Basic Hydroponics Air Quality Controls

Basic Hydroponics Air Quality Controls

Garden Air Quality Indoors & Greenhouse Growing

Breathe Easy: Basic Hydroponics Air Quality Controls

Ever been in a room where you couldn’t quite breath? It’s stuffy, hot and muggy, maybe it had that weird, doctor’s office smell… and all you can think about is getting out for some fresh air. Now imagine that you live in a pot and you can’t move. That is basically how your plants feel when we don’t take care of the air in our grow rooms. Carbon dioxide, temperature, and humidity all play an important role in the success of your hydroponic crop, as each of them affects photosynthesis. Increasing photosynthesis will give your crop the energy it needs to grow, fight stress and disease, and increase yields. A few simple control measures are all it takes.

[quote]Basically, understanding your crop’s optimal grow temperature can save lives.[/quote]


The Golden Rule

The first 3 steps for taking control of grow room or greenhouse air quality are as follows: monitor, monitor, monitor! It is important to be aware of your baseline before you start running fans and humidifiers and who knows what else.  Keeping track of what changes, if any, are needed, along with which ones are effective, will prevent unnecessary expenditures and maximize efficiency. Most high tech digital controllers will come with various sensors and meters, but if you are trying to keep things simple, start with a small desktop monitor. 

Grower’s Secret: When using any sort of meter, place the sensor at the level of the plant canopy to take readings of what the plants themselves are experiencing.

Is it cold in here or is it just me?

How many of us can’t sleep at night when the air conditioner is broken? And how many of us have to bring sweaters to work so we don’t die of frostbite? It’s much the same for your plants. Dialing in the right temperature can make or break a plant’s growth efficiency.

Increasing the temperature will increase photosynthesis. However, if you run too hot, you run the risk of cell desiccation and scorch leading to a slow, sad death (plus photosynthesis STOPS at 90° F).  Similarly, if your grow room is too cold, the chill will cause cell membranes and enzymes to break down faster, leading to another slow, sad death. Basically, understanding your crop’s optimal grow temperature can save lives.

Most hydroponic crops can be sorted into 2 groups: warm season and cold season. Check this quick guide for their favorite growing temperatures.

Cool season plants:

  • daytime temperatures of 60 to 70° F (15 to 21° C for our Canadian friends)
  • nighttime temperatures of 50 to 60° F (10 to 15° C).

Warm season  crops:

  • daytime temperatures of 70° to 80° F (21° to 26° C)
  • nighttime temperatures of  60° to 70° F (15 to 21° C)

Grower’s Secret: If you are uncomfortable with the temperature in your grow room, odds are so are your plants.


During the day cycle, your grow lights will naturally increase the temperature of your grow room. The higher wattage the bulbs, the greater the heat. The obvious solution is to vent: hot air out, cool air in. There are a thousand different way to vent your space, but here are a few basic rules. (NOTE: the following is for grow tents and grow rooms separate from habitated areas of the house.)

  • Start with the right fan: take the volume of your space (height x width x length). Your extraction fan should be able to move at least that many cubic feet of air per minute (CFM).  If you’re getting hot spots under your grow lights, try adding a small oscillator at plant level.
  • Don’t let your fan run wild: put fans on the same time cycle as your lights to cool when they are on, or use an analog thermostat for more exact target temperatures.
  • Where to vent hot air: outside of course, preferably from the wall opposite the intake vent to encourage even air flow. Or, if you live in a cold climate, create a backup vent that will channel that warm air back into your home during the cold winter months. Free, warm, fresh air! No space heater required.
  • Where to get cool air: from the coolest place possible, either outside or inside. If you live in a hot climate and worry about your central AC working too hard, consider sealing your room up and using a mini AC unit.

Grower’s Secret: When venting back into your home, use an inline filter to keep out dust, odors and mold spores, even if venting from sealed fixtures. Also, be aware that air from the plant environment could have higher humidity, so a dehumidifier might be needed to prevent excess moisture and condensation.

Other solutions to keep your lights from baking your babies are based in managing the lights themselves. Consider the following simple controls.


Generally speaking, you want night cycle temperatures to be lower because your plant is not photosynthesizing and elevated night temperatures will only cause you plant to lose more water to evaporation. That may not sound that bad until you check your reservoir, now half empty with the pH and EC all screwy (but that’s a conversation for another day).

Carbon Dioxide

Plants cells “inhale” CO2 to use during photosynthesis, and “exhale” oxygen as they expend energy, scrubbing carbon from the atmosphere.  Hence, there is a direct relationship between CO2 and Photosynthesis. Therefore the more CO2 in the air, the more photosynthesis a plant can carry out until saturation (or terminal velocity, if you will).


The average atmospheric CO2 level is about 350-400 parts per million (ppm). The more plants packed together, the more CO2 will be stripped from the air. In natural high density plant populations like forests and jungles, CO2 is replaced by decaying bio-matter and plant/animal respiration.  If you are venting your grow room, additional CO2 won’t be needed as fresh CO2 will be coming in all the time (and any excess you add will just be vented out anyway). However, if you are using a closed/sealed space, you will have to supplement it artificially, not only to boost photosynthesis, but also to maintain normal levels.

For smaller applications such as grow tents, closet growers, and small rooms, simple CO2 releasing bags and buckets can be very effective. They work by releasing CO2 from composted materials, mimicking the natural breakdown of bio-matter in nature, but minus the smell of decay, thankfully. For larger areas, tanks of concentrated CO2 can be combined with regulator valves that have built in timers, allowing you to dose automatically at the right time and concentration you desire.

Grower’s Secret: CO2 is heavier than other gasses and will sink down. Any CO2 bags or distributive tubing should be placed above plant canopies so that it moves toward and through the foliage.


When plants breathe, it is through small openings in the leaves called stoma. They take is CO2, and also transpire (exhale water into the air as a cooling method). If a plant gets too hot, transpiration can cause water stress, so the stomata close so that the plant won’t lose any more water. Unfortunately, this also stops CO2 intake. No CO2, no photosynthesis.  By upping the humidity in our grow area, we reduce transpiration, keeping the stoma open and the CO2 flowing.  But as is the case with extremes, too much humidity will saturate the air with water, making it impossible for a plant to transpire, causing even more stress. Excess humidity also encourages mildews and disease. The ideal range is about 50-85%. We don’t want to get too stuffy or too dry. Adding humidity can also have a cooling effect on the air, reducing heat from the grow lights.

It should be mentioned that humidity should be reduced at night with a dehumidifier or by venting. As temperatures drop, excess water condensates on plant leaves, increasing the risk of disease. Keeping the foliage dry will help prevent mildews, fungal spores, and bacteria.

Get a hold of yourself! And your grow room…

Don’t think that you must have all the biggest and most expensive gear when it comes to controls in your grow space. Start simple and you can always upgrade later when you’ve got the hang of it. Always remember that the first step is measuring and monitoring. By watching and maintaining temperature, CO2 and humidity in your grow space, you will maximize growth efficiency, which will have you and your plants breathing easy.

By: Susanna Whelan

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