Chapter 13: Air

Marijuana Horticulture: The Indoor / Outdoor Medical Grower’s Bible

by Jorge Cervantes

Fresh air is essential in all gardens. Indoors, it could be the difference between success and failure. Outdoor air is abundant and packed with carbon dioxide (CO2) necessary for plant life. For example, the level of CO2 in the air over a field of rapidly growing cannabis could be only a third of normal on a very still day. Wind blows in fresh CO2-rich air. Rain washes air and plants of dust pollutants. The outdoor environment is often harsh and unpredictable, but there is always fresh air. Indoor gardens must be meticulously controlled to replicate the outdoor atmosphere.

Carbon dioxide and oxygen provide basic building blocks for plant life. Oxygen is used for respiration – burning carbohydrates and other foods to provide energy. Carbon dioxide must be present during photosynthesis. Without CO2 a plant will die. Carbon dioxide combines light energy and water to produce sugars. These sugars fuel the growth and metabolism of the plant. With reduced levels of CO2, growth slows to a crawl. Except during darkness, a plant releases more oxygen than is used and uses much more carbon dioxide than it releases.

Roots use air, too. Oxygen must be present along with water and nutrients for the roots to be able to absorb nutrients. Compacted, water saturated soil leaves roots little or no air, and nutrient uptake stalls.

Air Movement

Air ventilation and circulation are essential to a healthy indoor harvest. Indoors, fresh air is one of the most overlooked factors contributing to a healthy garden and a bountiful harvest. Fresh air is the least expensive essential component required to produce a bumper crop. Experienced growers understand the importance of fresh air and take the time to setup an adequate ventilation system. Three factors affect air movement: stomata, ventilation, and circulation.


Stomata are microscopic pores on leaf undersides that are similar to an animal’s nostrils. Animals regulate the amount of oxygen inhaled and carbon dioxide and other elements exhaled through the nostrils via lungs. In cannabis, oxygen and carbon dioxide flows are regulated by the stomata. The larger the plant, the more stomata it has to take in carbon dioxide and release oxygen. The greater the volume of plants, the more fresh CO2 rich air they will need to grow quickly. Dirty, clogged stomata do not work properly and restrict airflow. Stomata are easily clogged by dirt from polluted air and sprays that eave filmy residues. Keep foliage clean. T avoid clogging stomata, spray foliage with tepid water a day or two after spraying with pesticides, fungicides, or nutrient solution.


Plants use all CO2 around the leaf within a few minutes. When no new CO2 rich air replaces the used Co2 depleted air, a dead air zone forms around the leaf. This stifles stomata and virtually stops growth. If it is not actively moved, the air around leaves stratifies. Warm air stays near the ceiling, and cool air settles near the floor. Air circulation breaks up these air masses, mixing them together. Avoid these would-be problems by opening a door or window and / or installing an oscillating circulation fan. Air circulation also helps prevent harmful pest and fungus attacks. Omnipresent mold spores do not land and grow as readily when air is stirred by a fan. Insects and spider mites find it difficult to live in an environment that is constantly bombarded by air currents.


Fresh air is easy to obtain and inexpensive to maintain – it is as simple as hooking up and placing a proper sized exhaust fan in the most efficient location. An intake vent may be necessary to create a flow of fresh air in the room.

A 10-foot square garden will use from 10 to 50 gallons or more f water every week. Plants transpire (similar to evaporation) most of this water into the air. Every day and night, rapidly growing plants transpire more moisture into the air. If this moisture is left in the grow room, humidity increases to 100 percent, which stifles stomata and causes growth to screech to a halt. It also opens the door for pest and disease attacks. Replace moist air with fresh, dry air, and transpiration increases, stomata function properly, and growth rebounds. A vent fan that extracts air from the grow room is the perfect solution to remove this humid, stale air. Fresh air flows in through an intake vent or with the hep of an intake fan.

Ventilation is as important as water, light, heat, and fertilizer. In many cases, fresh air is even more important. Greenhouses use large ventilation fans. Grow rooms are very similar to greenhouses and should follow their example. Most grow rooms have an easy to use opening, such as a window in which to mount a fan, but security or room location may render it unusable. If no vent opening is available, one will have to be created.

All grow rooms require ventilation. This system could be as simple as an open door or window that supplies and circulates fresh air throughout the room, but open doors and windows can be inconvenient and problematic. Most growers elect to instal a vent fan. Some growers need to install an entire ventilation system including ductwork and several fans.

A vent fan pulls air out of a room four time more efficiently than a fan is able to push it out. Vent fans are rated by the amount of air, measured in cubic feet per minute (CFM) or (cubic meters per hour) they can move. The fan should be able to replace the air volume (length x width x height = total volume in cubic feet or meters) of the grow room in less than 5 minutes. Once evacuated, new air is immediately drawn in through an intake vent or an intake fan. Covering the intake vent with fine mesh screen will help exclude pests. An intake fan might be necessary to bring an adequate volume of fresh air into the room quickly. Some rooms have so many crack for air to drift in that they do not need an intake vent.

Do not setup a circulation fan in the room and expect it to vent the area by pushing air out a distant vent. The circulation fan must be very large to adequately increase air pressure and push enough air out a vent to create an exchange of air. A vent fan, on the other hand, is able to change the pressure and exchange the air quickly and efficiently.

Squirrel cage blowers are efficient at moving air but are very loud. Blowers with a balanced, well-oiled wheel run most quietly. Felt or rubber grommets below each foot of the fan will reduce noise caused by vibrations. Run motor at a low RPM t lessen noise.

Online fans are designed to fit into a duct pipe. The propellers are mounted to increase the airflow quickly, effortlessly, and as quietly as possible. Inline fans are available in quiet, high-quality models that run smoothly.

Propeller or muffin fans with large fan blades expel air through a large opening, and are most efficient and quiet when operated at low RPM. A low moving propeller fan on the ceiling of a grow room will quietly and efficiently move the air.

Hot air rises. Adept growers locate air exit vents in the hottest peak of the room for passive, silent air venting. The larger the diameter of the exhaust ducts, the more air that can travel through them. By installing a biog, sow moving vent fan in this vent, hot stale air is quietly and efficiently evacuated. A fan running at 50 RPM is quieter than one running at 200 RPM. Smart growers install 12-inch ducting and inline fans.

Most often, the vent fan is attached to ducting that directs air out the grow room. Flexible ducting is easier t use than rigid ducting. To install, run the duct the shortest possible distance, and keep curves to a minimum. When turned at more than 30 degrees, much of the air that enters a duct will not exit the other end. Keep the ducting straight and short.

Intake Air

many rooms have enough fresh air coming in via cracks and holes. But other grow rooms are tightly sealed and require fresh air to be ushered in with the help of an intake fan. An intake fan is the same as the exhaust fan, except it blows fresh air into the room. The ratio of 1 to 4 should give the room a little negative pressure. Delivering fresh air to plants ensures they will have adequate CO2 to continue rapid growth. One of the best ways to deliver air to the plants is to pipe it in via flexible ducting to direct air where it is needed. The air is dispersed evenly throughout the room.

Always make sure fresh air is neither too cold nor too hot. For example, one friend that lives in a hot, arid climate brings col fresh air in from under the house, where the air is few degrees cooler than ambient air.


When installing a vent fan, security concerns dictate that no light or odor escape from the exterior vent while allowing ample air release. This can be accomplished in several ways. Baffle or turn the light around a corner to subdue brilliance. Many chimneys in British Columbia, Canada, shoot light out like a spotlight against the low cloud cover. You can walk down city streets where half the chimneys in the neighborhood are beacons of bright light. A 4-inch flexible dryer hose will subdue light in smaller grow rooms. larger, 8, 10, and 12 inch heat duct pipe is ideal for moving larger volumes of air.

Place one end of the duct outdoors. It should be high enough, preferably over 12 feet, so the odor disperses above most people’s heads. One of the best vents is the chimney. The outlet may be camouflaged by using a dryer hose wall outlet attached to a vent fan. The vent fan is then placed near the ceiling so it vents hot, humid air. Check for light and air leaks. Set up the fan an go outdoors after dark to inspect for light leaks.

Greenhouse fans are equipped with louvers (flaps or baffles) to prevent backdrafts. During cold and hot weather, undesirable backdrafts could alter the climate in the room and usher in a menagerie of pests and diseases. installing a vent fan with louvers eliminates backdrafts, but may present a security risk if it attracts the attention of the wrong crowd.


An accurate thermometer is essential to measure temperature in all grow rooms. Mercury or liquid thermometers are typically more accurate than spring or dial types, but ecologically unsound. An inexpensive thermometer will collect basic information, but the ideal thermometer is a day-night or maximum/minimum type that measures how low temperature drops at night and how high it reaches during the day. The maximum and minimum temperatures in a grow room are important for the reasons explained below.

Under normal conditions, the ideal temperature range for indoor growth is 72-76F. At night, the temperature can drop 5-10F with little noticeable effect on growth rate. The temperature should not drop more than 15F, or excessive humidity and mold might become problems. Daytime temperatures above 85F or below 55F will slow / stop growth. Maintaining the proper, constant temperature in the grow room promotes strong, even, healthy growth. Make sure plants are not too close to a heat source, like a ballast or heat vent, or they may dry out, maybe even get heat scorch. Cold intake air will also stunt plant growth.

Cannabis regulates its oxygen uptake in relation to the ambient air temperature rather than the amount of available oxygen. Plants use a lot of oxygen; in fact, a plant cell uses as much oxygen as a human cell. The air must contain at least 20 percent oxygen for plants to thrive. Leaves are not able to make oxygen at night, but roots still need oxygen to grow. A plant’s respiration rate approximately doubles every 20F. Root respiration increases as the roots warm up, which is why fresh air is important both day and night.

Temperatures above 85F are not recommended even when using CO2 enrichment. Under the proper conditions, which are very demanding to maintain, higher temperatures step up metabolic activity and speed growth. The warmer it is, the more water the air is able t hold. This moist air often restrains plant functions and decelerates growth rather than speeding it. Other complications and problems result from excess humidity and moisture condensation when the temperature drops at night.

Heat buildup during warm weather can catch growers off guard and cause serious problems. ideal grow rooms are located under ground, in a basement, taking advantage of the insulating qualities of Mother Earth. With the added heat of the HID and hot, humid weather outdoors, a room can heat up rapidly. More than a few American growers have lost their crops to heat stroke during the Fourth of July weekend, since it is the first big holiday of the summer, and everybody in the city wants to get away t enjoy it. There are always some gardeners that forget or are too paranoid to maintain good ventilation in the grow room while on vacation. Temperatures can easily climb to 100F or more in grow rooms that are poorly insulated and vented. The hotter it is, the more ventilation and water that are necessary.

The cold of winter is the other temperature extreme. Montreal, Quebec, Canada, growers will remember the year of the big ice storm. Electricity went out all over the city and surrounding areas. Water pipes froze, and heating systems failed. Residents were driven from their homes until electricity was restored some days later. Many growers returned to find their beautiful gardens wilted, stricken with the deepest, most disgusting green only a freeze can bring. Broken water pipes, ice everywhere! It is difficult to combat such acts of Gd, but if possible, always keep the grow room above 50F and definitely above freezing, 32F. If the temperature dips below this mark, the freeze will rupture plant cells, and foliage will die back or, at best, grow slowly. Growth slows / stops when the temperature dips below 55F. Stressing plants with cold weather conditions is not recommended; it may yield a proportionately higher THC content, but will reduce plants overall productivity.

A thermostat measures the temperature, then controls it by turning on or off a device that regulates heating or cooling, keeping the temperature within a predetermined range. A thermostat can be attached to an electric or combustion heater. In fact, many homes are already setup with electric baseboard heat and a thermostat in each room.

A thermostat can be used to control cooling vent fans in a but the coldest grow rooms. When it gets too hot in a room, the thermostat turns n the vent fan, which evacuates the hot stale air. The vent fan remains on until desired temperature is reached, then the thermostat turns off the fan. A thermostat controlled vent fan offers adequate temperature and humidity control for many grow rooms. A refrigerated air conditioner can be installed if heat and humidity are a major problem. Just remember, air conditioners draw a lot of electricity. If excessive heat is a problem, but humidity is not of concern, use a swamp cooler. These evaporative coolers are inexpensive to operate and keep rooms cool in arid climates.

Common thermostats include single stage and two stage. The singe-stage thermostat controls a device that keeps the temperature the same both day and night. A two-stage thermostat is more expensive but can be set to maintain different temperatures during day and night. This convenience can save money on heating, since room temperature can drop 5-10F at night with little effect on growth.

Many new grow room controllers have been developed in the last ten years. These controllers can operate and integrate every appliance in the grow room. More sophisticated controllers integrate the operation of CO2 equipment, vent, and intake fans. Relatively inexpensive computerized controllers are also available for grow rooms. If temperature and humidity regulation are causing cultural problems in the grow room, consider purchasing a controller.

Uninsulated grow rooms or grow rooms that experience significant temperature fluctuations require special consideration and care. Before growing in such a location, make sure it is the only choice. If forced to use a sun-baked attic that cools at night, make sure maximum insulation is in place to help balance temperature instability. Enclose the room to control heating and cooling.

When CO2 is enriched to 0.12-0.15 percent, a temperature of 80F promotes more rapid exchange of gases. Photosynthesis and chlorophyll synthesis are able to take place at a more rapid rate causing plants to grow faster. Remember, this higher temperature increases water, nutrient, and space consumption, so be prepared! Carbon dioxide enriched plants still need ventilation to remove stale, humid air and promote plant health.

The temperature in the grow room tends t stay the same, top to bottom, when the air is circulated with an oscillating fans. In an enclosed grow room, HID lamps and ballasts keep the area warm. A remote ballast placed near the floor on a shelf or a stand also helps break up air stratification by radiating heat upward. Grow rooms in cool climates stay warm during the day when the outdoor temperature peaks, but often cool off too much at night when cold temperature sets in.

Sometimes it is to cold for the lamp and ballast to maintain satisfactory room temperatures. grow rooms located in homes are usually equipped with a central heating and / or air conditioning vent. The vent is usually controlled by a central thermostat that regulates the temperature of the home. By adjusting the thermostat to 72F and opening the door to the grow room, it can stay a cozy 72F. However, using so much power is expensive, and could cause a security quandary. Keeping the thermostat between 60 and 65F, coupled with the heat from the HID system, should be enough to sustain 75F temperatures. Other supplemental heat sources such as inefficient incandescent light bulbs and electric heaters are expensive and draw extra electricity, but they provide instant heat that is easy to regulate. Propane and natural gas heaters increase temperatures and burn oxygen from the air, creating CO2 and water vapor as by-products. This dual advantage makes using CO2 generator economical and practical.

Kerosene heaters also work to generate heat and CO2. Look for a heater that burns its fuel efficiently and completely with no tell-tale odor of the fuel in the room. Do not use old kerosene heaters or fuel-oil heaters if they burn fuel inefficiently. A blue flame is burning all fuel cleanly. A red flame indicates only part of the fuel is being burned. I’m not a big fan of kerosene heaters and do not recommend using them. The room must be vented regularly to avoid buildup of toxic carbon monoxide, also a by-product of combustion.

Diesel oil is a common source of indoor heat. Many furnaces use this dirty, polluting fuel. Wood heat is not the cleanest either, but work well as a heat source. A vent fan is extremely important to exhaust polluted air and draw fresh air int the room heated by an oil furnace or wood stove.

Insect populations and fungi are also affected by temperature. in general, the cooler it is, the slower the insects and fungi reproduce and develop. Temperature control is effectively integrated into many insect and fungus control programs.


Humidity is relative; that is, air hold different quantities of water at different temperatures. Relative humidity is the ratio between the amount of moisture in the air and the greatest amount of moisture the air could hold at the same temperature. In other words, the hotter it is, the more moisture air can hold; the cooler it is, the less moisture air can hold. When the temperature in a grow room drops, the humidity climbs, and moisture condenses.

Relative humidity increases when the temperature drops at night. The greater the temperature variation, the greater the relative humidity variation will be. Supplemental heat or extra ventilation is often necessary at night if temperatures fluctuate more than 15F.

Seedlings and vegetative pants grow best when the relative humidity is from 60 to 70 percent. Flowering plants grow best in a relative humidity range from 40 to 60 percent. The lower humidity discourages diseases and pests. As with temperature, consistent humidity promotes healthy, even growth. Relative humidity level affects the transpiration rate of stomata. When humidity is high, water evaporates slowly. The stomata close, transpiration slows, and so does plant growth. Water evaporates quickly into drier air causing stomata to open, increasing transpiration, fluid flow, and growth. Transpiration in arid conditions will be rapid only if there is enough water available for roots to draw in. If water is inadequate, stomata will close to protect the plant from dehydration, causing slow growth.

When the relative humidity climbs beyond 70 percent, the pressure outside the leaf is too high and inside too low. This causes the stomata to close, which slows growth. For example, a 40 inch tall pant can easily transpire a gallon per day when humidity is below 50 percent. However, the same plant will transpire about a half-pint n a cool humid day.

Measuring Relative Humidity

Relative humidity control is an integral part of insect and fungus prevention and control. Humidity above 80 percent discourages spider mite but promotes fungus as well as rot and stem rot. Humidity levels below 60 percent reduce the chances of fungus and rot.

Measure relative humidity with a hygrometer. This extremely important instrument will save you and your garden must frustration and fungi. By knowing the exact moisture content in the air, humidity may be adjusted to a safe 40 to 60 percent level that encourages transpiration and discourages fungus growth.

There are two common types of hygrometers. The spring type is accurate within 5 to 10 percent. This hygrometer is inexpensive and adequate for most hobby growers whose main concern is to keep the humidity near 55 to 60 percent. The second type, a psychrometer, is more expensive but very accurate. A psychrometer that measures relative humidity with a wet and dry bulb is an excellent way to keep an accurate vigil on relative humidity. Today there are many exceptionally accurate high tech gadgets, plus they are equipped with memory!

A humidistat is similar to a thermostat, but regulates humidity instead of temperature. Humidistats are wonderful and make controlling the environment very easy. Humidistats costs less than $100 and are worth their weight in resin glands. A humidistat and thermostat can be wired “in line” to control a vent fan. Each can operate the fan independently. As son as the humidity (or temperature) exceed the acceptable range, the fan turns on to vent the humid (or hot) air outdoors.

The HID lamp and ballast radiate heat, which lowers humidity. Heat from the HID system and a vent fan on a thermostat / humidistat are all the humidity control necessary for most grow rooms. Other dry heat sources, such as hot air vented from a furnace or wood stove, dry the air and lower the humidity. be careful. Do not let piped in, warm, dry air blow directly on foliage. It will rapidly dehydrate plants.

Increase humidity by misting the air with water or setting out a bucket of water to evaporate into the air. A humidifier is convenient and relatively inexpensive. Humidifiers evaporate water into the air to increase humidity. Just set the dial to a specific humidity, and presto! the humidity changes to the desired level as son as the humidifier is able to evaporate enough water into the air. A humidifier is not necessary unless there is an extreme problem with the grow room drying out. Problems seldom occur that can be remedied by a humidifier. All too often, there is to much humidity in the air as a result of irrigation and transpiration. If a ventilation system is unable to remove enough air to lower humidity, a dehumidifier could be just the ticket!

A dehumidifier can be used anytime to help guard against fungus. Just set the dial at the desired percent humidity and presto, perfect humidity. Dehumidifiers are more complex, use more electricity than humidifier, and cost more, but to growers with extreme humidity problems not yet cured by a vent fan, they are worth the added expense.

Air conditioners draw moisture from the air and lower the humidity. The moisture condenses into water that is collected in a container or expelled through a tube outdoors. The condensed water carries the fragrance of cannabis. Sniffer dogs can easily smell the fragrance of cannabis in the tainted water expelled outdoors.

CO2 Enrichment

Carbon dioxide (CO2) is a colorless, odorless, nonflammable gas that is around us all the time. The air we breathe contains 0.03-0.04 percent CO2. Rapidly growing cannabis can use all the available CO2 in an enclosed grow room within a few hours. Photosynthesis and growth slow to crawl when the CO2 level falls below 0.02 percent.

Carbon dioxide enrichment has been used in commercial greenhouses for more than 35 years. Adding more CO2 to grow room air stimulates growth. Indoor cannabis cultivation is similar to conditions in a greenhouse, and indoor growers apply the same principles. Cannabis can use more CO2 than the 0.03-0.04 percent that naturally occurs in the air. By increasing the amount of CO2 to 0.12-0.15 percent – the optimum amount widely agreed upon by professional growers – plants can grow up to 30 percent faster, providing that light, water, and nutrients are not limiting. Carbon dioxide enrichment has little effect on plants grown under fluorescent lights. Fluorescent tubes do not supply enough light for the plant to process the extra available CO2.

Carbon dioxide can make people woozy when it rises above 5000 ppm and can become toxic at super high levels. When CO2 rises to such high levels, there is always a lack of oxygen!

Carbon dioxide enrichment does not make plants produce more potent THC; it causes more foliage to grow in less time. The larger the volume of THC-potent cannabis, the larger the volume of THC produced.

Carbon dioxide enriched cannabis demands a higher level of maintenance than normal plants. Carbon dioxide enriched plants use nutrients, water, and space faster than non-enriched plants. A high temperature, from 75 to 80F will help stimulate more rapid metabolism within the super enriched plants. When temperatures climb beyond 85F, Co2 enrichment becomes ineffective, and at 90F growth stops.

Carbon-dioxide enriched plants use more water. Water rises from plant roots and is released into the air by the same stomata the plant uses t absorb CO2 during transpiration. Carbon dioxide enrichment affects transpiration by causing the plants’ stomata to partially close. This slows down the loss of water vapor int the air. Foliage on CO2 enriched plants is measurably thicker, more turgid, and slower to wilt than eaves on non-enriched plants.

Carbon dioxide affects plant morphology. In an enriched growing environment, stems and branches grow faster, and the cells of these plant parts are more densely packed. Flower stems carry more weight without bending. Because of the increased rate of branching, cannabis has more flower initiation sites. Plants that sometimes do not bear from the first flower set are more likely to set flowers early if CO2 enrichment is used.

With CO2 enriched air, plants that do not have the support of the other critical elements for life will not benefit at all, and the CO2 is wasted. The plant can be limited by just one of the critical factors. For example, the plants use water and nutrients a lot faster, and if they are not supplied, the plants will not grow. They might even be stunted.

To be most effective, CO2 level must be maintained at 1000 to 1500 ppm everywhere in the room. To accomplish this, the grow room must be completely enclosed. Cracks in and around the walls should be sealed off to prevent CO2 from escaping. Enclosing the room makes it easier to control the CO2 content f the air within. The room must also have a vent fan with flaps or a baffle. The vent fan will remove the stale air that will be replaced with Co2 enriched air. The flaps or baffle will help contain the CO2 in the enclosed grow room. Venting requirements will change with each type of Co2 enrichment system and are discussed below.

Measuring CO2

Measuring and monitoring CO2 levels in the air is rather expensive and often unnecessary for small growers. Monitoring CO2 levels in grow rooms with ten or more lights really helps keep the levels consistent.

Disposable comparative colorimetry CO2 test kits are easy to use, accurate, and inexpensive. The test kits contain a syringe and test tubes and sell for about $30. To use the kit, break off each end tip of the test tube, and insert one end into the closed syringe. Pull 100 cubic centimeters into the syringe, and note the blue color change in the cylinder where the active ingredient reacts with the CO2 in the air drawn through the cylinder. These kits are reliable to within 40 ppm.

Electrochemical sensing systems measure electrical conductivity of an air sample in either an alkali solution or distilled or deionized water. These systems are relatively inexpensive, but they have drawbacks: limited accuracy and sensitivity to temperature and air pollutants.

Infrared monitoring systems are more accurate and versatile. They correctly measure CO2 and can be synchronized with controllers that operate heat, ventilation, and Co2 generators. Even though the initial cost for a monitor is high, they can solve many CO2 problems before they occur and can ensure optimum growing conditions. Specialty indoor garden stores sell the monitors for less than $1000.

Growers who do not want to spend the time and energy required to monitor CO2 can use a set of scales and simple mathematics to determine the appropriate amount of CO2 in the air, but this calculation does not account for ventilation, air leaks, and other things that could skew the measurement. It is easier to measure the amount of CO2 produced rather than to measure the amount of CO2 in the grow room’s atmosphere.

To measure the amount of fuel used, simply weigh the tank before it is turned on, use it for an hour, and then weigh it again. The difference in weight is the amount of gas or fuel used.

Producing CO2

there are many ways to raise the Co2 content of an enclosed grow room. The two most popular ways are to disperse it from a tank or burn a fuel to manufacture it. Carbon dioxide is one of the byproducts f combustion. Growers can burn any fossil (carbon-based) fuel to produce Co2 except for those containing sulfur dioxide and ethylene, which are harmful to plants. Carbon dioxide gas is a by-product of fermentation and organic decomposition. The CO2 level near the ground of a rain forest covered with decaying organic matter could be two to three times as high as normal, but bringing a compost pile inside to cook down is not practical. Dry ice is made from frozen CO2. The CO2 is released when dry ice comes in contact with the atmosphere. It can get expensive and be a lot of trouble to keep a large room constantly supplied with dry ice. It is difficult to calculate how much CO2 is released int the air without purchasing expensive equipment.

CO2 Emitter Systems

Compressed CO2 systems are virtually risk-free, producing no toxic gases, heat, or water. These systems are also precise, metering an exact amount of CO2 into the room. Carbon dioxide is metered out of a cylinder of the compressed gas using a regulator, flow meter, a solenoid valve, and a short range timer. Two types of systems are: continuous flow and short range. Metal carbon dioxide cylinders which hold the gas under 1000 to 2200 pounds of pressure per square inch (psi) can be purchased from welding or beverage supply stores. The cylinders are often available at hydroponic stores. Tanks must be inspected annually and registered with a nationwide safety agency. Welding suppliers and beverage suppliers often require identification such as a driver’s license. Most suppliers exchange tanks and refill them. Fire extinguisher companies and beverage supply companies normally fill CO2 tanks on the spot. If you purchase a lighter and stronger aluminum tank, make sure to request an aluminum tank exchange. Remember, the tank you buy is not necessarily the one you keep.

Buying a complete CO2 emitter system at a hydroponic store is the best option for most closet growers. These systems offer a good value for small indoor growers. You can make your own systems, but these systems often cost more than the pre-manufactured models.

Welding suppliers also carry regulators, and flow meters. Flow meters reduce and control the cubic feet per hour (cfh). The regulator controls the psi. Models with smaller flow rates, 10 to 60 cfh, are preferable for gardening purposes. buy a quality regulator-flow meter. Buy all components at the same time, and make sure they are compatible.

Carbon dioxide is very cold when released from a bottle where it has been kept under pressure. Even a quick blast can do damage to skin or eyes. If the flow rate is above 20 cfh, your regulator might freeze.

A regulator and flow valve are essential, but the solenoid valve and the timer are optional. However, growers who do not use a solenoid valve and timer waste CO2. The solenoid valve and timer regulate the flow of Co2. A solenoid valve is electrically operated and is used to start and stop the gas flow from the regulator and flow meter. The least expensive timer is plastic and is commonly used for automatic sprinkler systems.

Control the exact amount of CO2 released into the garden room by altering the flow and duration of CO2. To determine how long the valve should remain open, divide the number of cubic feet of gas required by the flow rate. If the flow meter is set at 10 cfh, the valve will need to be pen for 0.1 hours (1 divided by 10) or 6 minutes (0.1 hour x 60 minutes) to bring the room up to 1500 ppm. Remember, CO2 leaks out of the grow room. On average, the CCO2 level of the room returns to 300 ppm in abut three hours due to plant usage and room leakage. To maintain a steady level of CO2, split the amount of CO2 released per hour int two to fur smaller increments dispersed more frequently.

Distribute the CO2 from the tank to the grow room by using a tube or a fan. Suspend lightweight perforated plastic from the ceiling to disperse the CO2. The tubing carries CO2 from the supply tank to the center of the grow room. The main supply line is attached to several smaller branches that extend throughout the garden. CO2 is heavier and cooler than air and cascades onto the plants below.

To make sure the CO2 is dispersed from the tubing evenly, submerge the lightweight plastic tubing in water and punch the emission holes under water while the CO2 is being piped into the line. This way you know the proper diameter holes to punch and where to punch them to create the ideal CO2 flow over the garden.

Overhead fans help distribute CO2 evenly throughout the room. The CO2 is released directly below the fan into its airflow. This evenly mixes the CO2 throughout the air and keeps it recirculating across the plants.

Compressed CO2 is expensive, especially in large grow shows. At roughly $0.50 per pound, compressed gas is much more expensive than fuels used in generators. Cost of equipment and fuel make compressed CO2 enrichment systems less economical than generators.

CO2 Generator Systems

CO2 generators are used by commercial flower, vegetable, and marijuana growers. Green Air Products has introduced a complete line of reasonably priced CO2 generators that burn natural gas or LP (propane) to produce CO2. However, heat and water are by-products of the combustion process. Generators use a pilot light with a flow meter and burner. The inside of the generator is similar to a gas stove burner with a pilot light enclosed in a protective housing. The generator must have a cover over the pen flame. You can operate the generators manually or synchronize them with a timer to operate with other grow room equipment such as ventilation fans.

CO2 generators produce hot exhaust gasses (CO2 + H2O). Even though CO2 is heavier than air, it is hotter and therefore less dense and rises in a garden room. You must have god air circulation for even distribution of CO2.

Carbon Dioxide generators can burn any fossil fuel – kerosene, propane, or natural gas. Low grades of kerosene can have sulfur content as high as 0.01 to 1 percent, enough to cause sulfur dioxide pollution. Use only high-quality kerosene even though it is expensive. Always use grade “1-K” kerosene. Maintenance cost for kerosene generators are high, because they use electrodes, pumps, and fuel filters. For most grow rooms, propane and natural gas burners are the best choice.

When filling a new propane tank, first empty it of the inert gas which is used to protect it from rust. Never completely fill a propane tank. Propane expands and contracts with temperature change and could release flammable gas from the pressure vent if too full.

Generators burn either propane or natural gas, but must be setup for one or the other. They are inexpensive to maintain and do not use filters or pumps. Hobby CO2 generators range from $300 to $500, depending on size. The initial cost of a generator is slightly higher than a CO2 emitter system that uses small, compressed gas cylinders. Nonetheless, growers prefer propane and natural gas generators, because they are about four times less expensive t operate than bottled CO2 generators. One gallon of propane, which costs about $2, contains 36 cubic feet of gas and over 100 cubic feet of CO2 (every cubic foot of propane gas produces 3 cubic feet of CO2). For example, if a garden used one gallon of propane every day, the cost would be about $60 per month. In contrast, bottled CO2 for the same room would cost more than $200 per month!

CO2 generators are less expensive to maintain and less cumbersome, but they have some disadvantages, too. One pound of fuel produces 1.5 pounds f water and 21,800 Btu of heat. For grow rooms less than 400 cubic feet, this makes generators unusable. Even for larger garden rooms, the added heat and humidity must be carefully monitored and controlled so as not to affect plants. Growers in warm climates do not use generators, because they produce too much heat and humidity.

If fuel does not burn completely or cleanly, CO2 generators can release toxic gasses – including carbon monoxide – into the grow room. Nitrous oxide, also a by-product of burning propane, can grow to toxic levels, to. Well-made CO2 generators have a pilot and timer. If leaks or problems are detected, the pilot and timer will turn off.

A CO2 monitor is necessary if you are sensitive to high levels of gas. Digital alarm units or color change plates (used in aircraft) are an economical alternative. Carbon monoxide is a deadly gas and can be detected with a carbon monoxide alarm available at most hardware and building supply stores.

Check homemade generators frequently, including kerosene or gas heaters. Propane and natural gas produce a blue flame when burning efficiently. A yellow flame means unburned gas (which creates carbon monoxide) and needs more oxygen to burn cleanly. Leaks in a system can be detected by applying a solution of equal parts water and concentrated dish soap to all connections. If bubbles appear, gas is leaking. Never use a leaky system.

Oxygen is also burned. As it becomes deficient in the room, the oxygen / fuel mixture changes. The flame burns too rich and yellows. This is why fresh air is essential.

Turn off CO2 generators at night. They create excess heat and humidity in the grow room, and they need oxygen to operate. At night, roots need the extra oxygen in the room for continued growth.

if you are using Co2 and the yield does not increase, check to make sure the entire grow room is running properly and that plants have the proper light and nutrient eves, as well as the correct temperature, humidity, grow-medium moisture, and pH. Make sure roots receive enough oxygen both day and night.

Other Ways to Make CO2

There are many ways to make CO2. You can enrich small areas by burning ethyl or methyl alcohol in a kerosene lamp. Norwegians are studying charcoal burners as a source of CO2. When refined, the system will combine the advantages of generators and compressed gas. Charcoal is much less expensive than bottled CO2 and is less risky than generators in terms f toxic by-products. Others are studying the use of new technology to extract or filter CO2 from the air.

Compost and Organic Growing Mediums

Decomposing organic materials like wood chips, hay, leaves, and manures release large amounts of CO2. Although you can capture CO2 from this decomposition, it is most often impractical for indoor growers. Piping indoors the CO2 and fumes from a compost pile is complicated, expensive, and more work than it is worth.


Small scale growers use fermentation to produce CO2. Combine water, sugar, and yeast to produce CO2. The yeast eats the sugar and releases Co2 and alcohol as by-products. Growers who brew beer at home can use a small scale system to increase the Co2 levels in a room. Non-brewers can mix one cup of sugar, a packet of brewer’s yeast, and three quarts of warm water in a gallon jug to make Co2. You will have to experiment a little with the water temperature to get it right. Yeast dies in hot water and does not activate in cold water. Once the yeast is activated, CO2 is released into the air in bursts. Punch a small hole in the cap f the jug, and place it in a warm spot in your grow room. Many gardeners buy a fermentation lock. Such locks prevent contaminants from entering the jug, and they bubble CO2 through water so the rate of production can be observed. The hitch is that you must change the concoction up to three times a day. Pour out half the solution, and add 1.5 quarts of water and another cup (24cl) of sugar. As long as the yeast continues to grow and bubble, the mixture can last indefinitely. When the yeast starts t die, add another packet. This basic formula can be adapted to make smaller or larger scale fermenters. Several jugs scattered around the garden room have a significant impact on CO2 levels.

Fermentation is an inexpensive alternative to produce CO2. It releases no heat, toxic gases, or water and uses no electricity. But because it stinks, it is unlikely that a gardener cud tolerate a large scale fermentation process. In addition, it is difficult to measure CO2 production from this system, making it difficult to maintain uniform levels throughout the day.

Dry Ice

Dry ice gets very expensive with prolonged use. Two pounds of dry ice will raise the CO2 level in a 10 x 10 foot grow room to abut 2000 ppm for a 24 hour period. One chagrined grower remarked, “I can’t believe that stuff melts so fast”.

Growers have long used large, insulated tanks filled with dry ice to add CO2. Dry ice is carbon dioxide that has been chilled and compressed. As it melts, it changes from slid to gas. Gaseous CO2 can be mixed into the air with fans that circulate it among the plants. Dry ice works well on a smaller scale without a tank and converter. It is readily available and inexpensive. Because Co2 has no liquid stage, the transformation from solid to gas as the ice melts is clean and tidy. It’s also easy to approximate the amount f CO2 being released. A pound of dry ice is equal to a pound of liquid CO2. Determining the thawing period for a particular size of dry ice will all allow you to estimate how much CO2 is released during a particular time period. To prolong the thawing process, put dry ice in insulating containers such as foam ice coolers, and cut holes in the top and sides t release the CO2. The size and number of holes allow you to control the rate at which the block melts and releases CO2.

Dry ice is economical and risk free; it releases no toxic gases, heat, or water. Although dry ice is easier to handle than compressed CO2 tanks, it is difficult to store. The melting can be slowed through insulation, but it cannot be stopped. Because it is extremely cold, dry ice can also cause damage or burn the skin after prolonged contact.

Baking Soda and Vinegar

Consider using vinegar and baking soda to produce CO2 in a small grow room. This method eliminates excess heat and water vapor production and requires only household items. Create a system that drips vinegar (acetic acid) into a bed of baking soda. The main disadvantage of this system is the erratic level of CO2 produced. It takes a considerable amount f time for the CO2 to build up to a level where it helps plants. however, once it reaches an optimum level, it can contribute to rise until it reaches levels detrimental to plants. If you have time to experiment, it is possible to setup a drip system operated by a solenoid valve and a short term timer. With such a system, CO2 could be released periodically in small increments and coordinated with ventilation schedules.


A good exhaust fan, vented outdoors, is the first step in cannabis odor control and the easiest way to keep the house from reeking of fresh marijuana. If the odor is strong and venting is a problem, a negative ion generator (deionizer); deodorizing liquid, gel, puck, or spray; ozone generator; activated charcoal filter; or a combination of two or more of these will solve fragrance problems.


Kill odors by changing their structure at the molecular level. Products such as Odor Killer, Ona, VaporTek, and Ozium, are made from essential oils that kill odors by creating a neutral atmosphere at the atomic level. Such products are usually available in gel and spray. Many growers prefer to use the gel over the long term and spray for emergency situations, such as when unexpected guests stop by during harvest.

The deodorizers can be set out in the room, around the house, and near doorways. Several companies offer products that stick t the wall. One ingenious grower I interviewed stuck one such deodorizing puck t the inside of the front door, just below the mail slot to keep the house fresh. Other products are designed to be attached to the ventilation ductwork system. Often these products are used not only to alter the odor of marijuana, but also to alter the telltale odor produced by an ozone generator. Other companies offer aerosol spray cans with a dispenser that periodically meters out a burst of spray.

Negative Ion Generators

Negative ion generators are small and somewhat efficient to control odors, smoke, airborne pollen, mold, dust, and static electricity. They pump negative ions int the atmosphere. The negative ions are attracted to positive ions containing odors and other airborne pollutants. The negative ions attach to positive ions, and the odor becomes neutralized. The particles fall to the floor and create a fine covering of dust on the ground, walls, and objects in the room. These devices work fairly well for small grow rooms with minimal odor problems. The generator uses very little electricity and plugs into a regular 115-vlt current. Visually check the filter every few days, and make sure to keep it clean.

Ozone Generators

Ozone has many applications including food and water sterilization and removing odors from the air at the molecular level. Some growers even use high levels of ozone to exterminate grow room pests.

ozone generators neutralize odors by converting oxygen (O2) into ozone (O3) by exposing the stinky air to ultraviolet (UV) light. The extra molecule is always a positively charged ion that is predisposed to attach to a negatively charged cations. When the extra oxygen ion attaches to the cation, they neutralize one another and the odor, too. Once the extra molecule is shed, O3 is converted back into O2. The chemistry takes a minute or longer to occur, so treated air must be held in a chamber to be converted effectively.

Ozone has an unusual odor similar to the smell of air after a good rain. Anyone who has ever smelled the air in the room recently treated with ozone knows the smell and will never forget it. Make sure not to produce too much zone, and give it enough time to mix with smelly air to neutralize odors. The distinctive odor of excess ozone exiting a building will tip off narcs and thieves. For this reason, many growers also use carbon filter to further scrub the air.

There are many ozone generators available. When shopping for an zone generator, look for one that has been on the market for a few years and has an established track record. Watch for important features such as self-cleaning (or easy to clean) and easy, safe bulb replacement. When UV light encounters moisture in the air, nitric acid is produced as a by-product. This white, powdery nitric acid collects around the lamps at connection points. This is an unpleasant, very corrosive acid that will severely burn skin and eyes. Verify that the ozone generator has proper safety features built in, such as a switch that turns off the lamp for maintenance, making it impossible to look at the retina-searing UV rays. Legal exposure for humans is about 0.1 ppm for a maximum of 8 hours. Most grow room ozone generators produce about 0.05 ppm at timed intervals.

UV light is very dangerous. In a flash, intense UV light can burn your skin and the retinas in your eyes beyond repair. Never, under any circumstances, look at the UV lamp in an ozone generator. Sneaking a peak at a UV lamp in an ozone generator has cost more than one aspiring grower their eyesight@ Ozone is also capable if burning your lungs. At low levels, there is no damage, but at higher levels, danger is imminent. Never, never use to much!

Ozone generators are rated by the number of cubic feet they are able to treat. Some growers setup the ozone generator in the grow room and let it treat all the air in the room. They add a timer so the ozone generator intermittently disperses ozone in the room to maintain a relatively constant level. This practice can diminish the fragrance of the bud. Smart growers set up an ozone generator in a spare closet or build an zone exchange chamber and route fragrant grow room air through the closet for ozone treatment before being evacuated outdoors. Other growers set up ozone in ventilation ductwork to treat air before it exits. Once generated, ozone has a life of about 30 minutes. It takes a minute or two for the O3 molecules to combine with oxygen to neutralize odors.

Ozone Damage

For best results, keep the ozone generator in another room or isolated from the growing plants. Ozone causes chlorotic spots on leaves. The mottled spots that appear at first to be a Mg deficiency, increase in size and turn dark in the process. Most often, the symptoms are found on foliage near the generator. leaves wither and drop, and overall plant growth slows to a crawl.

Activated Carbon Filters

Activated charcoal filters are fantastic, and they work! The charcoal is “activated” with oxygen, which opens millions of pores in the carbon. The activated charcoal absorbs door molecules and other pollutants in the air. The mechanics are simple, and there are three important things to remember when using a charcoal filter. First, keep room humidity below 55 percent. At about 65-70 percent relative humidity, the charcoal absorbs moisture and cogs. At 80 percent humidity, it stops removing odors. Second, air must move slowly through charcoal filters to extract odors. The fan on professional units lets just enough air through the filter so the odors have enough (dwell) time to be absorbed by the carbon filter. Third, use a pre-fiter. The pre-filter catches most of the dust and airborne pollutants before they foul the carbon filter. Change the pre-filter regularly – every 60 days, or more often if the room is dusty. Carbon lasts about a year. Many growers prefer activated carbon made from coco. Do not use activated carbon that is crushed, because it is less efficient than charcoal pellets.

Install an intake a screen that filters out large particles of dust to prolong the life of the activated charcoal filter. Whether the intake is passive or brought in by a fan, use a filter for incoming air to minimize pollutants in the grow room.

Check with filter manufacturers or retailers about venting specifications for your grow area. A more powerful exhaust fan will be necessary to draw an adequate volume of air through the activated charcoal filter. An adequate airflow is imperative to keep a high CO2 content in the grow room.

Setting Up the Vent Fan – Step by Step

Step One – Figure the total volume of the grow room. Length x width x height – total volume. For example, a grow room that is 10 x 10 x 8 feet has a total volume of 800 cubic feet. A room measuring 4 x 5 x 2 meters has a total volume of 40 cubic meters.

Step Two – Use a vent fan that will remove the total volume of air in the room in less than five minutes. Buy a fan that can easily be mounted on the wall or inline in a duct pipe. Quality inline fans move much air and make very little noise. It’s worth spending the extra money on an inline fan. Small rooms can use a fan that can be attached to a flexible 4 inch dryer hose. Many stores sell special ducting to connect high speed squirrel cage fans with the 4 inch ducting.

Step Three – Place the fan high on a wall or near the ceiling of the grow room so it vents off hot, humid air.

Step Four – If possible, cut a hole in the wall, and secure the fan in place over the hole. Most locations require special installation.

Step Five – To place a fan in a widow, cut a 0.5 inch piece of plywood t fit the windowsill. Cover window with a lightproof dark colored paint or similar covering. Mount the fan near the top of the plywood so it vents air out of the grow room. Secure the plywood and fan in the windowsill with sheet rock screws. Open the window from the bottom.

Step Six – Another option to make a lightproof vent is to use a 4 inch flexible dryer ducting. Vent the hose outdoors, and attach a small squirrel cage fan t the other end f the ducting. Make sure there is an airtight connection between the fan and hose by using a large hose clamp or duct tape. Stretch the flexible ducting s it is as smooth as possible inside. Irregular interior surfaces cause air turbulence and seriously diminish airflow.

Step Seven – Another option is to vent the air up the chimney or into the attic where light leakage and door are seldom a problem. If using the chimney for a vent, first clean out the excess ash and creosote. Tie a chain to a rope. Lower the chain down the chimney, banging and knocking all debris inside to the bottom. There should be a door at the bottom to remove the debris. This door is also used as the exhaust vent.

Step Eight – Attach the fan to a thermostat / humidistat or another temperature / humidity monitor / control device to vent hot, humid air outside. Set the temperature on 75F and the humidity on 55 percent in flowering rooms and 60 to 65 percent in vegetative rooms. Most control devices have wiring instructions. More sophisticated controllers have built in electrical outlets, and the peripherals are simply plugged into them.

Step Nine – Or attach the vent fan to a timer and run it for a specific length of time. This is the method used with CO2 enrichment. Set the fan to turn n and vent out used CO2 depleted air just before new CO2 rich air is injected.

Marijuana Horticulture

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