Photoperiodic Greenhouse Crops
Competencies 14.0016.00
Photoperiodism
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The response of different periods of day
and night in terms of plant growth and
maturity
Long day plants flower when days are
long and nights are short
Short day plants flower when days are
short and nights are long
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Chrysanthemums
Poinsettias
Chrysanthemums
Chrysanthemums
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Most popular cut flower sold in the U.S.
Popular as a potted plant for Mother’s Day,
Memorial Day, and Easter
Second only to poinsettias in dollar value for
potted flowers
Many different cultivars are grown
Planting Chrysanthemums
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Rooted cuttings are available from wholesale
growers
The number of plants to use depends on the
pot size
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4” pot has 2 plants
5” pot has 4 plants
6” pot has 5 plants
Chrysanthemum Temperatures
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During the first two
weeks
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Greenhouse temperature
should be 63 degrees F
at night and on cloudy
days
On sunny days the
temperature should be
70 degrees F
Chrysanthemum Temperatures

After the first two weeks
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Greenhouse temperature should be kept at 62
degrees F at night and on cloudy days
On sunny days temperature should be kept at 70
degrees F
Chrysanthemum Temperatures
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During the last three weeks
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Greenhouse temperatures should be kept at 55
degrees at night and on cloudy days
On sunny days greenhouse temperatures should
be kept at 65 degrees
Uneven bud set is caused by improper
temperatures
Pinching Chrysanthemums
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Pinching is done to make a more compact
plant with more flowers
Disbudding is done to control or reduce the
number of flowers per plant
Media and Fertilization
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Media must be free of
diseases and insects
Soil must be porous
and well drained
Regular fertilization
should be done with a
liquid 20-20-20 fertilizer
at 250 ppm at each
watering
Chrysanthemum Photoperiod
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Short-day plant
Bud set light period is 14 ½ hours or less of
light, and flower development takes place
when daylight is 13 ½ hours or less
Vegetative growth takes place when light
period is less than 12 ½ hours
Chrysanthemum Photoperiod
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From August to May 15, light is used during
the middle of the night to break the long dark
periods
From March 15-October 20, black cloth is
used to give about 12 hours of darkness from
about 5 p.m. to 7 a.m.
Poinsettias
Poinsettias
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Most important pot plant grown for the
Christmas season
Many different cultivars are grown
Planting Poinsettias


Wholesale growers supply 2 ¼” rooted
cuttings
Number of plants per pot depends on pot
size, whether plants will be pinched or grown
as a single stem, and size of flower
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4” pot has 1 plant for pinched or for single stem
6” pot has 2 plants for pinched or 3-4 for single
stem
8” pot has 3-4 plants for pinched and 6-7 for
single stem
Poinsettia Temperatures
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Greenhouse temperatures should be 65
degrees F at night and 70-75 degrees F
during the day
Some cultivars vary slightly
If night temperatures get above 80 degrees
F, flowering will be delayed or may not flower
at all
Pinching Poinsettias
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Pinching may be done
to increase the number
of bracts on the plant
and to make the plants
appear fuller.
Media and Fertilization

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Media must be free from insects and
diseases.
Soil must be porous and well-drained
Large amounts of fertilizer are needed from
propagation until sold.
Poinsettia Photoperiod
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Short-day plant
Bud set light period is from 12 hours to 12
hours, 15 minutes
Vegetative growth light period is longer than
12 hours
Two hours of light from about 11 p.m. to 1
a.m. each night will cause short day effect