How Important Is Airflow?
 
*These videos were meant to be placed throughout the post, but Patreon made me combine them all and post as one video. I shall try to find a better solution for next week!

Importance of Air Circulation:

A common misconception that people have is that they think the most important part of growing are the lighting, some will say the nutrients. These are both vital, but air circulation is just as important in terms of having a healthy crop. Photosynthesis is something that is easy to understand in its basic form. Lights hits the plants leaves, the plant grows. Simple. Which is why it is easy to think lighting is the most important. But for the light to be used appropriately by the plant, there are more complex reactions at hand that cannot be easily seen until issues become present. For the plant to be able to use light correctly, it needs to have a food source for the roots to use to aid in its development, thus nutrients becoming an important factor. Let’s take a look at why air circulation is such an important aspect of growing!

Carbon Dioxide:

Plants also need C02 (Carbon Dioxide) to be able to perform photosynthesis. This is something that they take up from the environment around them through the air. If their is a lack of airflow, the availability of C02 around them decreases, as there is nothing to move other C02 closer to the plant once it has converted everything within its vicinity. In most small scale setups, additional C02 is not required, but in sealed rooms and bigger grow spaces, like an enclosed greenhouse/hoophouse, the availability of overall C02 within the space will drop to a point that it negatively affects the plants ability to grow, which is why you will find people in these situations using C02 injection in their grow space. Being able to keep fresh air constantly circulating around your plants means you are continuously giving them access to more C02!

Leaf Moisture Content Level:

The leaves of the plant act similar to the way skin works. When it is hot and dry, the leaves allow for moisture (and air) to be released through the leaves stomata. If it is really hot and dry with minimal air exchange, the plant is forced to push out more water through its stomata than it would ideally like to, drying the plant out. This could lead to the plant burning from not being able to cool itself quickly/efficiently enough. This state allows the plant to become light and/or heat burned more easily. Increasing the airflow in a hot and dry situation cools the plant off, giving it a better chance at success. The ultimate fix lies within getting the humidity and heat levels within check, but increased airflow will help the situation. 

Humidity and Heat Levels:

If you are growing within an enclosed space with no air circulation or exhaust, the problem you will be most likely facing is a hot room with high humidity. Having solid airflow throughout the room with a proper air exhaust setup should result in lower heat and humidity levels. A general rule that people follow is having relatively high humidity during the veg state (60-70%) and then continually lowering it throughout the plants flowering cycle until it is harvested (25-30%). If you have proper air circulation, your veg plants can get away with pretty high humidity levels. 

Diseases and Pests: 

Once you get to the later stages of flowering, bud sites act as a moisture catch, so if the humidity is too high for too long, you risk facing things like Bud Rot (Botrytis) and Powdery Mildew (PM). If there's a lack of good airflow, your chances of catching these things drastically increases in late flower.
Having proper air circulation decreases your risk for pest issues. Pests of all kinds prefer a humid, warm space that is lacking adequate air flow. If you bring a pest into your grow space, having proper air circulation will not magically get rid of the pest, but it will decrease its ability to reproduces as quickly, making it a more manageable situation.  The worst bug issues usually occur in the center of a dense plant that has no/minimal air movement within its canopy/insides. 

How Much Air Flow Do I Need?:

Most growers utilizing an indoor grow space, like a tent, will suggest that you have an exhaust fan that moves 2-4 times the amount of air than the dimensions of the space. If you have a 4x4x6 ft tent, then it will be (4x4x6=) 96 cubic feet. With this in mind you would want a fan that can push 192-384 Cubic Feet per Minute (CFM). If your exhaust is a straight shot and there is no need to bend your ducting from the exhaust fan, then you can get away with using a fan on that lower end of ~200 cfm’s. If your exhausted air has to travel through 1 or 2 bends, you will want a fan that pushes closer to 300 cfm’s. If it has to travel through more than 3 bends, you will be wanting a fan that can push closer to 400 cfms. 

I personally enjoy having a seperate variable controller that I can hook my can (exhaust/intake) fans up to. This way I can taylor the air flow depending on what stage the plant is in and depending on what the external ambient air temperature is. Having a fan that can only go max speed can be a disadvantage at times. I do not recommend (even though I use one myself and have never had an issue with it [knocks on wood]) fans with built in variable speed controllers. KISS (Keep It Stupidly Simple). Using equipment that perform multiple actions (in this case a fan with built in controller), increases the amount of parts that can potentially break. Using an external variable controller that is known to work with your brand of fan decreases your chances of things going wrong and allows for easier fixing when things do go wrong. 

All of this air movement has the added advantage of increasing branch strength. When the plant is hit with the moving air, it is forced to react to the movement and does so by beefing up its branch strength to be able to take the force from the moving air. This translates to bigger buds as the plant realises it is now able to hold more weight as a result of the bigger branches and pushes more weight into the buds. Having a plant in a grow space with no air flow tells the plant is does not need to put energy into makings branches stronger, which means it will not be able to support as big of buds. More air circulation means bigger, stronger buds! But be careful, as too much of a good thing can be bad. Find the balance for your specific situation.

How Much is Too Much?:

When a plant receives too much air, it is subject to dehydration and wind burn. Wind burn can express its symptoms in a few different ways that usually appear in conjunction with one another. One symptom is the leaves developing white patches in between its veins near the tips and edges. This is the result of the plant being stripped of its moisture on these leaves too quickly, drying it out, and becoming burned as a result.  If you see this in a space with a high amount of air movement, move the fan further from the plant or move the affected branches, if possible, further away from the fan. Leaf drooping is another sign of too much air movement. This is the plants attempt at shielding itself and trying to move further away from the air movement. If this goes on for too long, it can lead to the above symptoms of leaf burn. 

Another issue that you may face is branches breaking. If the air flow is too strong and the buds/branches are not supported well enough, you may encounter a broken branch or two when you check on your girls. This is not as normal of an issue as wind burn, but can still happen easily if not enough attention is paid.

Negative vs Positive Air Pressure

This is pretty straight forward. Negative air pressure results in sealed grow space (like a tent) when more air is trying to be pulled out of the space than is being pumped/sucked in. Positive air pressure is the result of more air being pulled into a grow space than is being pushed out. In a tent, negative air pressure looks like the tent is sucking in on itself, positive air pressure looks like the tent is filling up like a balloon (air pushing out on the walls). We are looking for a slight negative pressure within our grow space. The main reason most people want this is to be able to control odor. If using a carbon filter on the exhaust fan, you want all air to to go through the filter. Positive air pressure will push excess air out out though gaps in seams and zippers, allowing odor to escape. Having negative air pressure results in air being sucked in through these seams instead, and forces all air through the filter. If smell is not of a concern, I recommend a slight negative pressure or neutral pressure. I have found no benefit from having positive pressure. I suppose one potential benefit from positive pressure would make it so it is harder for any bugs/diseases to get in through the seams/zippers, which would make it so the only air entering the tent is the air going in through your filtered intake fan. I have no experience with trying this out however.

Examples: 

-Exhaust fan pushing 300 cfm out of the tent, with a passive intake (no fan), will result in negative pressure within the tent

-Exhaust fan pushing 300 cfm out of the tent, with an intake fan pulling 250 cfm will result in negative pressure within the tent

-Exhaust fan pushing 300 cfm out of the tent, with an intake fan pulling 350 cfm will result in positive pressure within the tent

-Exhaust fan pushing 300 cfm out of the tent, with an intake fan pulling 300 cfm will result in neutral pressure within the tent

What Kind of Fans and Where?:

Oscillating fans 

^20 Inch Oscillating Fan by Lasko^

These are your bread and budder. Being as that you cannot see your air flow throughout an entire grow space, it can be hard to detect dead spots. Having stationary fans will make it so the air will consistently following the same path, 24/7. This is no what we want. Adding one or a few oscillating fans will encourage the air to consistently change its path, covering more space, reducing the spots with no air movement. Having at least two of these with different paths really gets your air moving in different ways, keeping the whole grow space fresh.

The most commonly used oscillating fans on the market are the 16 inch fans by Hurricane, with Active Air and Lasko also being popular, well made fans.  Others will do, but these are not something to go cheap on. You will pay the price with a lack of quality air flow and quicker dying fans if you go cheap on this. These fans can be found with a stand or as a wall mounted fan, around the $40 price range, usually a bit less. If higher than that, look elsewhere.

It is a good idea to have one of these every 5 feet of wall space if you are able to afford it. For this I will use standard tent dimensions but applies well to rooms and hoop houses as well. In a 4x4 space and smaller, you can get away with just one and some smaller supplemental fans. Once you get to a 5x5, two would be more ideal. I would suggest 4-6 in a 4x8 space and 9-12 fans in a 10x10. Put these fan where you feel they have the most effect, but typically i put them in the corners of the room if it is small. In bigger rooms I have them along the wall and even coming down from the ceiling in the middle of the room. In all of these sizes, I would suggest a couple small fans to move air above the lights (if not using hooded fans) and down by the soil level.

Stationary fans

^6 Inch Stationary Fan^

If i could only use oscillating fans, I would, but sometimes we are strapped for cash or space and will have to make do with a stationary fan or 2. I have actually grown found of the small 6 inch fans that can be found for cheap on ebay/amazon, as they fit well into tight spaces that may need a little extra airflow (by soil level, above lights, in the corner of a tent/room, etc), where you don’t want to have to sacrifice canopy space. They are not the most powerful and need to be cleaned off frequently (I found removing the faceplate drastically improves performance), but they do a good job of killing off dead spots. 

Inline Fans (aka Can Fans)

^4 Inch Exhaust Inline Fan^

I went my first 6 months of growing without using one of these. Needless to say, I could not keep the bugs, diseases, and mildews away. Once I started the journey of informing myself on the importance of air flow, I realized I was shooting myself in the foot. Having one of these for exhaust is a must, having a second for air intake was an even bigger level up. If using a tent, you can get away with using just one for exhaust, leaving flaps/zippers open to allow air to passively come into the tent. If you really want your space to perform well, I recommend using one for air intake as well. Depending on your grow space, you can sometimes get away with just having the air exhaust into the same room. If the temp/humidity is getting to high, you may need to exhaust your fans out of the house or into another room. DO NOT pump this air into your attic. This air is humid and will cause issues if pumped into an attic or crawl space that will be a pain to manage/fix. 

The important thing about these can fans is that you want to have a minimal amount of bends in your ducting. The more bends, the more energy is required to move the air. The ideal space for your exhaust fan is going to be up above the lights, where all of the hot air collects. The ideal spot for your intake fan is going to be as close to the bottom of the space as possibly. This increase the overall air movement in your space. Having the intake next to the exhaust will result in the air coming in and going right out. Spacing these out is key for quality air flow and having multiple fans between its entrance and exit points will allow the most amount of fresh air to fill the space as best as possible before exiting. 

I highly recommend mushroom filters instead of HEPA filters for your intake. HEPA filters put a high amount of stress on the fans and shorten their life. Mushroom filters restrict air less and do a great job of keeping out the things you don’t want in there, and they are washable and reusable. Using a carbon filter on your exhaust is up to you. If smell is not an issue, skip this heavy, expensive item and put that money into something like oscillating fans. 

^6 Inch Dust Shroom by Horti Control^

Light Fans (Inline again)

^4 Inch Inline Fan^

These are the same inline fans mentioned above, but usually do not need to be as big as there is much less total air that is being moved around. They are used with hooded lights so that way the heat from the light is taken out of the room directly, rather than entering the grow space and having to be dealt with the main exhaust fan.

Duct boosters


You can pretty much just skip these unless you have ducting with multiple bends in it. Do not use this in place of an inline fan, they are not the same, regardless of how they are advertised. Not a bad addition to pull a little extra air out of hot spots, but do not rely on them as your main fan.

Passive In/Outtake

I do not recommend you rely on passive air movement to get your job done, but in addition to your can and oscillating fans, having spots where air can escape your space without any extra power consumption is a definite plus. This can be things like vents above your lights or near the ceiling in a hot corner. This is more useful in big rooms and greenhouse style set ups, but can be applied to smaller tents as well.

Techniques and Tips to Improve Air Circulation:

Lollipopping is the concept of removing all of the inner branches and leaves that are not getting adequate light or airflow, leaving just the canopy. It is useful for a variety of reasons, but the most important reason, in my opinion, is to allow better airflow through your plans, improving its air circulation. This will decrease the chances of getting bug issues, diseases, and mildews/molds, while increasing the overall airflow in the room. Air getting caught up on one plant prevents it from being able to pass through to your other plants. 

Look for dead spots after you have your fans set up and your plants in place. This can be hard to do by just looking and the moving branches. One way to be able to view the air paths within your space is by smoking in it. Light up a joint and blow smoke into all corners and nooks of your space. The smoke should dissipate instantly. If any smoke lingers anywhere, you run the risk of encountering issues. I do not vape, but I have noticed it is easiest to see the air paths when someone blows a thick cloud of vape smoke into a grow space. 

Watch your fans for a couple minutes and adjust them as needed. If you see that two of your oscillating fans are fighting one another, try adjusting the timing on one and see if that helps, or you may need to move it to a space with less air flow. Too much air flow in one spot as a result of multiple oscillating fans hitting the same spot at the same time may result in wind burn, and being as it damage will not be consistent (because the fans path changes), it may take longer to notice the damage you are doing. Watching all your equipment for a couple minutes per item can really help you in the long run. 

Next Week: We will be covering the advantages and disadvantages of different types of lighting and what you should use for your grow space to be able to utilize your space and bills efficiently!

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