Marijuana plants grown either in soil/soilless or with hydroponics consume carbon dioxide (CO2) through their leaves, and expel oxygen in the daytime hours. During the dark period they consume oxygen and expel carbon dioxide.
In a process called photosynthesis, light energy is converted to chemical energy via carbon dioxide and water. Plants use this chemical energy to grow.
CO2 is measured in parts per million, ppm. The amount of carbon dioxide present in outdoor air will generally range from 350-450 ppm.
Plants do well if the amount of CO2 that is supplied to them remains in the range of outdoor air, 350-450 ppm, during the day.
As the level drops below 350 ppm, plant growth slows down. When the level reaches about 200 ppm, plant growth stops because the amount of carbon dioxide available is too low.
When the amount of available carbon dioxide rises back over 200 parts per million, to more acceptable levels, growth of the plant will resume again.
Since plants consume carbon dioxide, in a grow area with little or no replacement carbon dioxide, they can consume what is available causing growth to slow or stop.
To prevent the carbon dioxide levels from dropping too low, letting in fresh air that contains carbon dioxide, or generating it, and keeping it circulating around the leaves with a fan is a good idea.
CO2 below 200 ppm - plant growth stops.
CO2 below 350 ppm - plant growth slows.
CO2 350-450 ppm - good for marijuana.
CO2 450-1500 ppm - optimal for marijuana.
CO2 above 1500 ppm - extra CO2 is not utilized.
Grown under strong light and warm temperatures, carbon dioxide levels between 450-1500 ppm encourage faster growth rates. At levels over about 1500 ppm, the extra CO2 can not be utilized.
In order to maximize yield and minimize the length of time till harvest, some growers increase the amount of CO2 so the plants have a steady concentration between 450-1500 ppm during the day period.
The optimal amount of CO2 will increase relative to the amount of light available, and the temperature of the air that surrounds the leaves.
As the amount of light increases over 250 watts of metal halide or high pressure sodium, and temperature increases to over 70°F/21°C, the plant is able to process CO2 at levels over 350-450 ppm.
These three factors must all increase together for any benefit to the plant. There is no reason to increase CO2 levels to over 450 ppm if either the amount of light is below 250 watts or the temperature is below 70°F/21°C.
The stronger the light is over 250 watts, the higher the temperature should be over 70°F/21°C and the higher the carbon dioxide levels should be over 450 ppm.
When possible, if you are growing under strong sunlight, like in a greenhouse, aim for carbon dioxide levels of 1250-1500 ppm and temperatures between 80-85°F or about 26-29°C during the day period.
If you are growing indoors with over 1000 watts of light, aim for carbon dioxide levels of 850-1500 ppm and temperatures between 75-85°F or about 24-29°C during the day period.
If you are growing indoors with 1000 watts of light or less, carbon dioxide levels between 450-850 ppm and temperatures between about 70-80°F or about 21-26°C are closer to optimal, during the period the light is on.
Aim for CO2 at levels of:
About 750-850 ppm with 1000 watts of light.
About 650-750 ppm with 600 watts of light.
About 550-650 ppm with 400 watts of light.
About 450-550 ppm with 250 watts of light.
These carbon dioxide levels have worked for the author of this guide, but you might want to experiment and only produce the minimum amount of carbon dioxide you feel your plants need.
Higher levels of carbon dioxide than needed won't harm plants, but will go to waste if the plant can't utilize them. If you are producing CO2, anything in excess of what is consumed gets wasted, that might work out to a financial loss.
Even when CO2 levels are raised to optimum, too much heat can slow down growth or kill a plant. The higher the temperature gets over the recommended amount, the more detrimental it is.
When increasing CO2 to optimal levels:
Always try to keep temperatures at or below 85°/29°C with strong sunlight or grow lights over 1000 watts.
Always try to keep temperatures at or below 80°/26°C with grow lights 1000 watts or under.
Adding carbon dioxide to a grow room during the period when the light is on is not something for the beginner on a budget to engage in, unless you have a cheap and easily obtainable supply, with a means of measuring it.
List Of Materials
carbon dioxide meter
carbon dioxide generator
*** carbon dioxide meter: A carbon dioxide meter will allow you to monitor the CO2 level in your grow room.
A reliable CO2 monitor will usually cost $100 or more. A very good model that is fairly accurate and relatively inexpensive is located here.
Have had this model in the flowering room since 2010 with excellent results. Besides carbon dioxide it monitors the temperature and humidity of the room it is in, also includes a clock.
It has an alarm that allows you to set low and high carbon dioxide warning levels. When programmed to do so, if the carbon dioxide level gets too low or too high an alarm will sound.
*** carbon dioxide generator: If you are interested in ensuring your plants get enough CO2, get a carbon dioxide meter and monitor your grow area for a time.
If necessary there are various kinds of carbon dioxide generators available. They will raise the carbon dioxide in the grow area they are put in.
Marijuana Grower's Handbook
Very good book by Ed Rosenthal that encompasses indoor and outdoor marijuana cultivation with soil and hydroponics. A reference manual with over 500 pages and color images throughout.
Marijuana Grower's Handbook
The Cannabis Encyclopedia
Easy to read with comprehensive focus on growing and consuming for medical and recreational purposes. This will be of interest to growers, patients, caregivers, consumers, or anyone interested in consuming, growing, or producing cannabis products.
Over 2,000 color images on 596 pages. Loaded with recent information covering all aspects of marijuana. The grow section explains hydroponics and soil growing, growing indoors, outdoors, and in greenhouses.
The Cannabis Encyclopedia