Your plants need to breathe: Indoor garden ventilation

People tend to overlook the fact that plants need air to grow well… You put a plant outside, and it grows. You put in a greenhouse and it grows, right? Well, yes. The plant sits outside and air currents do all that work for you. It’s the natural way. The thing is that, when you bring the garden indoors, you determine you indoor garden ventilation. By default, rooms don’t have much air flow… and if you grow a few herbs or a couple small plants next to your kitchen, you’ve got this covered. But…

…if you are going to be serious about a basement or a tent grow, you cannot leave this to chance because your plants will simply not grow very quickly.

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How much air flow do you need?

Your plant’s size and availability of other growth factors (nutrients, water, light) determine its CO2 needs. Your lazily-growing habanero needs an air change every 20 minutes. If you’re growing a bumper crop of cannabis, your air must be turned over every minute.

The avid gardener knows for sure if ventilation is adequate by using a CO2 sensor like this Hydrofarm CO2 logging sensor.However, for the $75 dollars that the sensor costs, you can simply add more ventilation to a small space. My rule of thumb is that I want my plant’s branches moving just a little bit before adding a supplemental fan. If your plants are sitting still, your air flow should be increased.

Your ultimate goal: Pull air with CO2 into the grow space, and exhaust the oxygen-rich air to where it will not be immediately recycled back into the grow space.

Plants are aerodynamic… to a fault (in our case)

In nature, plants are very good at managing a breeze. Unfortunately, unless your grow area reproduces that parallel-to-the-ground motion of the air, you get pockets with limited air exchanges. This leads to visibly smaller growth.

Most tents pull air in an upward motion, which is not great for mixing that CO2-rich air with the leaves. You can see from this animation that the plant impedes flow and most of the air goes around the plant.

By adding a fan, we create a turbulent air space which is far better at distributing the CO2 around the leaves where they are processed into more plant.

{{Plant model provided by heraSK. Thanks. Plants are hard to model.}}

Filters and scrubbers

Since you have control of your grow space, you control the air quality. This comes down to filtering incoming air and processing outgoing air.

Incoming air should be screened. A simple window mesh keeps out bugs and debris. Because they have a coarse mesh, they let in microbes that want to eat your plants, and infect your soil. Your input should have a HEPA filter over it if you can afford it. These remove pretty much all the tiny stuff (mold, bacteria, stray pollen, dust, etc) that can hurt your crop. Should you use one, remember to add about 20% to your air flow needs because they reduce flow. Remember to change the filters according to the manufacturer’s directions because old filters impede air flow and negate all your ventilation work.

Outgoing air doesn’t require any treatment unless you want to avoid a smell emanating from your grow site (I’m looking at you, cannabis guys). Carbon filters are great at eliminating VOCs (like paint fumes and terpenes), and are readily available in a variety of sizes. Prepared to replace them when they wear out, which with continued use, is about every 1-2 years. Again, adding any filter impacts air flow, so you should add another 20% to your needs.

Next steps

Hopefully that’s a good overview of what you need to get you thinking about ventilation. If you need additional support, check out our Designing Indoor Garden Ventilation article (coming soon) for detailed instructions.

• Lighting
• Setting up your grow area
• Growing Media
• Ventilation
• Water and Nutrients