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 successful harvest. Fresh air is the least expensive and an essential component required for maintaining a healthy pest free grow room. Experienced indoor horticulturists understand the importance of fresh clean air and take time to setup an adequate ventilation system.

Fresh air not only needs to be provided to the grow room but it also needs to be circulated around the room so that the garden is constantly being refreshed and the air never has a chance to stagnate around plants. Plants use all of the CO2 around the leaf within a few minutes. When that CO2 isn’t replenished, a dead air zone forms around the leaf. This stifles the stomata and virtually stops growth. If it is not actively moved, the air around the leaves stratifies. Warm air in the grow room will stay near the ceiling and cool air settles near the floor. Air circulation breaks up these air masses by mixing them together. Circulating fans are cheap so take care of your plants and install one before your garden gets ravaged by powdery mildew or grey bud mold.

The stomata are microscopic pores on the leaf undersides that regulate oxygen and carbon dioxide flows within the plant. The larger the plant, the more stomata it will have in order to take in carbon dioxide and release oxygen. In rooms with many plants there is a greater need for CO2 rich air so that plants can maintain vigorous growth. When the air in the grow room stagnates, the stomata on the plants will close or become clogged, severely limiting growth.

Activated Charcoal filters are great and they really work well. The charcoal is activated with oxygen, which opens millions of pores in the carbon. The activated charcoal absorbs odor molecules and other pollutants in the air.

There are a few general guidelines to follow when using carbon filters in the grow room:

• Keep the grow rooms relative humidity below 55%. At 65 to 70% humidity, the charcoal absorbs moisture and clogs. At 80% relative humidity, it stops removing odors all together.
• Air must move slowly through charcoal filters to extract odors. I would suggest that you purchase a speed control for your inline centrifugal fan that you will be connecting to the filter, this will allow you to dial in your air flow setting to the cfi setting that works best.
• Use a prefilter. Almost all of the carbon filters on the market today come with a cotton pre-filter that slips over the unit to filter out dust and other airborne pollutants before they enter and foul the filter.
• Change the pre-filter every 60 days or more often if your space gets dusty easily.

Fresh air is easy to obtain simply by hooking up and placing the proper sized exhaust fan in the most efficient location. An intake vent might be necessary to create a flow of fresh air into grow room.

A 10×10 foot garden will use from 10 to 50 gallons or more of water every week. Plants transpire most of this water back into the air. Every day and night, rapidly growing plants transpire more moisture into the air. If this moisture is left inside the grow room, humidity can increase to 100%, which stifles stomata and causes growth to come to a screeching halt. It also opens the door for pest and disease problems to surface. Evacuate the moist humid 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 stale air.

Ventilation is just as important as water, light, heat, and nutrients. The most efficient fans used in the grow room today are the inline centrifugal fans. These high performance fans have replaced the inefficient squirrel cage fans and move a lot more air. They are available for many different duct sizes, CFI ratings, and decibel ranges. When shopping for centrifugal fans make sure they move enough air for the size of your space. I would advise looking at the fans decibel rating as well if you are trying to manage the noise level in the grow room. You can download decibel measuring apps for your phone to give you an idea of how much noise any particular fan is going to generate at full performance.

An accurate hygrometer is essential to measuring temperature in the grow room. You will want one that can measure day-night or maximum/minimum temperature ranges. This will help you see how low the temperature drops at night and how high it gets during the day.

Under normal conditions, the ideal temperature range for indoor growth is 72°-76° F (22°-24° C). At night the temperature can drop 5°-10° F (2°-5° C) with little noticeable effect on growth rate. The temperature should not drop more than 15° F (8° C), or excessive humidity and mold might become problems. Daytime temperatures above 85° F (29° C) or below 60° F (15° C) will slow growth. Maintaining the proper temperature in the grow room promotes strong, even, healthy growth. Make sure that your plants are not too close to a heat source or they might dry out or get scorched from the heat. Cold intake air will also stunt growth.

Relative humidity means that air holds 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 can hold at the same temperature. So the hotter it is, the more moisture the air can hold; the cooler it is, the less moisture it can hold. when the temperature in the grow room drops, the humidity climbs and the moisture condenses. For example, an 800 cubic foot grow room (10x10x8) (21.5 m²) will hold about 14 ounces (414 ml) of water when the temperature is 70° F (21° C) and relative humidity is at 100%. When the temperature is increased to 100° F (38° C), the same room will hold 56 ounces (1.65 L) of moisture at 100% relative humidity. That is four times more moisture. When the temperature drops, the moisture condenses onto the surfaces of walls and plants.

Seedlings and vegetative plants grow best when the relative humidity is from 65 to 70%. Flowering your plants in a lower relative humidity range (from 55 to 60%) will discourage pests and diseases. As with temperature, consistent humidity promotes healthy, even growth. Relative humidity levels affect the transpiration rate of the plants stomata. When humidity is high, water evaporates slowly, the stomata close, transpiration slows, and as a result growth slows as well. When the air in the grow room is drier, water evaporates quickly causing the stomata to open, transpiration increases, and healthy vigorous growth occurs. This vigorous growth can only occur when the plants are provided with enough water for the roots to draw in and process. If water is inadequate, the stomata will close to protect the plant from dehydration, causing growth to slow.

When the relative humidity climbs beyond 70%, the pressure outside the leaf is too high and inside too low. This causes the stomata to close, which slows growth. For best results use the following guidelines for proper humidity levels in the different stages of plant development:

• Clones: 80-95% relative humidity – use a humidity dome to maintain this high level
• Vegetative Growth: 60-70% relative humidity
• Flowering: 40-60% relative humidity

Carbon dioxide enrichment has been used in grow rooms and greenhouses for more than 30 years. Adding CO2 to a grow room stimulates growth. Cannabis can use more CO2 than what is naturally found in the air. By increasing the amount of CO2 from the approximately 350 ppm found naturally to 1200 – 1500 ppm, the optimum amount widely agreed upon by professional growers, plants can grow up to 30% faster, providing that light, water, and nutrients are not limiting. Carbon dioxide enrichment has little or no effect on plants grown under low light conditions, but under intense HID lights they can take in large amounts.

Carbon dioxide does not make plants produce more THC or CBD; it causes more foliage to grow in less time. The larger the volume of healthy foliage will however help your plants to produce bigger and more robust flowers. CO2 enriched plants demand higher levels of daily maintenance than normal plants. Plants given higher doses of CO2 take up more space and use more water and nutrients. Keeping the grow room temperature a little warmer at around 75° to 80° (24° to 26° C) will help to stimulate a rapid metabolism within the highly enriched plants. When temperatures go beyond 85° F (29° C), CO2 enrichment becomes ineffective, and at 90° F (32° C) growth halts.

To maintain a CO2 level of 1000 – 1500 ppm in the grow room, the space will need to be completely enclosed. Anywhere that air can escape will need to be sealed to prevent the added CO2 from escaping and being wasted. Enclosing the room will make it easier to control the CO2 content of the air within the space. The grow room will also need a baffled vent fan so that stale air can be removed while preserving the CO2 within the garden.