October 2022 - 50

technically speaking
BY ERIK RUNKLE
Far-red Light in Greenhouse
and Indoor Farming
Far-red is a waveband of light that is marginally visible to
us but has profound effects on plant growth and development.
Far-red light is often described as radiation with a wavelength
between 700 and 800 nanometers (nm). However, the waveband
between 700 and 750 nm is of most interest because of its
potential to increase plant growth. As described below, far-red
increases both plant elongation and biomass accumulation.
By definition, photosynthetically active radiation (PAR)
is light with wavelengths from 400 to 700 nm. Thus,
far-red is outside of the PAR waveband. One way to
quantify far-red is by its percentage relative to PAR plus
far-red (i.e., 400 to 800 nm). About 25% of sunlight
within the 400 to 800 nm waveband is as far-red.
Incandescent lamps emit a lot of far-red (about 52%),
but those are increasingly obsolete. Other conventional
electric lighting fixtures emit relatively little far-red light,
such as high-pressure sodium (6%), metal halide (8%)
and cool-white fluorescent (2%).
Light-emitting diode (LED) fixtures usually emit very
little far-red light. Most horticultural LED fixtures emit
less than 2% as far-red and many (especially ones
with a purplish spectrum) emit less than 1% of light as
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.
far-red. The major exception is some LED products that have been
designed specifically to regulate flowering. These are all products
that screw into a socket and emit a low-light intensity. Examples
include the deep red/white/far-red GreenPower LED flowering
lamp from Philips, the Arize Greenhouse Pro LED lamp from GE
Current and the Pure Flowering 200 Lamp from TotalGrow.
Far-red increases extension growth. In contrast to PAR, leaves
do not absorb far-red light well. A majority of far-red light that
strikes a leaf is reflected back or transmits through the leaf.
However, this light then becomes available to other leaves and
plants nearby. Generally, increases in far-red light proportionately
increases extension growth. In particular, far-red light increases
internode elongation (the distance between each leaf on the
stem) and leaf length. It can also suppress branching, leading
to more pronounced growth on the primary stem. Many growers
know that plants " stretch " under incandescent lamps, and that's
because of their rich emission of far-red.
Far-red increases photosynthesis. A lot of research has been
performed in the past five years on the effects of far-red light on
plant growth. Far-red light indirectly increases plant growth by
increasing leaf size, enabling plants to capture more available
light than plants with smaller leaves. Studies have also shown
direct increases in the rate of photosynthesis, particularly with
wavelengths between 700 and 730 nm. In fact, many horticultural
scientists are suggesting that the definition of PAR be extended to
include far-red light. Stay tuned for more information on this topic.
Most high-intensity light-emitting diodes (LEDs) emit very little
far-red light, but there can be compelling reasons to include far-red
in a light spectrum.
Far-red sometimes promotes flowering. Studies performed
in the 1950s and 1960s showed that, in some cases, including
far-red light in day-extension or night-interruption lighting can
accelerate flowering of some species. The effects of far-red light
on flowering have become clearer with the use of far-red LEDs in
controlled lighting experiments. In general:
* Across a wide range of species and growing conditions,
lighting fixtures used at night that emit both red and far-red
light are the most effective at promoting flowering of
long-day plants.
* Far-red light alone is not effective at regulating flowering of
short-day or long-day plants.
* Adding far-red to low-intensity red light at night has no
significant effect on inhibiting flowering of short-day plants.
* The promotive effect of far-red light on flowering of
long-day plants decreases as the daily light integral (DLI)
increases. Once the average DLI is at least 12 mol∙m-2
∙d-1
there is usually little to no benefit of far-red in a lowintensity
light spectrum.
* Far-red has no consistent effect on flowering of day-neutral
plants.
There are compelling reasons to have some far-red in a light
spectrum, especially when crops are grown indoors, without
sunlight. Far-red light typically increases growth both directly
and indirectly and, in some cases, accelerates flowering.
However, crops grown with far-red typically have larger leaves
and longer stems, making plants less compact. Whether a taller
crop is desirable or not can depend on the species, container
size and market.
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50 OCTOBER 2022 GPNMAG.COM
http://www.GPNMAG.COM
October 2022

Table of Contents for the Digital Edition of October 2022
