I
n photoperiodistic plants, specialized
photoreceptors called phytochromes
keep track of how long the plants have
been in the dark. Phytochromes switch
back and forth between two states. In one
state, the phytochrome is sensitive to red
light (Pr), and in the other it is sensitive
to far red light (Pfr). In the case of short
day plants, far red light absorbing phyto-
chromes will slowly switch to being sensi-
tive to red light when in darkness. The
ratio of Pr:Pfr will indicate how long the
plant has been in the dark. If a lot of the
far red light has converted into red light,
then the plant has been in uninterrupted
darkness for a long time and will tri gger
flower development.
However, these changed phytochromes
will revert back to the far red light
state in the presence of light. While
the conversion of far red light to red
light in darkness takes many hours, the
switch back from red light to far red light
happens in a matter of minutes. Even a
brief interruption of the dark period is
all that is needed to raise far red light
levels high enough to interrupt the flower
trigger in long night (short day) plants.
OUTDOORS VS. IND o ORS
When using the natural outdoor seasons
without artificial lighting, there is little
effective difference between thinking
“short day” and “long night”. In outdoor
gardens, spring, fall, and winter all have
longer nights than the short nights of
summer. Short day/long night plants
bulk up during the growth stage during
summer, until the longer nights (not the
shorter days) of fall trigger flowering. If
these plants are set outside too early in the
spring when the nights are still long, they
can take the cue from the long nights to
start flowering (in mild enough climates,
this can allow for a spring harvest as well).
Part of the reason for the impor-
tance of the solstices and equinoxes
to outdoor gardens is that the solstices
mark the longest and shortest days
(nights) of the year, and at the equi-
noxes, day and night are approxi-
mately the same length (12 hours each).
Indoors, lighting schedules are at the
pleasure of the gardener. To keep a short
day (long night) plant in growth, timers
can be set to allow only a few hours of
darkness, or even none at all. While the
usual schedule for such plants includes
a long-lit period followed by zero to six
hours of darkness, any combination of
light and dark can be used as long as
there isn’t a long (12-ish hour) continu-
ous dark period. It would even be possi-
ble to keep a short day plant in growth
with only four hours of light a day (one
out of every five hours), but the plants
wouldn’t get very big on only four hours
of light a day. On the other side of the
spectrum, 24 hours of light can be used,
although any dissipation problems can
be exacerbated by leaving the lights on
continuously. Eighteen hours of light is a
commonly used compromise.
When the indoor gardener decides it
is time to initiate flowering of short day
plants, the dark periods are increased
to 12 hours or so. Electric timers can be
helpful in turning off lights (and back
on again) at the appropriate times since
remembering to do it every day by hand
can be tedious and prone to errors,
though either method will work if
done with regularity.
By triggering flowering earlier, harvests may
be moved earlier in the season, potentially
avoiding harsh fall weather and allowing for
flowering under the brighter summer sun.”
GREENHOUSES
Greenhouses can use a similar tactic
with the use of blackout covers. Nights
are artificially lengthened by using
an opaque cover to block the sun for
enough additional hours to cause
flowering. By triggering flowering
earlier, harvests may be moved earlier
in the season, potentially avoiding
harsh fall weather and allowing for
flowering under the brighter summer
sun. Conversely, artificial lights in an
otherwise naturally lit greenhouse can
be used to break up the long nights to
maintain vegetative growth during the
otherwise flower-inducing fall, winter,
and spring months.
Understanding the rules under which
photoperiod dependent plants flower
can allow a gardener to customize
lighting schedules to fit a given need
while still maintaining control of the
timing of flowering.
grow cycle
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