When all of the beverage from a drinking cup has been imbibed, it is considered to be “empty”, and admittedly that definition tends to work as far as drinking needs go in gardening. It is sometimes helpful to remember that it is not empty at all, but rather full to the brim with air.
In a natural setting, such as a meadow, an ocean of air washes over and though the plant life. Waves of air push away excess humidity and oxygen, and they carry carbon dioxide in. When a plant is removed from its native habitat, the natural sources of light, rain, and wind are lost, and the gardener becomes responsible for meeting the environmental needs of the plant. Indoor gardeners take their plants away from the wind, and fans or other environmental measures are used to replace it.
Plants grown in still air tend to have more problems with molds, insects, and disease. Moving air from fans helps to keep moisture from collecting and encouraging mold spore growth. Fans can also help remove excess heat, to help defend against heat loving spider mites.
Indoor gardens tend to have more trouble with heat than cold. This makes sense if you consider you are adding energy into a closed room in the form of electricity, which converts into light and heat by the lighting fixtures. The plants absorb and use some of that energy, but the rest can cause heat buildup if the amount of heat generated by the equipment is higher than the amount of heat removed from the room.
Hot air is just high energy air, and by removing it close to the heat source, you can pull the energy out of the garden before it transfers much of its heat to the surrounding air. To lower the garden temperature to growing temperatures it is more efficient to vent off the hottest air than it is to try to dilute it with cold air.
Since lights are likely your largest source of waste heat in the garden, they are one of the biggest climate concerns. Whenever possible, keep the ballast outside of the garden proper. As part of their functioning, ballasts generate a fair amount of waste heat, and any heat you can avoid putting into the garden, is heat you don’t have to worry about dealing with. Tents in particular should have the ballasts located outside of the garden environment.
Using sealed and vented hoods are a way to remove the waste heat from bulbs quickly and efficiently. A fan and ducting brings cooler outside air to the hood, then across the bulb, where it captures heat, before being sent out of the garden. Ideally this creates a closed path for outside air to pass through the garden and return without mixing with the air in the garden room itself. The total amount of air moved is small, and concentrated around the heat source, so you’ve dealt with much of the heat before it spreads.
When designing ducting layouts, keep in mind that wide turns are better than sharp turns, and that unwanted hot air rises. If a passive air vent is required to replace vented air (depending on how airtight the garden is), the incoming air needs to be taken from cooler air from the outside of the garden, and located opposite to the exhaust vent.
To calculate how many cubic feet per minute of ventilation you need, start with calculating the size of the garden in cubic feet. You do this by multiplying the height, length, and width of the room. A 8’x10’x10’ room would make for 800 cubic feet of garden space. A single 800 cfm fan would move the room’s area air mass every minute, and a 400 cfm fan would do the same every two minutes. Two 400 cfm fans would again clear the air once a minute, and so on. Read the listed cfm on the box, but keep in mind that fans are usually rated while under a very low load, so any ducting can reduce cfm dramatically.
Since the lights are usually the largest source of waste heat in the garden, when you’ve dealt with, then controlling the temperatures in the rest of the garden usually becomes a lot simpler. If hot air is collecting in a particular area of the room, try to vent it off. Although more expensive initially, using solar-powered fans to give an extra boost during the hottest part of the day without adding to the electric bill. If the garden still needs more cooling, bring in cooler outside air, or air-conditioned cold air.
One of the reasons that the temperature around the lights is so important, is that the cooler the lights are, the closer you can get them to the plants. Since light is broadcast across an area; a plant at 1’ away from the light source receives 4 times as much light energy than one at 2’. Ideally, you want the tops of the plants in the “sweet spot” where they are close enough to the light to grow well, without being so close to the light that the heat damages them.
Fortunately, with a couple reasonably priced meters, it is easy to find exactly how far away the “sweet spot” for plant growth is. The two devices you need are a light meter ($20-$100), and a quick acting digital indoor thermometer ($10-$20).
Put the temperature sensor next to the light sensor, and hold them below the light. Ideal temperatures are between 70-85°F, so raise and lower the sensors until you find the place where it reads 80°F, and check to make sure the light levels are at least 25,000 lux (in general, more is better). That is as close to the light as the tops of your plants should get. Then lower the meter until the light level drops to 20,000 lux or so, that is about the bottom of the “sweet spot” where unblocked light is still strong enough that it’s useful.
By using this pair of meters, it becomes very clear that having the light cool enough placed reasonably close to the tops of the plants that has a strong benefit in the amount of light available to the plants. Bright sunlight is about 100,000 lux and free, which is why even partial natural lighting can often be cost-effective. Aside from transporting excess heat out of the garden, air circulation is also important for evaporation and available carbon dioxide (CO2).
Evaporation in a garden is important to remove stray droplets of moisture that may collect. This moisture, if left unchecked, can encourage unwanted mold and bacterial growth. Mold does not grow as well in low humidity, so one of the first steps to treat a mold issue in an
indoor garden is to increase air circulation (although you must do so with care and not to spread the pathogen in the process).
Excessive evaporation however, indicates low humidity, which can attract dreaded pests. Too low of a humidity level can lead to spider mites, and too high leads to molds. If the garden is kept between these two extremes, things tend to go a lot smoother. If a problem with one or the other arises, change the environment to disfavor the pathogen. Mist spider-mites regularly, and let areas of mold outbreaks dry out a bit more than usual.
Photosynthesis in plants requires light and heat energy, water and carbon dioxide (CO2). Chlorophyll collects light in leaves. Plants use part of this energy to split water molecules into free oxygen gas (O2), and hydrogen (H). The hydrogen is then bonded with carbon dioxide (CO2) to form the sugars the plant can use to grow. If light and water are already present in sufficient quantities, then you can increase the heat slightly as carbon dioxide levels will raise. Without air movement this process can result in the plants sitting in a cloud of oxygen and not receiving enough carbon dioxide.
CO2 is naturally occurring in fresh air, but it is also sometimes added to the garden environment artificially. In the garden, CO2 is usually generated by chemical reaction, combustion, biological reaction, or slowly released from pressurized tanks.
My preference is for the gardener, who is as much a part of the system as anything else, to spend time in the garden, and contribute their breath to the system. If you think that your garden isn’t getting enough CO2, consider bringing a friend in and do some heavy breathing
together. Human beings are fairly large animals, and generate quite a bit of carbon dioxide when exercising, just make sure there is enough fresh air to be healthy for the humans involved.
The “empty” space around us isn’t all that empty. Atmospheric gases like oxygen, carbon dioxide, and water vapor are all transparent to our eyes, and therefore easy to forget, but these gases in the proper amounts are important for proper plant health and growth. We live in an ocean of air, complete with currents, flow, and volume. By giving plants the fresh air and airflow they need, not only will the plants be happier, but it can help keep molds and moisture loving fungus gnats at bay.
[alert type=white ]This article was originally published in Garden Culture Magazine, Issue 3 under the title, “Looking At Air: Climate Techniques”.[/alert]