May 2024 - 60

TECHNICALLY SPEAKING | BY ERIK RUNKLE
Controlling greenhouse temperature
for successful production
The single greatest reason we grow crops inside
greenhouses is to regulate the temperature. In addition,
the primary determinant of crop timing is the average
temperature. Despite this, it can be easy to overlook
temperature and focus on greenhouse crop production
challenges, especially ones that create obvious plant
symptoms. In this article, I discuss the fundamentals of
greenhouse temperature and why its management is so
critical to success.
Temperature controls crop production time. All plants
have the same general response to temperature: they
develop faster at high temperatures and slower at low
temperatures. What does vary from one crop to another
is what constitutes a high or low temperature. We have
categorized crops based on research that has estimated the
base temperature for each. The base temperature is the low
temperature below which plants stop developing. " Coldtolerant
plants " are those with base temperatures of 39° F
or lower and " cold-sensitive plants " are those with base
temperatures of 46° F or higher.
Seemingly small differences in base temperature can have
surprisingly large effects on crop production time (Table 1).
For example, lowering the average daily temperature from
68° F to 62° F delayed time to flower of the cold-sensitive
crop angelonia by 16 days, but the delay in flowering of the
cold-tolerant crop snapdragon was only five days. These
values will depend on the maturity of the plug at transplant
as well as average daily light integral, but the relative effect
of temperature on flowering does not change.
AVERAGE AIR TEMPERATURE
°F
62
64
66
68
70
72
74
°C
16.7
17.8
18.9
20.0
21.1
22.2
23.3
For more information on the effects of temperature on
flowering time for a wide range of crops, download the
FlowersOnTime grower tool under the " Grower resources "
tab at floriculturealliance.org. Production time information
specific to Wave petunia is online at panamseed.com/
northamerica/brands/wave/smartschedule.
Temperature influences crop heating costs. When it's
cold outside, energy consumption for heating increases as
the greenhouse temperature setpoint increases. It can be
tempting to lower temperature setpoints to decrease energy
costs on a daily basis, but that also delays flowering, which
can be counterproductive.
The online tools listed above can be used to gauge
the effect of decreasing (or increasing) temperature on
flowering time. Virtual Grower, available at virtualgrower.
net, can estimate the effect of changing the temperature on
greenhouse heating costs, among many other uses. It is a
powerful tool that can help growers make more informed
decisions on how to manage temperature based on energy
consumption as well as crop timing.
Temperature influences relative humidity. The
temperature of the air influences the amount of water vapor
it can hold. Specifically, the water-holding capacity of air
decreases with temperature, which means that relative
humidity goes up as temperature goes down. Except during
propagation, a high humidity (less than 80-85% relative
humidity or greater than 0.5 kPa vapor-pressure deficit)
should generally be avoided, especially at night. When
making temperature decisions, don't ignore its effect on
COLD-TOLERANT CROP
Snapdragon 'Montego'
(Tb
=36° F)
34
32
30
29
28
27
26
COLD-INTERMEDIATE CROP
Impatiens 'Accent'
(Tb
=43° F)
52
47
43
40
37
35
32
COLD-SENSITIVE CROP
Angelonia 'Serena Purple'
(Tb
=50° F)
59
52
47
43
40
38
36
The effect of air temperature on days from plug transplant to first flowering with different base temperatures (Tb
daily light integral (e.g. 10 mol∙m-2
60
MAY 2024 GPNMAG.COM
) under an intermediate
∙d-1). The three crops represent each temperature-response category. Data courtesy of Erik Runkle.

http://www.floriculturealliance.org http://www.panamseed.com/ http://www.GPNMAG.COM

May 2024

Table of Contents for the Digital Edition of May 2024