Maximum Yield USA February 2019 | Page 32

Ventilation and Air Flow “THE EXACT TEMPERATURES YOU MAINTAIN DEPEND ON THE CROPS YOU GROW AND THEIR PREFERENCES.” 32 Maximum Yield Evaporative cooling systems, also known as swamp coolers, turn hot air into a cool breeze by passing air through a soaked screen, pad, or spray of water. These systems are most effec- tive in environments with low humidity as they add moisture to the air. Fogging and misting systems are also often used to cool greenhouses. These two systems are similar in that they both spray water to cool the air. The main difference being the size of the water droplets. Foggers create a very fine mist, while misters spray larger water droplets. These systems can be put on a controller to spray water when temperatures get too high. While effec- tive, the main drawbacks to these types of systems are excess humidity causing fungal issues within the greenhouse and clogged emitters from water with high mineral content. You can also control temperature by keeping a black reservoir filled with water in your greenhouse. During the day, the water will absorb heat. When it cools down at night, heat is released and warms up the green- house. This is the principle of thermal mass at work. You can even put your tank to good use and grow fish while you’re at it. As a rule of thumb, fill your reservoir with two to three gallons of water per square foot of greenhouse space. Finally, if you need to increase temperatures in the winter, you can use heaters or even wood stoves. Air flow is essential for temperature control and fresh air exchange. Without fresh air, there will not be enough carbon dioxide for photosynthesis unless you install a CO 2 generator. Even with a generator, you’ll need fans to move air within the greenhouse to help prevent fungal disease. Vents, exhaust fans, and oscillating fans can be used for fresh air exchange and circulation. You can also install roof vents that need to be opened and closed throughout the day depending on the temperature. Roof vents are often operated by hand, but you can automate them with a thermostat controller to eliminate the manual work. Exhaust fans can also be set on a timer so air is exchanged as often as once per minute. If it is cold outside, you will want to exchange the air less often, about a few times per hour. You can also hook your fans up to a thermostat, so air is exchanged when temperatures start to rise. Mount these fans high on the end walls and make sure they have a protective hood so that rain is not blown into the greenhouse. Ceiling fans or oscillating fans keep air circulating within the greenhouse. This will control humidity levels and strengthen your plants’ stems, just as the wind strengthens plants outdoors. Finally, if you are going low-tech, you can install screened doors on the ends of your greenhouse to create cross-ven- tilation and prevent stagnant air.