November December 2023 - 14

LIGHTING
Similarly, knowing the root zone temperature can inform
about plant status and, if controllable, can be used for
optimal control at minimal costs. There is a large variety of
equipment and approaches available for controlling the key
greenhouse temperatures. The best solutions depend on
crop requirements, cost, reliability, grower preferences and
maintenance requirements.
HUMIDITY CONTROL
Humidity control of the greenhouse air is closely tied
to air temperature control. Warmer air can hold more
moisture (water vapor), causing the relative humidity to
go down (assuming no additional water is evaporated into
the air as it warms). Like temperature, plants perform best
when the humidity of the surrounding air is maintained
within an optimal range.
When the surrounding air is too dry, plants lose more
moisture to the point that they begin to wilt. High humidity
levels make it more difficult for plants to
transpire (reducing water and nutrient
movement from the roots to the shoots and
limiting the plant's ability to cool itself) and
can increase the risk of fungal diseases.
Adding more moisture to the greenhouse
air is relatively straightforward - a misting
system is effective and relatively low-cost.
Removing moisture from the greenhouse
environment can be done by replacing
humid air with drier air (e.g., through
ventilation) or by removing the moisture
through a cooling process (colder air can
hold less moisture so it will condense out
during the cooling process). The latter can
be expensive when technology such as
mechanical refrigeration is involved.
While traditional humidity control has focused on
maintaining specific relative humidity values, from the
plant's perspective it makes more sense to focus on the
control of the vapor pressure deficit (VPD). VPD is the
difference between the saturated vapor pressure of the air
inside the stomatal cavities (i.e., at the canopy temperature)
and the partial vapor pressure of the surrounding air
(which can be calculated using the temperature and relative
humidity of that air). It's the VPD that determines that
plant's ability to move water vapor to the surrounding air
and is therefore a better indicator of plant status than the
relative humidity of the surrounding air.
LIGHT CONTROL
Solar radiation (consisting of ultraviolet, visible and
infrared radiation) and supplemental lighting (if used)
provide the energy source for photosynthesis. We use
the instantaneous intensity and the daily light integral
(DLI) across the
Photosynthetically Active
Radiation (PAR) waveband
to quantify the impact of
light on plant growth and
development.
Supplemental lighting
A greenhouse
equipped with
shade and energy
curtains as well as
supplemental lighting
to control the amount
of light reaching the
plant canopy.
requires electricity and can
be expensive due to the
large number of fixtures
needed to deliver an
adequate intensity and/or
DLI. As a result, accurately
controlling the amount
of light needed is key to
maintaining optimum plant
growth and development,
while minimizing electricity
costs. There are times
of the day/year when
A misting system is used to increase the greenhouse humidity,
thus promoting the root growth of these geranium cuttings.
14 NOVEMBER/DECEMBER 2023 GPNMAG.COM
greenhouse crops receive too much solar radiation,
requiring the use of shade curtains. A sophisticated light
control system relies on measurements of the light reaching
the crop canopy in order to operate the supplemental
lighting and shading system.
Depending on the light fixtures installed, the intensity
and/or spectral output can be adjusted based on the
desired light targets. The latter is only practical when
light-emitting diode (LED) fixtures are used. The newest
LED fixtures are more energy efficient compared to the
traditional high-intensity discharge (metal halide and
high-pressure sodium) fixtures but are more expensive
to buy. The (waste) heat LED fixtures produce is also a
little easier to deal with than the radiant heat produced
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November December 2023

Table of Contents for the Digital Edition of November December 2023
