February 2025 - 40

TECHNICALLY SPEAKING | BY ERIK RUNKLE
Getting the most out of energy
and shade curtains
Retractable energy and shade
curtains offer potential advantages
when producing crops inside
greenhouses. In the summer, a curtain
system can limit the greenhouse
temperature rise by decreasing the
solar load, which also decreases plant
water use. Curtains can also decrease
loss of heat, which is especially useful
on cold nights, and increase light
uniformity. However, these benefits are
not fully realized when a curtain system
is not well managed or maintained.
Curtains that primarily function
to retain heat, which are known as
energy or thermal curtains or screens,
utilize a closed-weave material,
meaning that air does not readily
flow through them. Curtains whose
primary function is to decrease
sunlight usually have an open-weave
construction so warm air is not
trapped below them.
Most growers in the U.S. with
curtain systems have one screen and
need to prioritize energy retention
or shading. In countries where fuel
is relatively expensive, two or three
interdependent curtain systems
are often employed. While multiple
screens are obviously more costly
to install, they enable a more finely
tuned environment and greater
energy savings.
In temperate locations, curtain
systems are positioned inside the
glazing material to avoid problems
with snow and provide an additional
layer of insulation. Below are some
considerations to successfully
manage an internal and retractable
40 FEBRUARY 2025 GPNMAG.COM
one-layer greenhouse curtain system.
Curtain deployment often depends on
multiple parameters, such as solar light
intensity, outside temperature and time.
SOLAR RADIATION
There are two primary reasons to
use curtains based on solar light levels.
The first is when propagating young
plants, especially unrooted cuttings
and tissue culture plantlets. From the
time they are stuck until they have
started to root and reacclimate, the
photosynthetic photon flux density
(PPFD) should generally not exceed
150 to 200 µmol∙m-2
∙s-1 at crop level,
depending on the species. Since peak
propagation of garden plants is in the
winter and spring, shade materials
with 40% to 50% light transmission is
suggested. A material that also scatters
the light is recommended to create a
more uniform light environment.
Ideally, light intensity should be
measured by quantum sensors placed
close to the plants, just like aspirated
boxes that measure greenhouse
temperature for environmental control
systems. However, most environmental
control systems measure light on
a weather station outside of the
greenhouse. Thus, growers need to
determine how much sunlight reaches
plants inside the greenhouse, which is
variable and highly situational. When
feasible, growers are encouraged
to add quantum sensors inside
greenhouse production areas.
Retractable curtains are also
useful during high sunlight levels
and high outdoor temperatures to
mitigate the temperature rise inside a
greenhouse, prevent excessive plant
temperatures and limit plant water
loss. In this situation, shade curtains
should only be deployed when solar
radiation inside the greenhouse is
high (e.g., > 500 µmol∙m-2
∙s-1
) and
the outside temperature is above the
desired greenhouse setpoint. For this
application, a slightly higher screen
shading factor is typically used, for
example 50% to 60%. Curtains
with even higher shade factors are
usually not recommended because the
combination of high temperature and
low light can lead to low-quality plants.
OUTSIDE TEMPERATURE
As noted above, deployment
of curtains also depends on the
temperature outside. To limit heat
loss, curtains should close when it's
dark and cold outside. For example,
when the outdoor temperature is
moderately cooler (e.g., 10° F) than
the greenhouse setpoint and when
there is little or no light from the sun
(e.g., <20 µmol∙m-2
∙s-1), the curtain
should be closed to save energy.
Shade screens with an open-material
design can still provide energy
savings, just not to the extent as a
closed energy screen.
Plant temperature is typically
warmer below a closed curtain
compared to plants exposed to a
cold glazing material above, even
at the same air temperature. This is
especially true at night. Thus, during
cold weather, cropping time can be
shorter when grown under a well
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February 2025

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