If you’re a grower you already know about the scents your garden gives off all year round. Some are welcome, some are… less than invited into the nostrils of our neighbors. No matter what the scent or how strong, aromas in the grow room and the garden are something growers have to deal with during every grow cycle.

Whether it’s your blooming flowers, the humid air, or your soil, your garden’s full of scents that you’ll face. This week we’ll be going over grow room odors: where they come from, and how to handle them. Because we all love a great harvest, but not everyone appreciates the smells that come with them.

So what is a scent, anyway?
Let’s start with the basics: what is a scent?

A scent is what we smell when we inhale a certain concentration of “odorant molecules” that react with receptor cells in our noses. These molecules are only found in volatile organisms, which are organisms that can easily be turned in to gas. That’s why things that break down easily give off scents, versus materials like steel, rocks, and minerals that do not have a scent.

Scents vary in intensity depending on the type of odorant molecules the source gives off, and the conditions around the scent’s source. The smell of trash, for example, intensifies with increased heat so the hotter it gets the more intense the smell of trash becomes. Chemicals that dissolve in water (like dish soap) have inherently strong scent molecules which are meant to release strong scents once they’re broken down.

There are tens of thousands of scents in the world, but the human nose can only detect around 10000 of them.

Why does all this matter? Because even though you can’t smell the scent of your growing plants and their medium, pests and insects sure can Not only is odor control good for discretion, but it’s also a measure in pest control.

Know what your plants should smell like is a great way of understanding what your plants need. If your growing plants should have a certain smell at a point in its life and it doesn’t, chances are the plant’s suffering from some sort of deficiency.

Handling Strong Scents with Duct Fans & Carbon Filters
You know ‘em, you love ‘em: Carbon Filters. They’re pretty much everything you need to help neutralize the odors in your grow room all in one unit. Filters have activated carbon in them that helps clean the air that’s pushed into it, leaving you with fresh air that doesn’t contain all of those odor molecules.

When the funky air passes through the filter the carbon will absorb those smells, and the air that’s released will be free from smells. But to make them work you have to make sure you have the right size and the right setup in order for air to come into the filter and exit it neutralized.

Sizing Your Area for the Right Fan & Filter
The most common (and highly recommended) size for any indoor grow is 6” fan and filter combination, but you’ll also want to be sure you have the correct size for your growing space.

Filters will have a certain CFM (cubic feet per minute) they can handle, so you’ll want to make sure you calculate the volume of your growing area to make sure your filter can handle the odors you’re trying to eliminate.

How to Calculate CFM

Exhaust fans and filters are usually rated by CFM, or “cubic feet per minute”. The CFM rating is a reflection of the rate that the fan can replace stale air with fresh air or its “fresh air exchange”.

The first thing you do to figure out the CFM you need is to take the volume (LxWxH) of your grow room. Most growers want their grow room to exchange stale air with fresh air every 3 minutes, so take your volume and divide that number by 3.

After that, you’ll take into account light temperatures, the use of a filter, and the environmental conditions of your growing area.

Add 25-40% to the total CFM of your grow room for warm and humid temperatures. HID’s should account for about 15% to take their heat into account, and 20% should be added if you plan on using carbon filters (Note: if you’re using LED’s you do not need to calculate 15% for lights)

But no matter the size filter you have, we recommend replacing the filter rather than trying to clean them. Running your carbon and the metal filter through water will not do much to help the smells, and it’s likely to damage carbon, rendering it useless.

So let’s put this in the action…

Let’s say we’re in a grow room that’s 10x10x12ft, which gives us a volume of 1200 cu.ft. Now take that number and divide it by 3 for air exchange and you’ve got 400 CFM as a base number for the necessary CFM needed for your growing area.

If your grow room gets up to around 97°F so it’s pretty warm- that means we’ll add 25% of 400 (100) to the base CFM.
You’ll probably also use a filter, which is another 20% (80), and if you’re using a CMH light we’ll add 15% (60) to take light temp into account.
From there you’ll take 400 + 100 + 80 + 60 and add them up, giving you a total of 640 CFM.
That means your fans have to be able to move at least 640 CFM of air to give you the ventilation your plants need.
Odor Elimination Without Fans or Filters? Yup, it’s Possible
There are options out there that are more silent than running a duct fan and filter (trust us, we know how noisy those can get). There are gels like ONA Gel that help eliminate smells at the source. They’re used in tons of applications, from locker rooms to fruit and meat drying areas with strong odors. They work by attracting odor molecules, while at the same time releasing particles that help destroy smells at the source.

There are also air purification machines that help eliminate stale air. As air passes through the purifier particles are trapped on a sieve, and the more times air passes through the more particles will be trapped and kept from getting into the air. But while these sorts of air purification setups have their setbacks.

Gels Aren’t All They’re Cracked Up to Be– The first setback with filter-less odor elimination is that it’s really only useful for small-scale use. ONA gels can only eliminate some of the smell out of a smaller grow tent, and air purifiers will only pick up certain types of odor particles, not including terpenes which are what help give plants their smells (and some can even replace the odor with a new scent).

Moreover, ONA gels and the like will actually take flavor and smell out of flowers and fruits by attacking their terpenes at the source (i.e. the plant), so if you do end up going with ONA gels you need to be sure that if terpene preservation is a must you keep the gel as far away from your plants as possible.

We suggest putting them at the edge of where you don’t want the odor escaping. For example, instead of putting gel near your plants put it near the windows and in the threshold of your doors and let the gel catch odors on the way out.

Still, these types of odor reduction and elimination should only be used on a very small scale or as a short-term solution.