January 2023 - 12

technically speaking
BY ERIK RUNKLE
Several Consequences
of Growing Too Cool
An increase in natural gas
exports, geopolitical events
and strong demand have led to
a considerable increase in
fuel prices in the last two
years. Unfortunately, this
has made it substantially
more expensive to heat
greenhouses. Energy
consumption for heating
depends on several
factors; obviously, one of
them is the greenhouse
temperature setpoint. To
decrease energy costs,
it is tempting to lower
heating settings, but this
has several consequences
to the crops growing inside.
Slower rooting during
Erik Runkle is
professor and
floriculture
Extension specialist
in the department
of horticulture at
Michigan State
University. He
can be reached at
runkleer@msu.
edu. Reference
to companies,
commercial products
or trade names
does not imply
endorsement or bias
against those not
mentioned.
propagation. Whether you are
rooting cuttings or growing
seedlings, root development
is primarily determined by the
temperature of the root zone
and the daily light integral
(DLI). The air temperature
can also influence rooting;
active shoot growth typically
promotes root growth.
Therefore, maintaining both
desirable air and root-zone
temperatures, as well as
delivering sufficient light (at
least once root initials have
formed), generally leads to the
quickest production of plugs
and liners.
Recommended air and rootzone
temperatures can depend
on the crop and, in some cases,
the average DLI. In the absence
of crop-specific information, a
12
JANUARY 2023 GPNMAG.COM
root-zone temperature of 73 to
77° F and a slightly cooler air
temperature are recommended.
However, there are some crops
that root faster at a higher
air temperature, particularly
those that are cold sensitive.
For example, rooting of the
cold-sensitive crop thunbergia
was much more rapid at an
air temperature of 70° F than
at 60° F, regardless of the
root-zone heating temperature
(Figure 1).
Longer crop production
time. The rate of plant
development increases linearly
with temperature, until some
crop-specific maximum value.
Thus, it takes longer for a plant
to become marketable - such
as to fill its container or to
flower - as the average daily
temperature is decreased.
The magnitude of the delay
depends on the crop, and
is much greater for coldsensitive
crops such as celosia,
thunbergia, and vinca than for
cold-tolerant crops such as
osteospermum, snapdragon,
and marigold.
Although counter-intuitive,
in some situations lowering
the temperature can actually
increase heating costs on
a per-crop basis. Over the
years, we have done many
heating cost simulations using
the Virtual Grower computer
software developed by the
USDA-ARS. We estimated time
to flower and heating costs to
produce various bedding plant
crops at different temperatures
in different locations in the
Thunbergia 'Sunny Lemon Star'
Root-zone temperature (°F):
70
75
80
Figure 1. Rooting of the cold-sensitive crop thunbergia was more rapid at
the warmer air temperature. Photo courtesy of Annika Kohler and Roberto
Lopez, Michigan State University.
Air temperature (°F):
70
60
http://www.GPNMAG.COM
January 2023

Table of Contents for the Digital Edition of January 2023
