Garden & Greenhouse June 2018 Issue | Page 12

fresh air. This is why portable evaporative coolers work best when placed near the greenhouse entry or a fresh air intake vent. Like the wet wall systems, portable evaporative coolers are thermostatically con- trolled. Unlike the wet wall systems, portable evapo- rative coolers can easily be stored under a bench or somewhere out of the way when not in use. Portable evaporative coolers offer small greenhouse growers a great way to maintain cooler temperatures during the hot summer months of the growing season. is drawn into the greenhouse, it passes through the wet pad and is cooled. This system works in a way that resembles a radiator cooling the engine of a car. The powered fans, which are placed on the wall op- posite the wet wall system, are the driving forces of the air movement. Motorized shutters are positioned over the wet wall system to allow air to enter the greenhouse. These shutters are usually controlled by a thermostatic controller. This means they will open and close depending on the temperature. Wet wall systems are fairly expensive and require more exten- sive plumbing which is why they are generally re- served for large, commercial greenhouse applications. Portable Evaporative Coolers Similar to portable air-conditioners, portable evaporative coolers are self-contained, cooling de- vices that are commonly used by greenhouse hobby- ists. Portable evaporative coolers con- tain a similar pad to that used in a wet wall system. A mechanical fan draws air over the wet pad to create an evaporative cooling effect. For a portable evaporative cooler to work most efficiently, it should have access to 12 Fogger or Humidifier For some small greenhouses, a fogger or hu- midifier can serve as an evaporative cooling system. A fogger is a device that emits water in a fine fog. These devices can be placed directly behind a circu- lating fan so the water-cooled air is blown around the greenhouse. As the water evaporates, it also cre- ates a cooling effect. A humidifier found at a big box store can be set up in a similar fashion. When placed behind a circulating fan, the cool, moist air will be blown across the greenhouse for an evaporative cool- ing effect. Both foggers and humidifiers emit water droplets so small that the plants within the green- house do not get wet. Instead, the water quickly evaporates and cools the greenhouse’s climate in the process. Foggers and humidifying systems should not be confused with mist systems. Misting systems emit larger sized water droplet that will settle on plant leaves and actually make the plant wet. This extra moisture, combined with warm temperatures, can cause a whole new series of problems, including pathogenic molds and fungi. Passive cooling offers an inexpensive way to re- duce a greenhouse’s heat signature. In some cases, passive cooling on its own will be enough to keep a greenhouse in the desired temperature range. In other cases, when mechanical evaporative cooling or mechanical fans are also required, passive cooling techniques can still increase overall efficiency and, in turn, a grower’s return on investment. Gardeners who implement evaporative cooling techniques are essen- tially mimicking the plant’s natural cooling method known as transpiration. Not only are evaporative cool- ing techniques effective, but they are also efficient ways to maintain healthy growth in a greenhouse even during the mid-summer heat. Ultimately, the geograph- ical location, the type of greenhouse, and the crop being grown should all be considered when determin- ing which cooling technique or which combination of cooling techniques a horticulturist should use. GG Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse senior editor and can be contacted at Ehop@ GardenAndGreenhouse.net. www.GardenandGreenhouse.net June 2018