Most of our world is a layered mass of solids and liquids. Stacked on top is a thin layer of invisible gas we call air. While mostly inert nitrogen, air contains the elements needed by almost every living thing and plants use both oxygen and CO2 depending on the day/night cycle. Plants breathe using the stomata, or pores, underneath their leaves. Over time these pores can get clogged by dirt, dust, or other air-borne particles pretty easily. Usually, rain and wind wash and dry the leaves to keep them clean while moving fresh air around them, but in a greenhouse or grow room you have to replicate the effect using ventilation systems and sprays.
Why Circulation is Important
CO2 and Oxygen Replacement
You may be thinking, “Air is everywhere, this room isn’t hot so it’s fine.” But you’d be wrong. Plants are excellent CO2 scrubbers (absorbers that take in CO2 and remove it from the atmosphere). Their leaves greedily take in all of the CO2 around them and begin releasing excess oxygen (during the day). This rapid removal of CO2 is only replenished by moving air. Without circulation of the room’s internal atmosphere, plant growth can become stunted. It’s like trying to breathe after heavy exercise; the plants can’t take in enough CO2 for photosynthesis to occur properly. They can’t create the sugars they need and so development slows or halts. In some extreme cases, larger plants will even lose leaves. Proper air circulation keeps fresh CO2 moving past the leaves of the plants. Remember, you don’t need a strong wind, just a gentle breeze to replenish the air in the room.
Circulating the air also allows you to control relative humidity. The heat inside a grow room can cause a large amount of moisture as condensation – especially when it’s colder outside – and that leaves your plants more vulnerable to pests and fungus. Your plants also give off (transpire) a lot of moisture that will build up in a room without air circulation. Replenishing fresher air and cycling out the moist air will prevent this buildup. On the other hand, cycling more humid air into a dry room can keep your plants from drying out in a reverse situation.
For hot or cold grow rooms, you’ll find that ventilation is the primary method for controlling temperature. Whether you use natural (passive) or mechanical (powered) systems, you’ll gain access to adding or removing heat from your grow room. Extreme temperatures are damaging to plants and can inhibit growth or kill the plants. Even then, every plant is different on what temperature it prefers (varying even by life-stage) so you’ll want to plan accordingly.
Ways to Ventilate Your Garden
The system you use for ventilation depends on where you’re growing your plants. For an outdoor greenhouse you can use a natural ventilation system. A sloped roof with exhaust vents at the top and intake vents at the base is the most common method. The warm internal air of the greenhouse rises and leaves through the exhaust vents creating a low pressure area that pulls in cooler air through the intake vents at the base. This updraft keeps a steady cycle of air moving through the greenhouse. The alternative is a powered system using filters and fans. A powered system is almost always required for growing indoors, especially if you’re using a grow tent. In-line fans in the vents usually lead into and out from the room and work to cycle fresh air through the room. With in-line fans you can also set up a separate cooling unit for your grow lights to keep them from overheating. It’s not a bad idea in either case to install an oscillating fan in the room to improve air circulation within the room.
In the end, whether you use passive or powered ventilation is up to you. Just make sure you ventilate wherever your plants are growing to ensure proper CO2 intake, maintain even humidity levels, and optimize ambient temperature for your plants. All of these will help you get the most out of your garden.