October 2023 - 6

PRODUCTION
Photoperiod
MANAGEMENT
Flower induction of specialty cut flowers
BY ROBERTO LOPEZ
D
ue to consumer demand for local, unique, fresh and
high-quality cut flowers, domestic production of
specialty cut flowers continues to increase across
the country. Those interested in growing specialty cuts are
finding that the premium prices one can receive for stems
and pre-arranged bouquets at farmers markets, u-pick
and non-traditional florists, especially off season is quite
lucrative. However, there are several factors that influence
the flowering responses of cut flowers and growers must
consider how juvenility, photoperiod (day length), light
quantity and temperature influence off-season and yearround
production in greenhouses and high tunnels.
Over the past 10 years, our group has been conducting
specialty cut flower research in the three production
systems - field, high tunnels, and greenhouses - to:
1) determine the flower initiation and development
requirements of challenging crops and new introductions
and 2) compare production systems that results in high
yields and stem quality in the mid-western U.S.
In this article, we will focus our attention on photoperiod
and how it should be managed during the young plant (ie.
seedlings or rooting of cuttings) and production stages for
some of the most popular specialty cut flowers produced.
WHAT IS PHOTOPERIOD?
Photoperiod can influence many aspects of plant growth
and development including dormancy, storage organ
formation and, most importantly, flowering.
Many cut flowers, potted flowering, bedding plants and
herbaceous perennials flower in response to day length.
While we refer to photoperiod when discussing flowering
responses, it is actually the uninterrupted period of
darkness that controls plant responses to day length. The
natural dark period, or skotoperiod, can be extended (to
create a shorter day) or shortened by providing electric
light (to create a longer day) to manage photoperiodic plant
responses such as flowering. For most crops, a short day
and long day are perceived when the night length is at least
12 hours and less than 10 hours, respectively.
PHOTOPERIODIC RESPONSE GROUPS
There are three main photoperiodic response groups:
6 OCTOBER 2023 GPNMAG.COM
Figure 1. Stock Champion
'Lavender' grown under an
11-h short day (left) and
a 4-h night interruption
(right). Photo taken 15
weeks after the plants
were placed under their
respective photoperiods.
Photos by Roberto Lopez.
short-day plants (SDP), long-day plants (LDP)
and day-neutral plants (DNP). SDPs flower when the
daylength is at or shorter than a certain length specific
to each plant genera. For example, dahlia is classified as
SDP (Table 1). Whereas LDPs flower when the daylength is
longer than a certain length.
Day-neutral plants flower regardless of the daylength.
Furthermore, the LDP and SDP responses may be classified
as either obligate (qualitative) or facultative (quantitative).
An obligate photoperiod response means that the plant
must be exposed to SDs- or LDs to flower or it will
remain vegetative.
Alternatively, a facultative LDP or SDP will flower more
quickly when placed under LDs or SDs, respectively, though
flowering will eventually occur regardless of daylength as is
the case with stock (Figure 1). Photoperiodic response groups
of many common specialty cut flowers are listed in Table 1.
CRITICAL DAYLENGTH
How do you know how " long " or " short " a day needs to
be to induce flowering? Earlier we mentioned flowering
when photoperiods are above or below specific lengths for
LDP or SDPs, respectively. This specific daylength is often
called the critical daylength (CDL) and is the photoperiod
above- or below-which flowering occurs for LDP or SDP,
respectively. For African marigold an facultative SDP, the
CDL is around 12 hours and flowering occurs when the
photoperiod is 12 hours or shorter.
CREATING ARTIFICIAL SHORT DAYS
A SD can be created in the greenhouse by using an opaque
woven material such as black cloth or blackout cloth. Black
cloth can be pulled manually or with an automatic curtain
system over benches or an entire greenhouse section at a
specified time each day to truncate the natural day length.
The cloth is then retracted at a specified time in the morning
to expose plants to light. Materials used for black cloth are
generally synthetic black fabric with reflective outer surfaces
or plastic. Black plastic is not widely used due to the lack of
durability and potential for heat build-up and condensation,
which can cause the plastic to collapse on top of the plants but
may be useful when providing SDs to a small number of plants.
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October 2023

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