Airflow covers two jobs. Circulation stirs air inside the grow space so there are no hot, humid or CO2-depleted pockets. Exchange replaces room air with fresh air to remove heat and moisture and bring in CO2. Both work by reducing the boundary layer, the thin film of still air around each leaf that slows gas and heat exchange.
In practice
Kitaya and colleagues (2003) showed that leaf gas exchange improves sharply as air speed rises from still air to around 0.5–1 m/s, with little extra benefit beyond that. A practical target is canopy leaves that flutter gently, not whip. Constant strong wind dries leaves, can cause windburn (clawed, papery leaves on the fan side) and wastes energy.
Good airflow means air moving above the canopy, through it and below it. Dense flower rooms need fans under the canopy too, because the inside of a cola is where botrytis starts. If leaves at the back of the tent are still while the front ones dance, add or reposition a fan.
Don't confuse with
- Exhaust (ventilation): removes air from the space; circulation fans only mix it.
- CFM: a fan rating, not a measure of air speed at the leaf.
CO2 enrichment raises carbon dioxide in a sealed grow space from about 420 ppm to roughly 800–1,500 ppm, letting plants use higher light intensities productively.Leaf damage from a fan blowing directly and constantly on the same leaves: curled, clawed or crispy leaves on the side facing the fan, sometimes with brown, dried patches.A large palmate cannabis leaf with several long leaflets on a long petiole; fan leaves do most of the plant's photosynthesis and carry few trichomes.The dense, elongated cluster of flowers that forms at the end of a stem or branch; the main cola tops the leader.Botrytis cinerea, the fungus behind bud rot or grey mould, which rots dense colas from the inside in cool, humid conditions late in flower.An exhaust fan pulls air out of the grow space through ducting, removing heat, humidity and odour and drawing in fresh air.An oscillating fan sweeps back and forth to circulate air over and through the canopy, mixing the room and preventing still, humid pockets.CFM (cubic feet per minute) is the volume of air a fan moves per minute; growers use it to size exhaust fans to their grow space volume.A microclimate is a small zone within the grow space whose temperature, humidity or airflow differs from the room average, such as inside a dense cola or a corner.
