The number one mistake most growers make is underestimating and undersizing the ventilation in their grow-rooms, causing major heat and humidity problems. Your ventilation system is literally the lungs of your grow system. It must be, at the very least, adequate for your needs, allowing for more than sufficient movement of air going in and out. To calculate what size ventilation fans to use, first find out the cubic footage of your grow chamber. Use this formula: height × width × length. Fans are sold using cubic foot per minute (CFM) as a measurement. Oversize your fans—get something bigger than you need. Ultimately, you want to be able to completely exchange and evacuate all air in 3 to 5 minutes.
For safety, you must filter all incoming and outgoing air. Filtering incoming air decreases the risk of airborne contaminants. Filtering outgoing air is simply necessary for odor control. This is very important. I can’t emphasize this enough: you must control and eliminate all odors of cannabis!
Urinal cakes (the white, hockey puck sized, odor-eliminating cakes you see in urinals) are one option. Placing them in your final exhaust air ducts masks and eliminates any residual trace of cannabis odor. They are inexpensive, readily available, and work well for a long time. Placed in the duct after the filter masks any odors, they make it seem as though the vented air is coming from a bathroom, not a growroom.
Environmental controller visible in lower left hand corner.
Photo: Freebie
Charcoal filters can be used for outgoing air, and are available at most grow stores in many sizes and for many different applications. However, for incoming air, charcoal filters will not eliminate mold or mildew, so you must get air scrubbers, which are widely available.
In-line fans are perfect for moving air in smaller chambers, and roof ventilators are great for medium-sized chambers. For large chambers, you will need industrial-duty exhaust fans with motorized dampers. Evaporative coolers can be used in the vegetative chambers, but not in flowering rooms, because they increase humidity, which can cause mildew and mold.
There are standard equations for calculating the exhaust rate in a given room; one of them is to multiply the area of the room (think back to grade school: (length × width) by height). The resulting figure is the number of cubic feet per minute (CFM) you will need to evacuate. Fans are provided with these measurements, so all you need to do is match the correct fan up to your figure. Remember to oversize, as you never want to overload the fans. They should come on and evacuate all the air while replacing with new in just three to five minutes.
The opposite of exhaust is intake, and unless you have access to fresh, filtered air in your growroom, you’ll need to set up some form of intake fan. The general rule is that for every one CFM of exhaust, you need one CFM of intake to replenish the exhausted air. Note: all intake air should enter at ground level and exhaust air should exit from the top of the room at the opposite ends of the room, creating a complete air transfer.
Preferably, you should have slightly more air exiting the growroom than entering, to create a negative pressure situation in order to stop errant odors from escaping into the surrounding environment. To achieve this, use a fan speed controller (available from grow stores and some hardware stores) and simply plug the intake fan into the speed controller, then plug the controller into a power outlet. Turn on the fan and adjust the dial so that the speed of the intake fan is slightly less than the exhaust fan—thus ensuring that no odors escape the room without going through the charcoal filter. If you don’t care about escaping odors, a positive pressure is preferred. It is difficult for mold spores and pests to enter a positive pressurized greenhouse or grow-room. This is how legal plant producers operate their ventilation systems.
Air movement within the chambers is also very important. Oscillating wall-mounted fans keep air and CO2 moving around the chambers perfectly. Don’t force too much wind on the plants—a nice, even breeze is the goal, not a hurricane.
Heat around plants can be a problem. The more lights there are, the more heat is being generated. Heat is measured in British Thermal Units (BTU). To give you an idea of how much heat is produced, a 1,000-watt light gives off approximately 3,400 BTU per hour, which could boil approximately 2 gallons of water in an hour. The larger and more lights you use, the hotter your environment gets.
Temperature is the most easily manipulated factor. Heat will cause or aid internodal stretching / stem elongation, root borne disease, molds, and mildews; it will also stop plant development. The perfect temperature for cannabis is between 70 to 75°F, and 80°F if using CO2-enriched air. These are acceptable parameters. Every cultivar of cannabis prefers a different humidity and temperature range: experiment and find the perfect humidity and temperature range for your specific needs. As a guide, humidity must be kept at 74 to 80% in the cloning chambers. In the vegetative stage, chambers should range from 60 to 70%. The flowering chambers require from 50 to 60%.
Charcoal filter.
Photo: Samson Daniels
Window air conditioners are fantastic growroom refrigerators (again: oversize, not undersize). They are also rated in BTU capacity; for example, two 1,000-watt lights generate 6,800 BTU per hour, requiring at least a 12,000-BTU air conditioner.
Air intake, exhaust, and temperature controllerX.
Photo: Hydrofarm
Cold winter days and nights can cause growroom temperatures to drop to undesirable levels—excessively cold nighttime temperatures will slow plant growth and sometimes stunt the plant completely, resulting in diminished yields. You must monitor these temperatures. A thermostat-controlled heater will rectify this situation—either place the thermostat to your desired temperature and let it come on and off automatically as needed, or plug the heater into a good quality climate controller and let it maintain ideal temperatures for you. Grow stores sell inexpensive digital hi/lo temperature displays with a sensor probe you can place where you want in the medium.
Note: Remember there is a difference between ambient temperatures (surrounding air in the growroom) and radiant heat (heat directly under the lights). You must also measure the temperature at the plants’ canopy, as excessive radiant heat will burn plants, dry out their leaves, and diminish the potency of your buds by destroying resin glands at the bud tips (or whatever is closest to the heat source). Never place lights close to plants; use the digital temperature display’s probe to monitor temperatures at the canopy.
Caution: Do not point heaters at plant reservoirs or any combustible materials or surfaces. Simply let the heater warm the air. Don’t point it at anything that will melt or burn! Don’t point it at anything, period! Be safe.
Basic Grower’s Tip: Always keep oscillating fans on at night to keep air circulating
Grow Room Monitoring Systems
Inside of a dehumidifier.
Freebie
Monitor both high and low temperatures as well as humidity.
A monitor that keeps track of pH fluctuation, nutrient PPM, and reservoir temperature.
There are many recording devices available to you.
Stoned Rosie
These are utilized for temperature and humidity control, and work by turning fans on and off when either level exceeds pre-set points. Regular air changes are required for optimum plant health. A combined humidistat / thermostat and a speed controller allow you to control the airflow and the number of air changes per hour. They also compensate for hot and cold outside climates by increasing and decreasing the air exchange rates. Fans will run faster / slower, yet continuously reach full potential when needed to control temperature and humidity.
Carbon, hydrogen, and oxygen make up approximately 90% of the dry material in a finished bud. CO2—or carbon dioxide, which is made up of both carbon and oxygen—in the air supplies all of the carbon to a plant. Thus, it makes perfect sense that you can accelerate your plants’ growth by increasing the amount of available CO2. Like humans, plants breathe air—the only difference between our respiration and theirs is that their roots take in oxygen and the leaves take in CO2 and expel oxygen, whereas we take in oxygen and expel CO2. A plant utilizes CO2 gas to produce sugars needed for photosynthesis. At night, the plant expels any unwanted CO2. The CO2 gas is the carrier source of the carbon that plants manufacture into organic compounds. Thus, increasing CO2 levels will accelerate plant growth dramatically.
The introduction of CO2 can accelerate growth by as much as 30 to 35%. The more available light there is, the higher the PPMs of CO2 must be utilized to compensate. A plant at two months requires approximately ten times the amount of CO2 than that of a clone / seedling. Simply put, the more light there is, the more CO2 must be utilized to expedite manufacture of sugar via photosynthesis and, thus, increased cellular division / plant growth.
Besides inadequate ventilation, lack of CO2 is the biggest mistake growers make concerning air quality. Also, you must employ oscillating fans to keep air / CO2 moving across the plants’ leaves to properly reap the benefits of CO2 injection; pure CO2 is heavier than air. You must supply large amounts of air and CO2 for maximum growth and production. Larger environments need CO2 injection via air tanks and regulators, but small areas can use massive air-in and -out ventilation systems.
The more light available to a plant, the more CO2 it needs for photosynthesis. Atmospheric CO2 levels can be as high as 400 to 450 PPM. If you ensure that your growroom has very good ventilation and good circulation fans, the plants should have more than enough CO2. Depending on the size of your chambers, your needs can range from 500 to 2,000 PPM. However, more than 2,000 PPM of CO2 does not do the plants any good. Extremely high light densities require 2,000 PPM of CO2. Work up to the higher levels slowly, because all other factors must be in excellent working order for the plants to photo-synthesize these higher amounts.
The wiring here is acceptable, although on an industrial scale, hardwiring is preferred. Equipment visible here includes reservoir monitors and environmental controllers.
Photo: Freebie
Your plants do not need CO2 in the dark. Switch on the CO2 half an hour after the lights come on—after the fans’ intake and exhaust go off (they should be turned on first thing in the morning) – and shut it off half an hour before the lights go off. This ensures that the CO2 is used up before the lights go off. Cease CO2 injections 7 to 14 days prior to harvest, to ensure complete ripening of flowers/buds.
Oscillating fans must also be placed at the floor level to circulate the heavier CO2 to the plants. Some growing chambers may need a complete air change every hour to make sure that CO2 and air are constantly being exchanged. Weather-stripping on doors can eliminate CO2 loss and leakage. You must circulate CO2 up from the floor or inject it directly into the fan to ensure good coverage among the plants.
Again, if your garden is not completely fine-tuned, 2,000 PPM of CO2 can be toxic. Increase levels slowly until you completely understand how it works. If you do not get a 25-30% increase in plant growth, examine the levels of light and root oxygen available to your plant. Higher levels of CO2 require higher temperatures—80°F. Be aware: this will dramatically increase the water and nutrient usage.
CO2: Distance / Level
Note: these figures are based on running all plant resources available at maximum and at a temperature of 80°F. Also note that you would never place a plant within a foot of a light unless it was air-cooled.
* Ambient CO2 in the cities is between 400 to 450 PPM.