July 2024 - 59

TECHNICALLY SPEAKING
Flower induction
can be incomplete
or delayed if the
duration of cooling
is too short, the
temperature is too
high or both.
production time is often shorter late in the spring
compared with early in the spring.
When crops are to be marketed in flower when the
DLI is low, maximize transmission of sunlight not
only through your greenhouse structure but to your
crops. Placing hanging baskets overhead can delay
flowering time of crops below when the DLI is not
sufficiently high.
Light pollution. For short-day plants, flowering can
be delayed or prevented when light comes through
gaps in blackout curtains or from a nearby lighted
area. Plants are very sensitive to low-light intensities,
and less than 1 µmol∙m-2
∙s-1 (the lowest reading by
a quantum sensor) is sufficiently bright to delay
flowering in many cases.
When light pollution is suspected even when a
quantum sensor measurement reads zero, consider
obtaining a quantum light pollution sensor by Apogee
Instruments. This sensor measures light in nanomoles
per square meter and second (nmol∙m-2
∙s-1
), which is
one thousandth of a micromole. The light perception
threshold has not been determined for most crops,
but some are sensitive to 50 nmol∙m-2
∙s-1
∙s-1
µmol∙m-2
) of light.
Excessively high temperature. A high temperature
can delay flowering of a wide variety of crops. What
constitutes a high temperature depends on the crop
and can be as low as 73° F for some potted flowering
plants or cold-tolerant annual bedding plants.
For some crops, this " heat delay " can occur
specifically during the day or night. For example,
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GREENHOUSE PRODUCT NEWS 59
(0.05
a night (but not day) temperature above 73° F delays
flowering of poinsettia. The magnitude of the delay
increases as temperature and duration increase.
Insufficient vernalization. Many bulb crops (e.g.,
Easter lily, tulip and daffodil) and herbaceous perennials
require, or flower earlier, following a vernalization
treatment. The vernalization process is complete
following an extended period (5-12 weeks) of low
temperature treatment (35° F to 55° F, depending on the
crop). Flower induction can be incomplete or delayed if
the duration of cooling is too short, the temperature is
too high or both.
To conclude, there are several potential explanations
for why a crop is not in flower on time, and some of the
major ones are listed here. Knowing the history of the
crop is sometimes needed to determine the reason(s)
why a crop is not in flower on time.
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 commercial
products or trade names does not imply endorsement or bias
against those not mentioned.
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July 2024

Table of Contents for the Digital Edition of July 2024
