July 2020 - 42

technically speaking
BY ERIK RUNKLE
Indoor Propagation
We use greenhouses primarily to regulate the environment
inside, creating conditions suitable for plant growth most or all
of the year. However, greenhouses are still subject to weather
conditions and daily and seasonal changes in temperature, light,
and other environmental parameters.
Although we can temper some of these changes by cooling
and heating systems, supplemental lighting, shade curtains, etc.,
conditions are still not controlled in a predictable and consistent
manner. As a result, there are still differences in greenhouse crops
produced from one location to another, from one month to the next.
When grown
indoors, inside
refrigerated growth
rooms and without
sunlight, crops
can be propagated
uniformly and
predictably yearround.
Indoor
propagation is
certainly more
expensive than in
greenhouses, but it
can be economical
in some situations,
such as with highvalue
crops like
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.
young plants (plugs and
cuttings), and especially
for difficult-to-propagate
varieties. This article briefly
lists some of the important
considerations for an indoor propagation facility.
Lighting. Light-emitting diodes (LEDs) are the clear favorite for
indoor propagation because of their energy efficacy and range of
spectra available. Most young plants grow well under a daily light
integral of 10 to 12 mol·m-2
·d-1 and a photoperiod of 18 to 20
hours, which calculates to an instantaneous light intensity of
140 to 185 µmol·m-2
·s-1. (A lower intensity may be desirable in
some cases, such as when acclimating tissue-cultured plants
once out of the flask.) In addition to intensity, also consider
lighting uniformity, since that influences plant uniformity. Fixtures
that meet horticultural lighting specifications by the DesignLights
Consortium (www.designlights.org) are recommended because
they meet important technical performance requirements.
The " best " photon spectrum is situational, depending on
the crops and characteristics desired. Fixtures that include
white LEDs make it easier for us to see and interact with plants
(Figure 1). Elevated blue light can make leaves and stems more
compact, which can reduce or eliminate the need for plant
growth retardants.
A fixture that emits a modest intensity (10 to 20 µmol·m-2
·s-1
)
of far-red radiation can accelerate flowering of some crops,
especially long-day plants, but it can also increase extension
growth. For much more information on lighting for young-plant
production, visit www.endowment.org/series-on-leds.
HVAC. Indoor propagation requires a heating, ventilation
and air conditioning system to remove excess water vapor and
deliver the desired temperature setpoints. Work with an engineer
who has experience and
expertise in determining
the refrigeration capacity
needed considering the
heat generated by the
lighting fixtures as well as
water transpired by plants.
Engineers without sufficient
plant-based expertise often
under-estimate the demand
for dehumidification, which
can greatly limit production
capacity and/or create
unfavorable environments.
Design, layout and
Figure 1. Lighting is a major consideration when propagating plants
indoors. Here, a blend of white LEDs is used for seedlings grown at
Shenandoah Growers.
watering. One of the
biggest challenges is
designing an indoor growth
room that facilitates
access to plants and has
an effective and laborefficient
watering system.
Subsurface irrigation is usually preferred because leaves
don't get wet and much less water is lost to non-target areas,
which then needs to be removed by the HVAC system. Some
growers are using carts modified so that watering can be semiautomated
and recirculated.
Humidity and air movement. In some cases, a very high
humidity (near saturation) can be desired in propagation,
such as for callusing of cuttings or weaning of tissue-culture
plants. Rooted plants don't grow well at such a high humidity,
so a separate facility may be needed for these high-humidity
applications. Once plants have an initial root system, or for
seedlings, we have observed that an intermediate humidity
(60 to 75%) works well. Watering frequency increases when
the humidity is too low, and when humidity is too high, root
systems can be less developed and disease pressure increases.
In addition, as in a greenhouse, some air movement is desired to
create a more uniform aerial environment.
42 JULY 2020 GPNMAG.COM
http://www.endowment.org/series-on-leds http://www.designlights.org http://www.GPNMAG.COM
July 2020

Table of Contents for the Digital Edition of July 2020
