levels of carbon dioxide does not necessarily require expensive commercial equipment. You can build a simple C02 generator for your home greenhouse.
Make the Greenhouse Airtight
When you start to make deliberate changes to the atmospheric conditions in your greenhouse, you must begin with an airtight room. If the greenhouse is not airtight, all the carbon dioxide you so carefully produce will simply escape into the atmosphere.
Simply applying plastic window seal around the windows and doorway may be sufficient to keep the carbon dioxide in your greenhouse. An atmospheric ppm (parts per million) meter will be helpful in determining whether carbon dioxide levels in your greenhouse are sufficient to have an effect on your plants.
Dry ice is carbon dioxide that has been frozen to roughly 100 degrees below zero F. At such extreme temperatures, carbon dioxide becomes solid. When brought out of the deep freezer into normal atmospheric temperatures, the carbon dioxide slowly thaws and returns to its gaseous state.
Dry ice is useful in the greenhouse because it is inexpensive and relatively easy to use. Simply cut some air holes in a styrofoam cooler, drop in a large block of dry ice and let it evaporate. Ideally, you want to suspend the cooler of dry ice above your plants, so the carbon dioxide rains down over them.
Use caution when handling dry ice--it can cause frostbite extremely quickly. If you touch the dry ice, immediately soak the area in tepid water and seek medical attention if necessary.
When yeast reacts with sugar, it creates carbon dioxide. This is the chemical reaction responsible for many of humanity's staple foods, including bread and beer. You can harness the carbon dioxide produced by the yeast and vent it into your greenhouse.
You will need a large, airtight fermentation bucket such as those sold for home beer or wine making. Add a mixture of baker's yeast, warm (70 to 75 degrees F) water and sugar. You will need approximately 3 lbs. of sugar per gallon of water. The amount of yeast depends upon how fast you need the reaction to occur and how much carbon dioxide you want to produce per day. A few tablespoons of yeast is enough to produce carbon dioxide for a small greenhouse.
Connect a hose to the bucket to vent the carbon dioxide. Keep the loose end of the hose submerged in distilled water to prevent bacteria from entering the fermentation bucket. The carbon dioxide will bubble up through the water and enter the atmosphere in the greenhouse.