August 2019 - 50

technically speaking
BY ERIK RUNKLE
Managing Light Pollution
Greenhouse owners and operators are increasingly adding
supplemental lighting to increase plant growth. The virtues
of high-intensity lighting have been known for decades, but
installations are increasingly common as developments in
conventional and LED lighting fixtures advance. While there
are many compelling reasons to install lighting fixtures in
greenhouses, there are sometimes unintended consequences if
the light reaches non-targeted areas.
Light pollution can be considered electric lighting that has
unintended consequences to neighboring areas at
night. These areas could be other greenhouse sections,
or nearby homes or communities when neighbors
consider stray light to be undesirable. Light pollution
can come from fixtures designed for people (such as
new or brighter street lighting) or those designed for
horticultural applications. And, as a consequence of
increasing urban sprawl, some greenhouses that were
once located in rural areas now have homes nearby.
Human centric light pollution. The Netherlands,
which has both high densities of people and
greenhouses, has been dealing with greenhouse light
pollution for many years. There, greenhouse lighting
installations are at an especially high intensity because
of the types of crops grown (fruiting vegetables and
cut flowers, for example) and the low solar daily light
integral during the winter. Ordinances have been
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.
developed in which as much as 95 to 98% of the electric
lighting must be contained within a greenhouse facility.
There is only one practical way to prevent electric lighting
from coming through a greenhouse, and that's by using
retractable blackout screens and curtains along the sides
and roof. These can be expensive to install, but are typically
much less expensive when designed into a new structure
rather than added to an existing one. Therefore, when building
new greenhouses, greenhouse growers and operators are
encouraged to consider the installation of retractable curtains.
If there is not currently a compelling reason to include curtains
when a greenhouse is constructed, keep the idea in mind when
designing the greenhouse so that if curtains are needed later, a
retrofit will be easier and less costly.
In addition to cost, there are other considerations to
installing a blackout curtain. First, a blackout curtain can serve
as an excellent energy curtain, substantially reducing heat
loss at night during the winter. Also, consider the possible
implementation of state or local ordinances that could
Figure 1. The effect of light intensity from high-pressure sodium
fixtures, during an otherwise long night, on flowering of the
long-day plant campanula. Intensities of 0.1 µmol·m-2
·s-1
(and sometimes even lower) from adjacent greenhouses
can be sufficiently bright to influence flowering. (Photo: Cathy
Whitman and Royal Heins, Michigan State University)
Within a population of seedlings, there is often variability in
the sensitivity of plants to very low intensities of stray light.
When a population of plants is exposed to intensities (or
durations) of light that are not saturating for flowering, some
plants may be induced to flower over time while others may
stay vegetative. The result is a population of plants that is
induced to flower in varying degrees - some may be in flower,
some have initial signs of flowering, and some completely
vegetative.
force you to either install a blackout system or limit use of
supplemental lighting at night. Restrictions on supplemental
lighting translates into fewer hours of use, which reduces crop
yield, quality, or both.
Plant centric light pollution. Stray light can also have
unintended consequences on plants grown nearby. Many
ornamental crops have a photoperiodic flowering response,
and the night length is often managed to prevent or induce
flowering, whichever is desired. If supplemental lighting is
added to a greenhouse section, it can be sufficiently bright
to reach neighboring sections to modulate the photoperiod.
Some plants, such as poinsettia, coreopsis, echinacea and
campanula, are sensitive to extremely low light intensities,
such as 0.1 µmol∙m-2
light source) (Figure 1).
Campanula carpatica 'Blue Clips'
9-hour natural day with 7-hour day-extension lighting
provided by high-pressure sodium fixtures at (µmol·m-2
0.0
0.05
0.10
1.0
·s-1
):
2.0
·s-1 (about 0.5 footcandles from a white
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August 2019

Table of Contents for the Digital Edition of August 2019
