December 2019 - 42

technically speaking
BY ERIK RUNKLE
How Much Supplemental Lighting Do You Need?
From October to March, the daily light integral (DLI) can
be a limiting factor in the production of greenhouse crops,
particularly in the northern half of the U.S. and in Canada.
Because the DLI is an accumulated value (the quantity of
light delivered per square meter each day), the only way to
increase it is to deliver a high intensity of light from electric
fixtures for extended periods of time, typically up to 18 to
20 hours per day. This
delivery of supplemental
light is used to accelerate
rooting of
seedlings and
cuttings, increase
yield of fruiting
crops, as well as
increase plant
quality attributes
such as branching,
stem thickness,
leaf color and
flower number.
Several
LIGHT
INTENSITY
8
50 1.44
70 2.02
90
2.59
110 3.17
130 3.74
150 4.32
170 4.90
190 5.47
210 6.05
10
1.80
2.52
3.24
3.96
4.68
5.40
6.12
6.84
7.56
HOURS OF LIGHTING PER DAY
12
14
16
2.16
3.02
3.89
4.75
5.62
6.48
7.34
8.21
9.07
factors need to
be considered
when choosing
a supplemental
lighting system, and arguably the most important one is
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.
to determine what light intensity you want to deliver. This article
provides steps on how to do that.
Step 1: Determine what the solar DLI is inside your
greenhouse from fall through spring. Many modern greenhouse
environmental control systems measure this value, although
sometimes this is measured outside, not inside. If it is measured
outside, then estimate the transmission percentage indoors
by measuring light intensity on a sunny day outdoors, then
immediately indoors, and calculate the percentage difference.
Many commercial greenhouses have a light transmission of 60 to
70%. The DLI value should be reported in mol·m-2
·d-1
, or moles
per day. If it is reported in a different way, work with your climate
control company, a lighting professional, or a university extension
educator to convert values into moles per day. Alternatively, the
DLI can be estimated by following steps in this article: https://bit.
ly/2r3yhWu.
Step 2: Determine your target minimum DLI. This is highly
situational and especially depends on the crop. Suggested
minimum DLI values for a wide range of greenhouse crops can be
found in this article: https://bit.ly/2C4LvnX.
Step 3: Determine the difference between your target DLI
and solar DLI when those crops are grown. For example, if the
average solar DLI in your greenhouse in January is 4.4 moles/
day, and if your target DLI is 10 moles/day, then your objective is
to deliver supplemental lighting that delivers 5.6 moles/day.
42 DECEMBER 2019 GPNMAG.COM
2.52
3.53
4.54
5.54
6.55
7.56
8.57
9.58
2.88
4.03
5.18
6.34
7.49
8.64
9.79
Table 1. The daily light integral delivered by electric lighting depends on
the instantaneous light intensity (in µmol·m-2
and duration used each day. The values in the table are in mol·m-2
moles/day.
Step 4: Once you know by how much you want to increase the
DLI, determine over what time period (hours per day) you can
accomplish that. There can be barriers to the number of lighting
hours each day, such as if the crop(s) has a photoperiodic
response, or if you have peak electricity rate periods that you
want to avoid. In addition, consider how many hours each day
you typically have direct sunlight from mid-morning to midafternoon.
The utility of
supplemental lighting is
diminished when it is sunny
outside, so you probably
don't want to plan to
operate the fixtures then.
Let's look at a few
examples. If you can
deliver long days to your
crops, you can light them
for 18 hours per day and
sometimes even longer. If
you don't have peak charge
rates for electricity, and
on an " average " day in
January you receive two
hours of direct sunlight
(from mid-morning to midafternoon),
then you can
18
3.24
4.54
5.83
7.13
8.42
9.72
10.58
10.94
12.10
11.02
12.31
13.61
20
3.60
5.04
6.48
7.92
9.36
10.80
12.24
13.68
15.12
·s-1) delivered at crop height
·d-1
, or
plan to deliver lighting for 16 hours per day. However, if you are
growing a crop that needs to remain under short days, such as
12 hours per day, and if on average you receive four hours of
mid-day sunlight, then you should plan to light for only eight
hours per day.
Step 5: Use Table 1 to determine the light intensity needed
from electric lamps to deliver your desired DLI considering how
many hours you can light each day. Using the examples above
and Table 1, if you want to deliver 5.6 moles/day from electric
lamps and can light for an average of 16 hours per day, then
your objective should be to deliver a supplemental lighting
intensity of around 100 µmol·m-2
·s-1. If you want to deliver 5.6
moles/day but can only operate fixtures for an average of eight
hours per day, then the installed supplemental lighting needs to
be about twice that.
Supplemental lighting is expensive to install and operate, but
it's one of the best methods to increase crop quality and yield.
While you want to deliver enough light to reach your production
objectives, you also don't want to install more than is necessary.
For many growers of ornamentals, an installed lighting intensity
is between 70 and 90 µmol·m-2
·s-1, but it is twice that (or
more) for fruiting vegetable crops such as tomato. Work with
professional plant lighting companies to help you evaluate what
type of lighting fixture (e.g., LED or high-pressure sodium) is
most economical considering the intensity of supplemental
lighting you want to deliver.
https://bit https://www.bit.ly/2C4LvnX http://www.GPNMAG.COM
December 2019

Table of Contents for the Digital Edition of December 2019
