July 2024 - 58

TECHNICALLY SPEAKING | BY ERIK RUNKLE
Why crops don't flower (on time)
Many commercial greenhouse
growers of floriculture crops
are challenged with producing
hundreds, if not thousands, of
different species and cultivars.
This is incredibly challenging
because of their variation in plant
culture (fertility and pH), growth
habit and vigor, water demand,
temperature and light responses,
and factors that regulate
flowering. In addition, growers
need to have crops marketable -
often meaning in flower - for
predetermined dates.
Considering these challenges,
it's not uncommon for one crop
or another to not be in flower on
time. Sometimes the reason is
obvious, for example a crop was
transplanted late, but other times
it is elusive. This article discusses
some of the reasons why a crop
flowers late, or not at all.
Application of ethephon or
other PGR. The application of a
plant growth regulator (PGR), intentionally or not,
can delay flowering. Ethephon (e.g., Florel or Fresco) is
typically applied to inhibit extension growth, stimulate
branching and/or abort flowers. It's usually sprayed onto
young plants when flowering is not desired.
However, a late or inadvertent application can reset
the flowering clock and delay subsequent flowering
(Figure 1). Ethylene can also be released by faulty unit
heaters, which can abort flower buds and open flowers.
In addition to ethephon, a late application of a PGR -
especially the powerful growth retardants paclobutrazol
(e.g., Piccolo, Bonzi, or Paczol) and uniconazole (e.g.,
Sumagic or Concise) - can delay flowering. The potential
flowering delay is greater when applied as a foliar spray
directly to flower buds. Other types of PGRs, particularly
branching compounds, can also delay flowering.
Days are too short or too long. A large number of
ornamental crops flower in response to the day length,
or photoperiod. Many annual bedding plants and
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FEBRUARY 2024 GPNMAG.COM
Figure 1. Some
trays of lobelia were
inadvertently sprayed
with ethephon, which
delayed flowering
compared to the
young plants that
were not sprayed.
Photo courtesy of
Eric Runkle.
herbaceous perennials flower faster under long days
and short nights. Some common potted flowering plants
only flower, or flower earlier, under short days and long
nights. What constitutes a long or short day varies by
species and often cultivar and can be influenced by other
factors such as daily light integral.
If a photoperiod-sensitive crop is not in flower on time,
ensure lighting controls are set properly, the time on the
environmental control computer or timer is correct and
that fixtures and bulbs are working and delivering at least
1-2
µmol∙m-2
∙s-1 at plant height.
If blackout curtains are utilized to create short days,
check that they are closing completely. When some
varieties of the same species are in flower but others are
not, it's possible that the slower-flowering ones have a
different critical photoperiod than the others.
Low daily light integral. Many crops take longer to
flower under a low DLI (e.g., less than 8 mol∙m-2
than a higher one. This is one of the reasons why crop
∙d-1
)
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July 2024

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