The cooler the room becomes, the worse the problem gets. As temps climb above 80 degrees, another set of problems is encountered. The metabolism of garden plants becomes so high that C02 in the grow room is used up very quickly… a 6x8x10 garden area can become C02 deficient in approximately 45 minutes. This means that all photosynthesis (and therefore plant growth) stops, unless you are constantly exchanging the garden air or constantly adding additional C02. In extreme cases, high temps in the grow room combined with higher metabolism rates will simply cook the plants to death.
So, what is the ideal temperature for an indoor garden? That is a tricky question. In general, I like to keep my daytime temperature at about 75 degrees, and my nighttime temperature 5 to 10 degrees cooler. This is just about ideal in a garden that constantly exchanges the air with a powerful fan. If you are adding C02 to your garden, however, it is more beneficial to keep daytime temps closer to 82 degrees. The slightly warmer temperature speeds up plant metabolism slightly and allows you to take full advantage of the extra C02 by speeding up plant growth rates to their maximum.
It is important to note that photosynthesis (and plant growth) stop when the C02 in a garden drops below 250 ppm. With higher temps, the C02 in a fully enclosed garden is used very quickly and needs to be supplemented or replaced. That is how temperature affects C02 in an indoor garden. With higher temps the plants will consume more water. With more water, plants consume more nutrients. If you do not want your plants to become chemically burnt (over-fertilized), then you need to reduce the strength of your nutrient solution slightly whenever the garden is running at a warmer temperature.
Now that you know what temperatures are ideal for your indoor garden space and how it will affect the consumption of C02 and nutrients, let’s look at the most reliable methods for controlling the temperature in your garden. The very first thing I recommend is to have a large volume of cool air ready to exchange with the rest of your garden. A 4×8 foot garden area in a 20×40 foot basement is a good situation to have… the large volume of air can be constantly exchanged with the garden to keep it cool.
As your garden is confined to a smaller and smaller area temperature becomes more and more of a problem. If your garden is confined in a small space, you will want to ventilate your reflector separate from the rest of the grow room. This means having a fully enclosed reflector, pulling air from outdoors, through the reflector to cool it, then exhausting the (now warm) air back to the outdoors. This will prevent a lot of the heat from the light from entering into the rest of the garden air space.
The rest of the garden air space needs to be exchanged constantly with a powerful fan (either squirrel cage or centrifugal). Even then it may not be enough to keep high temperatures in check. This is especially true in the Summer months. If you are using this method and you find that it is not enough to keep your garden temps right, you only have one option… you need to introduce cold air into the garden. This is usually done with AC, but in Fall and Winter may be done in some areas simply opening a window and pulling air in from outdoors.
Those are the basics of indoor garden temperature control. I recommend always screening your air intakes to prevent outdoor pests from making their way into your garden. For more information, be sure to check out the included link, and happy growing!