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