Maximum Yield Australia/New Zealand March/April 2019 | Page 34

Ventilation and Air Flow
“THE EXACT TEMPERATURES
YOU MAINTAIN DEPEND
ON THE CROPS YOU GROW
AND THEIR PREFERENCES.”
While shading is effective at reducing temperature, it
also reduces the amount of light reaching your plants by
50-60 per cent, which can inhibit growth rates. Sometimes,
however, this trade-off is necessary to ensure your plants’
survival. To create shading, you can purchase shade
cloth, paint-on materials, or adhesive compounds that
can be applied directly to the glass for a more permanent
solution. There are also roll-up screens made of wood,
aluminum, vinyl, or plastic that come with pulley and
rope systems to allow for adjustment based on light and
outside temperature. You can grow trees and plants next
to the greenhouse as a source of shade too. You can even
grow trees or trellis grapes inside your greenhouse to
supply shade to other plants. The large size of the grape
leaves have the additional benefit of acting as a natural
evaporative cooling system.
Evaporative cooling systems, also known as swamp
coolers, turn hot air into a cool breeze by passing air through
a soaked screen, pad, or spray of water. These systems are
most effective in environments with low humidity as they
add moisture to the air.
Fogging and misting systems are also often used to
cool greenhouses. These two systems are similar in that
they both spray water to cool the air. The main difference
being the size of the water droplets. Foggers create a
very fine mist, while misters spray larger water droplets.
These systems can be put on a controller to spray water
when temperatures get too high. While effective, the main
drawbacks to these types of systems are excess humidity
causing fungal issues within the greenhouse and clogged
emitters from water with high mineral content.
You can also control temperature by keeping a black
reservoir filled with water in your greenhouse. During the
day, the water will absorb heat. When it cools down at
night, heat is released and warms up the greenhouse. This
is the principle of thermal mass at work. You can even put
your tank to good use and grow fish while you’re at it. As a
rule of thumb, fill your reservoir with two to three gallons of
water per square foot of greenhouse space. Finally, if you
need to increase temperatures in the winter, you can use
heaters or even wood stoves.
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Maximum Yield
Air flow is essential for temperature control and fresh air
exchange. Without fresh air, there will not be enough carbon
dioxide for photosynthesis unless you install a CO 2 generator.
Even with a generator, you’ll need fans to move air within the
greenhouse to help prevent fungal disease.
Vents, exhaust fans, and oscillating fans can be used for
fresh air exchange and circulation. You can also install roof
vents that need to be opened and closed throughout the day
depending on the temperature. Roof vents are often operated
by hand, but you can automate them with a thermostat
controller to eliminate the manual work.
Exhaust fans can also be set on a timer so air is exchanged
as often as once per minute. If it is cold outside, you will want
to exchange the air less often, about a few times per hour. You
can also hook your fans up to a thermostat, so air is exchanged
when temperatures start to rise. Mount these fans high on the
end walls and make sure they have a protective hood so that
rain is not blown into the greenhouse.
Ceiling fans or oscillating fans keep air circulating
within the greenhouse. This will control humidity levels
and strengthen your plants’ stems, just as the wind
strengthens plants outdoors.
Finally, if you are going low-tech, you can install
screened doors on the ends of your greenhouse to create
cross-ventilation and prevent stagnant air.
Humidity
Humidity levels should be maintained between
70 and 85 per cent. When levels begin reaching
90 per cent and higher, growth slows, fungal diseases start
to occur, and early bolting becomes a problem. Keep in mind
it can be a challenge to measure humidity levels when air is
constantly circulating. Placing multiple hygrometers around
your greenhouse can help you get a more accurate reading.
If humidity gets too high, you can add a dehumidifier to
dry it out. You might also try pruning your plants. Leaves
perform a process called transpiration in which they release
moisture into the air from pores on their surface, so reducing
the foliage should help reduce humidity levels. Proper
ventilation and air circulation are also important keys to
managing humidity levels in a greenhouse. So, open doors
or vents to reduce humidity. Controlling your temperature
will also help to manage moisture levels as heat dries up
moisture. If you live in an arid environment and need to
increase humidity, you can do this by spraying the floor with
water or installing a misting system.
Greenhouses can make a wonderful addition to your
garden as long as they are managed properly. When you
control your temperature, air circulation, and humidity, you
can take advantage of extended growing seasons and eat
fresh year-round.