May 2023 - 10

technically speaking
BY ERIK RUNKLE
Lighting Plants Indoors,
Without Sunlight
Greenhouse production
takes advantage of sunlight,
which usually provides most,
if not all, the light to crops
grown inside. Greenhouse
growers, however, are
increasingly amending
sunlight with low-intensity
photoperiodic lighting
to create long day,
high-intensity lighting
to increase growth and
yield, or both. Despite the
benefits of greenhouse
lighting, there are still
daily and seasonal
fluctuations in light (and
other environmental
factors) that affect crop timing
and quality.
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.
With indoor production, the
only light source available
to plants is from electric
(artificial) light. While the
capital and operational lighting
expenditures are usually
higher for indoor production
facilities, there are numerous
potential benefits to growing
indoors. This article provides
an overview of some major
considerations when developing
an indoor lighting system.
Photoperiod, DLI and light
intensity. Among the first and
most critical steps with an
indoor lighting system is to
determine your target daily
light integral (DLI). The DLI
refers to the total quantity of
photosynthetic light received
each day and has a unit of
mol∙m-2
∙d-1
10
MAY 2023 GPNMAG.COM
, or more simply
Electric (artificial) lighting indoors enables complete control of the
photoperiod, daily light integral and light spectrum to produce consistent
crops year-round.
moles per day. The DLI is a
product of the light intensity
(the photosynthetic photon
flux density, or PPFD) and the
duration that the lights operate
(the photoperiod). The choice
of photoperiod depends on the
crop and whether photoperiod
regulates flowering or not.
A common photoperiod for
many crops produced indoors
is 18 to 20 hours, but shorter
photoperiods (i.e., 12 hours)
may be desired to promote
flowering of short-day plants
such as cannabis.
As an example, many young
plants can be grown under
long days and a moderate
DLI, so one could choose a
photoperiod of 18 hours and
a target DLI of 12 mol∙m-2
∙d-1
.
The required PPFD to achieve
this DLI is calculated as
12 (DLI) × 277.8 (a constant)
÷ 18 (hours) = 185 µmol∙m-2
That may be too much light
∙s-1
.
for some applications, such
as when weaning plants from
tissue culture, or too little
for other crop types such as
fruiting plants (strawberry,
tomato, etc.).
Light spectrum. Since the
only light available to plants
grown indoors is from electric
lighting, the choice of light
spectrum is critical. Today,
practically all indoor lighting
fixtures are composed of red,
white and/or blue light-emitting
diodes (LEDs), although
sometimes far-red LEDs
are also included. The light
spectrum regulates leaf size
and color, stem elongation, and
other plant quality attributes.
A desirable light spectrum is
situational and could vary from
one crop to the next, or for one
market to another. Therefore,
discuss spectrum options
with plant lighting providers,
fellow growers and university/
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May 2023

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