Air-conditioning has moved into the greenhouse. With summer heat pushing against the glass outside, spring flourishes in the mild climate within.
You can grow asters, snapdragons, stock, carnations, and many foliage plants that languish or get buggy in high temperatures. And you find yourself taking refuge in the cool, humid air—even in mid-August when work for the winter display in your greenhouse must begin.
In the Northeast, the temperatures of houses and office buildings are lowered by refrigeration. Still, in dry climates, this is done by evaporative cooling, which has proved best for greenhouses because it adds needed humidity to the air instead of drying it out.
The same type of cooling occurs naturally at the beach, where the temperature of a breeze passing over the ocean is lowered as it takes moisture.
You are much more comfortable in the cooled, rapidly moving, moisture-laden air on the shore than in the still, humid air inland.
Evaporative Cooler In The Greenhouse
In the greenhouse, an evaporative cooler duplicates beach conditions in this way: air absorbs moisture as it is drawn through wet rot-resistant excelsior by an electric fan.
The moisture vaporizes in the warmer greenhouse air, thereby lowering the temperature. In addition, the cool, humidified air is expelled through roof vents fast enough to create a breeze.
In hot, dry weather, plants lose water through their leaves faster than their roots can take it up—with harmful results. But when the temperature is lowered and the air humidified, transpiration does not occur quickly, and plants thrive.
An evaporative cooling system usually makes it possible to keep the temperature in the greenhouse equal to the temperature in the shade outdoors without the help of shade on the roof. However, you can often maintain a temperature several degrees cooler with some light shading material.
The cooling unit is relatively inexpensive. One large enough to cool a 14’x 17′ greenhouse costs around $158. It requires about the same electricity as an attic fan or sunlamp.
The water is recirculated by a pump so that only enough new water is needed to replace what is lost through evaporation and some that you can run to wash away undesirable salts.
How To Install A Cooler
To install a cooler, have a 2’ foot square opening in your greenhouse foundation and supply an electrical connection for the motor that drives the fan. A ¼” inch flexible copper tube furnishes water to moisten the excelsior.
The cooler can be manually controlled and run only during the day or operated on a humidistat or a thermostat (most often used).
When your greenhouse begins to cool off in the late afternoon as the sun sinks, the thermostat can either shut off the fan and the water circulator or shut off the water circulator and keep the fan running.
The kind of crops you grow will dictate which is best, and your greenhouse cooling engineer will be able to advise you.
The equipment can also serve as a humidifier in spring, fall, and winter when there is no need to cool the air.
At that time, move it into the greenhouse under a bench and operate it as a recirculator to draw in greenhouse air, blow it through the wet excelsior, and back into the greenhouse. This requires humidistat control for the fan and water pump.
The high relative humidity is maintained without undesirable condensation or drops of water on plants which result from other methods.
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