When first setting up a small greenhouse, many gardeners do not take into consideration the heat which can build up in the greenhouse during the summer months. A nice, warm environment will generally promote healthy growth but once the temperature in a greenhouse rises above a certain threshold, it can cause serious damage. The heat that enters the greenhouse via the sun’s radiation gets trapped and can quickly build up if the grower has not taken some sort of precaution to combat it. There are many different ways a grower can effectively cool a greenhouse to maintain temperatures that promote robust growth. Some of these methods are simple and inexpensive while others are more complicated and costly. Learning more about some of the most common methods for cooling a greenhouse will help a horticulturist determine which, if any, the best fit for his or her situation is.
Perhaps the easiest way to make sure a greenhouse stays cool is to choose an advantageous location for the greenhouse. Many growers believe a south-facing greenhouse is the best option. Although a south-facing greenhouse will receive the most sunlight, it will also be exposed to the most heat. Many greenhouse experts actually recommend a west-facing greenhouse over a south-facing greenhouse. Just remember the saying, “west is best”. A west-facing greenhouse will still get plenty of sunlight for healthy plant growth and will automatically operate at cooler temperatures than a south-facing greenhouse in the same geographical location. A south-facing greenhouse has an increased likelihood of higher overall temperatures and hotspots which could damage plants. If a gardener does plan to have a south-facing greenhouse, he or she will most likely need to implement additional cooling techniques to maintain optimal temperature conditions.
Convection is the physical movement of a warm gas or liquid to a colder location. Some greenhouse growers rely on convection to help cool their greenhouses. This is typical of greenhouses in remote areas or greenhouses that do not have access to electricity. Strategically placed vents in a greenhouse will allow hot air to escape from the top of the greenhouse, while cool air replaces it, entering from the lower portion of the greenhouse. In many situations, convection alone will not be sufficient to maintain optimal temperatures in the summer months.
The most effective way to cool most greenhouses is with a powered fan. The purpose of a powered fan in a greenhouse is to pull fresh, cool air through the greenhouse (across the plants) and then exhaust the warm air outside of the greenhouse. Actively drawing cooler air from outside the greenhouse brings the temperature down within the greenhouse. A powered ventilation system will typically keep a greenhouse’s temperature about 10 degrees cooler than if the greenhouse is passively cooled (convection only). Before purchasing a powered fan, a gardener should “size” the fan for his or her particular garden space. Powered fans are rated by their CFM rating, which represents how many cubic feet of air the fan can ventilate per minute. Ideally, a greenhouse should have all of its air exchanged in 1-2 minutes. The simplest way gardeners can make their own calculations to determine the needed CFM is to multiply the length by the width by the wall height of the greenhouse.
Length x Width x Wall Height = Recommended CFM (cubic feet of air volume per minute). This calculation does not take into consideration the roof pitch so it is not an exact representation of the greenhouse’s cubic dimensions. However, this measurement is accurate enough to properly size a powered fan for a greenhouse. Once a grower has calculated the recommended CFM, he or she can find a fan that meets those criteria. For example, a greenhouse that is 30 feet long, 10 feet wide, and has a wall height of 10 feet will have a recommended CFM of 3,000 (30 x 10 x 10 = 3,000). The owner of this greenhouse should purchase a fan with a minimum of a 3,000 CFM rating.
Shade cloth is a fabric-like material that is placed next to the walls and ceiling of a greenhouse to help reduce the amount of the sun’s radiant energy from entering the greenhouse. Shade cloth has two distinct purposes: to protect plants from getting burned and to help keep the greenhouse cooler on hot, sunny days. Shade cloth can be made from a few different materials including aluminum alloys, polyester, or nylon. Shade cloth is sold by the percentage of shading that it provides. Higher or lower percentages are available but the shade cloths mainly used by horticulturists will shade 50%, 60%, or 70% of the sunlight. The higher the percentage shaded, the more protection and the cooler the greenhouse will be. Shade cloth is a great cooling option and in some cases may be the only cooling device required.
A popular cooling option among hobby greenhouse growers is portable evaporative coolers. Portable evaporative coolers are completely self-contained, movable, and relatively inexpensive. Portable evaporative coolers contain a wet pad that air is drawn over to create an evaporative cooling effect. Most portable evaporative coolers are equipped with a float valve and are connected to a water spigot to replace the water in the tank as the water evaporates. Portable evaporative coolers work best when they have access to fresh air so they should be placed near the greenhouse entry or a fresh air intake vent. Portable evaporative coolers are one of the best ways for home hobby greenhouse gardeners to maintain cooler temperatures during the hot summer months.
Wet wall systems are cooling devices that consist of cooling pads encased in aluminum housing. Wet wall systems are the most popular and efficient way to cool large, commercial greenhouses. The aluminum housing of a wet wall system appears similar to a honey comb. This “honey comb” design allows air to pass over the cooling pads which are kept wet with a water supply. The way a wet wall system works is very similar to the way a radiator cools an engine. The water in the pads cools the air as it passes through on its way into the greenhouse. Powered ventilation fans are placed on the wall opposite of the wet wall system. It is the powered fan(s) that act as the driving force for the air movement. Motorized shutters, which allow air to enter the greenhouse, are usually thermostatically controlled. In other words, they automatically open and close depending on the temperature. The main reasons why wet wall systems are used only by commercial growers are because they are expensive and more elaborate in their installation. The water used in a wet wall system is continuously recirculated and requires a fair amount of plumbing. All in all, wet wall systems are great for large commercial applications where their higher initial cost and more sophisticated installation can be justified.
Some small greenhouse growers will use humidifiers or foggers to aid in keeping the greenhouse cool. Humidifiers can be placed behind the greenhouse’s circulating fan so the cool, moist air is blown across the greenhouse for an evaporative cooling effect. A fogger is a device that emits water in a fine fog. Like the humidifier, fogger devices can be placed directly behind a circulating fan to blow the water-cooled air around the greenhouse. As the water evaporates, it creates a cooling effect. Please keep in mind that mist systems are not the same as humidifiers or foggers. Mist systems will actually cause water droplets to accumulate on the plants. This can cause a whole series of unwanted issues. Both foggers and humidifiers emit water droplets so small that the plants within the greenhouse will not get wet. Instead, the water quickly evaporates and cools the greenhouse’s climate in the process.
Most greenhouses require the use of some type of cooling method during the hot summer months. By implementing one or more cooling technique, a greenhouse gardener can ensure his or her plants will continue to perform and, most importantly, will not suffer significant damage that can occur from an overheated greenhouse. If you are thinking of setting up a greenhouse be sure to take into consideration how the location of the greenhouse will affect the atmospheric conditions. Shade cloth, powered fans, evaporative coolers, and/or humidifying devices are all effective techniques a hobby gardener can use to keep his or her greenhouse cooler. Implementing one or more of these options will help keep the plants within the greenhouse growing vigorously even during the peak heat of summer.
Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse contributing editor and may be contacted at Ehop@GardenAndGreenhouse.net.