Better Buds with Mr Bill: Stagnant Ventilation and Your Yield

Mr. Bill, our plant fix-it guy, helps readers to make the most of their air flow, which affects yield.

Mr. Bill is ready to assist you by drawing upon over twenty-eight years of active grow shop experience - including answering growers' questions about the hydroponic industry. Before Mr. Bill opened his first store he worked in the agricultural industry for five years, growing tobacco.

I find this information very important in gaining yield and thought I would share it with you.

We think of air as nothing because we walk thorough it with no problem; we understand that air has very little effect upon our movements. However air slows down cars, so we make them aerodynamic. Now, to give real substance to air: have you ever watched a plane fly overhead? That plane probably weighs around twenty tons and yet air is stable enough to keep the plane afloat - at least with enough speed.

Stagnant Ventilation Affects Marijuana Yield
Air movement around your plants is very important. Think of air as a solution of elements that surrounds everything, just like water does when you submerge objects. Air contains important elements for plants and needs to continuously be replaced from the leaf surface of your plants.

The composition of air in percent by volume, at sea level at 59° F/15° C (and standard atmospheric pressure of 101325 Pa.) is:

Nitrogen (N2) 78.084 %
Oxygen (O2) 20.946 %
Argon (Ar) 0.9340 %
Carbon dioxide (CO2) 0.0397 %
Neon (Ne) 0.001818 %
Helium (He) 0.000524 %
Methane (CH4) 0.000179 %
Krypton (Kr) 0.000114 %
Hydrogen (H2) 0.000055 %
Nitrous oxide (N2O) 0.0000325 %
Carbon monoxide (CO) 0.00001 %
Xenon (Xe) 0.000009 %
Ozone (O3) 0 to 7×10-6 %
Nitrogen dioxide (NO2) 0.000002 %
Iodine (I2) 0.000001 %
Ammonia (NH3) trace
Water vapor (H2O) ~0.25 % by mass over full atmosphere, locally 0.001%-5%.

These elements need to be replaced from around the leaf surfaces, just like the nutrient solution surrounding your roots. Within a closed room these elements can either become depleted or accumulate to toxic levels.

When setting up your grow room ventilation, we will need to know the size of the room, then figure that length times width times height equals the cubic footage of your room. Fans are sold on the bases of freely moving air, in and out of the fan with no duct work attached. Let's take a room that is L10 x W8 x H8 = 640 cubic feet of air. The things in your room such as reservoirs, lights, fan, tables and growing medium all take up part of the cubic area; we will temporarily ignore this point, as we will use the higher volume of air movement that these units displaced.

Choose a ventilation fan that removes more than the cubic air footage of your growing area. We want to be able to remove every cubic foot of air from the growing area within three minutes of 'on' time. Remember that these fans are measured with no duct work on them and we will need to direct this air from the room. Every bend that our ducting has to navigate around causes loss of air flow. Make sure that you still have enough air flow to remove all the air within the growing area within three minutes. This will help to remove the air within the body of the plant.

Plants have branches that spiral out from a central stem, creating a bit of a shield around the plants and we must remove the air from within this area. We will need three fans, with two being exhaust fans and one fan for oscillating the air within the room. You may use one exhaust fan for removing air and one exhaust fan to replace this air from outside ('air-in' and 'air-out'). We do not want to create a vacuum effect, though, as one fan pulling air from a source would do. Thus, pushing air into the room helps.

When using two fans, one in and one out, try not using the same window to exhaust and capture air, or you could actually be drawing in the air you just removed. Also, have the exhaust fan a bit larger than the return fan; this keeps a little negative pressure inside the room, not allowing the air from within the room to escape outside. This works particularly well when you do not want garden smells going into the rest of the building.

Besides non-helpful elements being removed, we will also need to remove the heat that builds up within the grow space. By choosing the right size of fan, we can efficiently remove unwanted elements, heat and humidity.

To test to see if we can remove all the air within a growing area within an allocated time frame, light incense or something that creates an odor that will leave a noticeable smell, then, turn on your ventilation fan to see if all traces of the smell are removed. If not, correct the ventilation before placing your plants into the growing area. If you walk into a growing area and you feel uncomfortable from being cold or hot, sweaty from humidity and cannot wait to get out of the room, think about your poor plants that are probably also very uncomfortable. They rely upon you to provide an ideal growing area. As they cannot leave the room like you did, your yields will suffer for it.

Nature is so vast that we live in an ecosystem; our sun heats up the air and surface of our planet, causing evaporation. Wind moves around the water vapor (gases); as it moves over cooler areas, the wind allows the water vapor to fall back to the ground as rain or snow. Since very few days of the year have windless days, it is the wind or water that recirculates the elements, allowing everything to be replenished.

We will need to set up more than one type of fan in our growing area because we need to move this air solution around and out of the growing area. Have oscillating fans set up in corners of your growing area and ensure that all plants are getting a direct hit of moving air onto them. This will bring fresh air into the interior of the canopy of growth. We are not trying to create a wind tunnel here but do want good air circulation. Plants have evolved in the fresh air and need a steady source of air movement, removing gases from the surface area of the plant that are being exchanged with the air solution.

Having an oscillating fan blowing air throughout your grow room strengthens the stems and can cause shorter, bushier plants. It is possible to cause wind burn on the leaves, so finding the right amount of air movement is critical to optimum growth. Too much wind can cause the stomata to close, slowing down photosynthesis and therefore stunting plant growth. Having no air movement will do similar things by not allowing fresh carbon dioxide to be in contact with the leaves. A nice, fresh movement of air works best; you may want to try different amounts of air movement to find your optimum growth pattern. Thus, it is just not what you feed the plants, but rather everything that you do to them that leads to your yield (aside from strain selection, of course).

Remember, plant leaves are continuously taking in and releasing gases which include carbon dioxide (CO₂), Oxygen (O), Hydrogen (H), water vapor (H₂0) and Nitrogen (N). As these elements are released and are depleted from the plant and build up within the growing area, you will need to remove them from the building. Just removing the air into another room is not enough, as this polluted air will seep its way back into the growing area as the air is extracted from the room. We need to send this air to the outside of building.

Cheers to your healthy plants!
Mr. Bill