similar to those needed by the plants.
Heated rooms may be a little low in
humidity, but the moisture level in the
micro-environment surrounding the plants
is usually higher. This is caused by
evaporation of water from the medium and
by plant transpiration.
Since hot air rises and cool air sinks, a
fan placed above the plants pulls out the
heated air. Squirrel fans and other
ventilation fans make these setups a snap.
Experienced gardeners choose fans with
the capacity to move the room’s cubic area
every 10 minutes. As an example a fan in
200 cubic foot grow space moved 20 cubic
feet per minute.
Increasing the rate of air change using
a fan has beneficial effects besides
controlling temperature and humidity. A
breeze which causes some movement of
the stem increases its strength. When a
plant moves in the wind, small tears
develop in the tissues. The plant quickly
grows new tissue, thickening and
strengthening the stem. A breeze also
increases the amount of CO 2 available to
the plant. This is covered in depth in
chapter 17 - CO 2.
Sensible growers know that open
windows are not as good a solution as fans
for several reasons. They present a new
problem regarding detection, both by light
and odor, and plant pests living outside
might use the passageway to find new
indoor feeding grounds.
Some growers use a closed system.
The air is cooled using an air conditioner,
the humidity is lowered using a dehumidifier