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

no air, and nutrient uptake stalls.
Air Movement

At least one good circulation fan and an extraction fan are necessary in grow rooms. The extraction fan in this grow room is attached to a carbon filter.
    Air ventilation and circulation are essential to a healthy indoor harvest. Indoors, fresh air is one of the most overlooked factors contributing toa 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 set up an adequate ventilation system. Three factors affect air movement: stomata, ventilation, and circulation.
Stomata
    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 the 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 CO 2 -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 leave filmy residues. Keep foliage clean. To avoid clogging stomata, spray foliage with tepid water a day or two after spraying with pesticides, fungicides, or nutrient solution.
Circulation
    Plants use all CO 2 around the leaf within a few minutes. When no new CO 2 -rich air replaces the used CO 2 -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.

This photo of a half-opened stomata, the mouthlike opening on leaf underside, was magnified 2500 times.

Microscopic stomata located on leaf undersides must remain clean and un-stifled by humidity to promote rapid growth.

Hot air flows upward naturally. Always design grow rooms and greenhouses to take advantage of this principle.

Floor-mounted oscillating fans stir the air in this room 24 hours a day.

Place circulation fans far enough away from plants to prevent too much airflow on any one portion of the garden.

This garden has several circulation fans to move air between dense, resinous buds.

CO 2 around leaves is used quickly and must be replaced every few minutes.

Here is an example of a heavy-duty extractor fan from a Dutch grow room.

A booster fan in the gray box pushes intake air down the duct to the bottom of the garden.

Avoid long, unnecessary ducting such as shown above. Keep ducting as short as possible.

Ballasts are kept in the box which contains the extra heat. Separate ducting carries out the hot air.

This blower is at the end of intake air ducting. The main fan is at the other end.

Roof fans and vents make grow room ventilation easy and inconspicuous.

Squirrel cage blower moves a lot of air, but it is noisy! This blower has been necked down to force more air through the ducting.

This fan is suspended by bungee cords to shunt noisy vibrations. Always keep your fans well lubricated so they run smoothly and quietly.

Ventilation
    Fresh air is easy to obtain and inexpensive to maintain–it is as simple as hooking up and placing the 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 (0.92 m 2 ) garden will use from 10 to 50 gallons (38 to 190L) or more of 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