September 2022 - 42

technically speaking
BY ERIK RUNKLE
Pros and Cons of Cool Nights
Temperature is the most manipulated environmental
parameter inside greenhouses and indoor farms. Temperature
controls the rate of plant development and thus determines
crop production time. It also regulates crop quality attributes
including stem elongation and,
for some crops, promotes or
inhibits flowering. Below are some
compelling reasons to deliver a
relatively low temperature at night,
but also some trade-offs that need
to be considered.
A low night
temperature:
Decreases respiration.
As discussed in my March
2021 article in GPN, plant
respiration uses sugars
(primarily generated
from photosynthesis)
as the energy source
for various maintenance
processes. The rate of
respiration (the rate of
sugar consumption)
generally increases with
Erik Runkle is
a professor and
floriculture
Extension specialist
in the department
of horticulture at
Michigan State
University. He can be
reached at runkleer@
msu.edu.
temperature. Therefore, during the
night, when most plants do not
perform photosynthesis, a lower
temperature decreases respiration.
Less respiration at night translates into a higher concentration
of sugars during the day, which can be used to support growth
processes such as the formation of new leaves, roots and flowers.
Avoids heat delay of flowering. Heat stress has a negative
effect on plant growth and flowering. Research has shown that
plants can tolerate heat stress much better during the day (in
the presence of light) than at night. What constitutes heat stress
therefore depends on the time it occurs as well as the crop.
One example of heat stress is the delay of flowering of some
greenhouse crops.
Short-day plants only flower, or flower earlier, when the night
length is longer than some species-specific duration, usually
more than 12 hours. Flowering of some short-day plants,
including chrysanthemum and poinsettia, is delayed when the
night temperature is high. What constitutes a high temperature
varies somewhat by species, but nights warmer than around
74° F can delay flowering of these two crops. Therefore,
providing relatively cool temperatures at night can prevent
flowering delays of sensitive short-day plants.
Lowers the greenhouse fuel bill. It is estimated that
around 75 to 80% of greenhouse heating occurs at night.
Decreasing the night temperature decreases the amount of
energy consumed for heating, even when the day temperature
42 SEPTEMBER 2022 GPNMAG.COM
There are advantages of lowering the temperature at night,
but also some trade-offs that need to be considered.
is increased to maintain the same average daily temperature.
The relative energy savings depends on location, time of year,
etc., but can be especially compelling in the fall and spring.
However, decreasing the night temperature without an offset to
the day temperature decreases the
average daily temperature, which
slows down plant development
and increases crop timing.
Therefore, if you lower the night
temperature without increasing
the day temperature, you will need
to add time to the crop schedule.
Creates a +DIF. Stem elongation
of many crops increases as the
difference (DIF) between the
day temperature is progressively
higher than the night temperature.
For example, a 72° F day and
63° F night temperature creates
a 9° F +DIF. Plants grown at this
+DIF will be taller at flower than
plants grown with less of a day/
night temperature differential,
especially compared to plants
grown at a day that is cooler than
the night. Thus, plants grown at a
+DIF might need higher rates of
plant growth retardants, or other
height-suppressing techniques, to
achieve target heights compared with plants grown at a lesser
DIF value.
Decreases the water vapor-pressure deficit (VPD). As
discussed in a December 2021 article, decreasing the air
temperature decreases the amount of water vapor it can hold,
which decreases the VPD (and increases the relative humidity).
It is critical that the air temperature does not decrease to the
dewpoint, which is when saturation occurs. When this happens,
the VPD is zero, water vapor condenses into liquid, and the
leaves become wet. This condition is favorable for the spread of
pathogens including bacteria and fungi. Generally, maintaining
a VPD of greater than 0.3 kPa is recommended for plant
production. This translates into a relative humidity of less than
85% at 63° F and 88% at 68° F.
In summary, lowering the night temperature decreases plant
respiration, prevents heat delay and decreases the amount of
energy consumed for greenhouse heating from fall through
spring. However, to offset a lower night temperature, the day
temperature needs to be increased, which creates a +DIF that
promotes stem elongation. Finally, consider the effects of
temperature on the amount of water vapor in the air to ensure
the vapor-pressure deficit is at least 0.3 kPa (relative humidity
is less than 88%).
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September 2022

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