November 2020 - 42

technically speaking
BY ERIK RUNKLE
Effective Long-Day Lighting Fixtures
A large number of annual bedding plant and herbaceous
perennial species flower earlier when grown under long days.
A long day can be as few as 13 hours of light for some crops,
while at least 15 hours is needed for the most rapid flowering of
other crops. Other crops have a short-day flowering response,
and when the days are short, the delivery of long days is used to
inhibit flowering.
Therefore, greenhouse growers
who want to regulate flowering of
photoperiodic plants need to
deliver lighting from around
September through April. Only
a low intensity of light (1 or
2 µmol·m -2
·s-1) is needed to
create long days and thus,
photoperiodic lighting fixtures
are much less expensive to
purchase and operate than
those for supplemental, highintensity
lighting.
There are many different
kinds of lighting fixtures on
the market, but some are
more effective and suitable for
greenhouse applications than
others. This article describes
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.
the major considerations when selecting
a photoperiodic lighting system.
Tolerance to growing conditions.
General purpose light bulbs are those
developed for indoor use, such as homes and office buildings.
Most are not built to tolerate outdoor or greenhouse conditions,
where they can get wet, be exposed to high humidity and high
temperature, etc. In greenhouses, water droplets, water and
chemical vapor, and dust can enter general use bulbs, which
can reduce light intensity and lead to early failure. Therefore,
it is important to use bulbs that are built to withstand plant
growing conditions.
LEDs can have an ingress protection (IP) rating, which refers
to the level of protection from solid objects (including dust) and
liquids. The ratings are in the form of " IP-XY " , where X and Y are
usually numbers that refer to protection level from solids and
liquids, respectively. Bulbs are increasingly more tightly sealed as
the IP rating number increases. A bulb with a rating of IP-44 is
susceptible to the entry of dust but is protected by the splashing
of water from any direction.
This rating is subjectively the minimum level of protection
needed for products with greenhouse applications. A bulb with
an IP-65 rating is dust tight and even more protected against
intrusion of water, and will likely perform better in growing
Figure 1. Several factors influence the efficacy
of photoperiodic lighting fixtures, including
tolerance to greenhouse conditions and the
method, intensity, and spectrum of lighting.
environments than bulbs with a lower - or especially no -
IP rating.
LEDs can also be certified by Underwriters Laboratories (UL)
and other nationally recognized testing laboratory programs.
UL recently developed standards specifically for horticultural
lighting. Among the requirements to be a UL listed horticultural
lighting product is the ability to tolerate
damp and/or wet conditions and high
temperature, as well as have ingress
protection from dust. Therefore, a
UL-certified lighting fixture for horticulture
is another way to have confidence in its
performance in growing environments.
Lighting method. The most common way
to deliver photoperiodic lighting is using
screw-in bulbs at fixed intervals, usually
every 6 to 10 feet and at least 4 feet
above the crop (Figure 1). This traditional
" mum lighting " method is effective when
used with incandescent bulbs, but is less
effective on some long-day crops when
compact fluorescents are used. LEDs
are now the preferred bulb type because
of their longevity and energy efficacy,
although as discussed below, the spectrum
influences their efficacy.
There are two other types of effective
lighting methods to deliver long days, and
both are a type of cyclic, or intermittent
lighting. One type is to use a high-pressure
sodium fixture with an oscillating reflector, which moves a beam
of light back and forth to crops below. More information on this
strategy can be found online at www.bit.ly/3i16Pxd.
Another strategy is to install lighting fixtures on irrigation
booms, and operate them at night with the lights on and water
off. For more information on this approach, visit www.bit.
ly/3mSaEZd. Regardless of lighting method, operate fixtures for
at least four hours, either at the end of the day or the middle of
the night.
Light spectrum. The influence of the spectrum of photoperiodic
lighting is somewhat situational. With short-day plants, the
spectrum doesn't really matter, as long fixtures emit at least a
moderate percentage of red light.
With long-day plants, the spectrum matters when plants are
grown under a low average daily light integral (DLI), such as
less than 8 mol·m -2
·d-1. In particular, for some long-day crops,
flowering occurs most rapidly when low-intensity lighting emits
similar amounts of red and far-red light. When the DLI is higher,
the inclusion of far-red light has little to no effect on flowering of
long-day crops.
42 NOVEMBER 2020 GPNMAG.COM
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November 2020

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