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Horticulture Lamps
Timing
Day and night
Photoperiodism
Short day (long night)
The perennial Chrysanthemum and the Poinsettia,
which flower in the autumn, are examples of
short-
day (long-night)
plants. They fail to flower when the
day length, or period of light , is extended beyond a
critical value.
Long day
Long-day
plants, such as the China Aster and
Tuberous Rooted Begonia, flower only with a day
length longer than a critical value.
Day neutral
Day-neutral
plants, such as the Rose and Carnation,
are not limited by photoperiod.
Understanding these principles enables commercial
growers to use artificial light profitably, so that
flowering and vegetable harvesting can be timed
for markets.
The relative length of day and night and the seasons is
important to plants. The number of hours of darkness in a
24-hour cycle is an important factor in determining blossoming
and growing time.
Night length triggers seed germination, tuber and bulb
formation, and other growth characteristics such as colour,
enlargement of leaves and stem size and shape. This rhythmic
characteristic is called photoperiodism and is of great value
to growers.
Plants can be classified according
to photoperiodicity.
Slow down
The
Perennial Chrysanthemum
is a short-day length plant
that will not flower when
the day is long (short-night).
To postpone flowering
Chrysanthemum growers,
instead of lengthening the
day, interrupt the night for
about four hours. This makes
the night appear short to
plants, which then continue to
grow vegetatively instead of
starting to flower.
A more economical method
of postponing flowering of
Chrysanthemums is to apply
cycles of light , switching light
on for 10 minutes and off for
50 minutes, for four hours
during the night , instead of
applying light continuously.
This is cyclic lighting. It is an
effective way of growing
flowers. If lighting levels are
higher then the grower will
see better stem and flower
quality and less opportunity
for disease.
Speed up
The
China Aster
is a typical
long-day (short-night) plant .
Long-day plants can be
brought to flower ahead of the
normal time by lengthening
the day. Relatively low
intensities of light are enough
to induce flowering, when
applied early in the morning or
at the end of the day. A dark-
period interruption - from a
few minutes to a few hours -
as with other long-day plants,
effectively induces flowering
just as it inhibits flowering of
short-day plants.
Poinsettias
must have
complete and continuous
darkness for about 12 hours
a day in order to flower. Even
1 minute of light in the middle
of the dark period will prevent
their flowering.
Tuberous Begonias
flower only
when daily dark periods are
short - less than 12 hours -
but they require long dark
periods for best production of
tubers. Flowering of tomatoes,
however, is not influenced by
photoperiod.