The greenhouse is probably one of the most expensive pieces of equipment on a farm, but once summer arrives, many growers give up on using it because of the heat. As temperatures outdoors rise into the upper 80s and 90s, the greenhouse can easily get too hot for plants to survive, let alone thrive. But greenhouses can be kept cool enough to be useful throughout the hottest summers. In this article, we’ll tell you what you need to know about greenhouse cooling – both for those planning to build a greenhouse, and for those whose greenhouses need some modifications to stay cool.

We’ve also got some good news about recent research that found a low-cost way to cool off the greenhouse when things get really hot. Greenhouse cooling falls into three broad categories: ventilation, shade, and evaporative cooling. Ventilation The first category, ventilation, is important when the weather outside is still in the optimum range for plant growth, below 85°-90°F. That includes spring and fall, but also morning and evening during the summer.

Proper ventilation allows the grower to keep the inside of the greenhouse as cool as the air outside, and it can include design features such as roll-up sidewalls, fans, endwall vents, roof vents, and motorized louvers. Ventilation is a basic design consideration when building a new greenhouse, and you should be able to get good advice from the person who sells you the greenhouse. You need to match ventilation equipment to the size of the structure in order to get adequate air exchange that will prevent air from slowing down and heating up inside the greenhouse. During the summer, a greenhouse gains 250 BTUs per square foot per hour, according to John Bartok, professor emeritus of agricultural engineering at the University of Connecticut. “It doesn’t take very long to build that heat up in the greenhouse,” he says. In his book, Greenhouse Engineering, Dr. Bartok recommends that you install fans to move 8 cubic feet per minute, per square foot of floor area. That should let the greenhouse get no more than seven degrees higher than the outside temperature. Greenhouse Engineering provides more specific data for determining what size fans and vents are needed to provide adequate air exchange for the configuration you’re planning, as well as other design considerations.

It is recommended for anyone investing in a new greenhouse, and is available from Growing for Market for $30 plus $3 shipping. (800-307-8949.) Shading As critical as ventilation is, it’s obviously no help once the outside temperatures get over 90°F. At that temperature, plants close their stomata and stop transpiration, and they can’t absorb enough moisture from their root systems. It’s particularly difficult for small plants in cells or pots – of heat-absorbing black plastic – to survive that kind of heat. The first line of defense against 90-plus temperatures is shade, and there are several options for shading the greenhouse. The cheapest is white latex paint, thinned with water at a ratio of one part paint to 10 parts water. This is useful only on poly that is at the end of its life span and was scheduled to be removed at the end of the season anyway. The paint can be sprayed or brushed on.

Greenhouse shading compounds are another cheap option. These paint-like materials are designed to wash off gradually over the course of the summer. There are several brand names, including Kool Ray Liquid Shade and Visor, and they can be purchased from greenhouse supply companies. (See resources at end of this article.) The greenhouse poly manufacturer Klerk’s sells a type of poly called Kool Lite Plus, which contains pigments that reduce infrared heat while allowing 85% transmission of PAR light, the part of the spectrum plants use. Klerk’s claims that Kool Lite Plus reduces heat 15 degrees when the temperature is 100°F. Kool Lite Plus costs about 13 cents a square foot, compared to about 7 cents a square foot for Klerk’s regular four-year poly. Shade cloth can be purchased custom-made to fit the greenhouse, with hems and grommets to insure a tight fit and long life. It’s available in various percentages of shade, and the percentage you choose depends on what you’re growing and how much sunlight you get. In general, vegetables and flowers do best with no more than 50% shade, and probably would prefer 30% shade, but in places with less cloud cover and high light intensity, you can go to 60% shade for cool-loving crops such as lettuce.

Daniel Willits, professor of Biological and Agricultural Engineering at North Carolina State University, recently measured the heat reduction from several types of shade cloth. He found that it doesn’t reduce heat as much as you would expect, based on the shade percentages. A 30% shade cloth reduced energy gain by 12% and a 50% shade cloth reduced energy gain by 22%. Shade cloth must be installed on the outside of the greenhouse, and most growers cover the roof and two-thirds of the way down the sides. Shade cloth used inside the greenhouse results in overheating because the light is still transmitted through the plastic, then captured by the black fabric. Shade cloth is available from farm and greenhouse suppliers. White and aluminum shade cloths are also available. Dr. Willits found that 40% white shade reduced leaf temperature at the exhaust end of the greenhouse by 4 degrees. Ted Dietrich of Green-Tec, a company that makes an aluminum shade cloth called Aluminet, says that black shade cloth is less effective because it absorbs heat. “The white and Aluminet, being more reflective, does not heat up as much, and contributes less heat to the greenhouse for the same amount of shading. There are no hard values correlating percent shade and degrees in the greenhouse, because the ventilation system has such an overriding effect on the greenhouse temperature (i.e., a good ventilation system can minimize the difference between black and white shade cloth). Dr. Dietrich adds that if you’re worried about your plants not getting enough light, you should select a lower-percent shade in white or aluminum coverings. Shade and mist In an effort to improve the somewhat disappointing cooling performance of the shade cloths, Dr. Willits tried misting them to see if the combination of evaporative cooling and shade made a difference. It did, at least with the black shade cloth. When the 50% black shade cloth was wet, it reduced energy gain by 43% and air temperature gain by 38%. Leaf temperature under the wet 50% shade cloth was reduced by 6 degrees. To keep the shade cloth wet, Dr. Willits used micro-irrigation sprinklers, the kind used in landscape irrigation, mounted in two PVC headers running the length of the greenhouse, one on either side of the ridge. The water was applied for very short periods of time, 30 seconds out of every 3 minutes. Misting black shade cloth was more effective than misting white shade cloth because the black holds more heat that can be evaporated, thus cooling the greenhouse.

Evaporative cooling
Evaporation is the key to successful cooling when big temperature reductions are required. Traditionally, greenhouses have been outfitted with evaporative cooling systems, sometimes called swamp coolers, that consist of cellulose pads through which water is pumped. Hot outside air is pulled by fans through the wet pads, and it is cooled by the evaporation of the water as it moves into the greenhouse. Evaporative cooling systems can reduce temperatures inside the greenhouse by 20 degrees. However, evaporative coolers are most efficient in dry climates, where the hot air is able to evaporate water. When humidity is high, the cooling effect is lessened because the air can’t hold much more water. Also, some greenhouse owners have experienced disease problems in the greenhouse because of too much humidity caused by the wet pads. They also require some maintenance to keep the water clean. The equipment and installation of an evaporative cooling system costs more than $1,000 for the average 96-foot greenhouse. Frank Arnosky, who writes the flower column in GFM, grows transplants all summer in a greenhouse outfitted with evaporative cooling pads. Frank cautions that the pads should be big enough to handle the volume of the greenhouse, and that they lose effectiveness in greenhouses longer than 100 feet. That’s just too far to move the cool air. Even in a 96-foot house, there can be a 10 degree difference between the air near the swamp cooler and the air at the other end. But that can be used to advantage, Frank says; he starts lettuce transplants in August at the cooler end of the greenhouse. Another option, not less expensive, but certainly more immediately useful, is the portable evaporative cooler. It contains the water unit and the fan in one big, moveable box that can be set up only when and where needed. A model big enough to cool a 3,000 square foot greenhouse 20 degrees costs about $2,200. The cheap option in evaporative cooling is to use mist or fog to cool the greenhouse. Some growers install hoses with mist nozzles right above the intake vents. Dr. Bartok said an even better way to have the same effect is to build a small lean-to shed, about 3 feet deep, outside the intake vents and run the mist line on the outside of the greenhouse, inside the shed. The shed prevents the mist from blowing away, and having the mist outside prevents too much excess water from dropping to the floor of the greenhouse, contributing to excessive humidity. Finally, Dr. Bartok recommends that you purchase the more expensive fog nozzles if using any of these ideas. Fog nozzles have an output of about one gallon per hour, compared to 5 gallons per hour for mist nozzles. Fog is more effective because the small size of the water drop allows more to be changed to vapor before it reaches the ground, resulting in greater cooling. The cheapest first aid for an overheated greenhouse is to regularly wet the floor and benches while running the fans to increase evaporative cooling. And finally, remember that a greenhouse full of plants will stay a lot cooler than a nearly empty one. An empty greenhouse can get up to 150°F on a hot day, when a full one won’t get above 100°F. That’s because all those plants are transpiring, releasing water vapor into the air. “That’s your natural evaporative cooling,” Dr. Bartok said. So use your greenhouse this summer. Fill it up with plants, and let it continue to make money for you. On the next page, you’ll find 10 suggestions for money-making summer greenhouse crops. Resources Greenhouse structures and equipment Stuppy (MO), 800-733-5025; www.stuppy.com Van Wingerden (NC), 828-891-7389; www.van-wingerden.com AgraTech (CA), 925-432-3399; www.agra-tech.com Rimol (NH), 877-746-6544 Nexus, 800-228-9639, www.nexuscorp.com Ventilation equipment/portable coolers Schaefer Ventilation, 800-779-3267, www.schaeferfan.com Shade Kool-Lite Plus – Klerk’s, 888-255-3757, klerksusa.com Shade cloth – G&M Agricultural Supply, 800-901-0096 Aluminet – Green-Tek, 800-747-6440, green-tek.com Kool Ray Liquid Shade – Continental Products, 800-305-5869 Visor shade compound – www.bucolor.com Mist and fog systems DripWorks, 800-522-3747; www.dripworks.com Harmony Farm Supply, 707-823-9125, www.harmonyfarm.com