Every plant for which flower initiation is light-dependent
(and not all plants are) has a critical day-length associated with it. The term long-day describes plants that begin forming flower
buds when the days are longer than their critical day length. The term short-day, on the other hand, describes plants that
begin flowering when the days are shorter than their critical day length.
Day-neutral plants form flowers independent of day length. Generally, long-day plants
flower in summer; short-day plants flower in spring or fall.
Here are some examples of light-dependent plants and their
approximate critical day lengths. There may, however, be cultivars of these plants which
have been bred to have shorter or longer critical day lengths, to meet market demand for
flowers at a certain timeor, in the case of spinach, to delay flowering to increase
the length of the harvest season. Also, in some cases other environmental factors such as
temperature can affect the time of flowering.
Two common long-day plants are dill and spinach.
Both these plants will initiate flowers when the day lengths are longer than their
particular critical day length:
Dill critical day length: 11 hrs.
Spinach (some types) critical day length: 13 hrs.
Two familiar short-day plants are chrysanthemum and
poinsettia. These plants will initiate flowers when the day lengths are shorter
than their critical day length:
Chrysanthemum (some types) critical day length: 15 hrs.
Poinsettia critical day length: 10 hrs.
Remember that the terms long-day and short-day dont
refer to the length of the critical day. Note that short-day chrysanthemums
have a critical day length that is longer than long-day spinach. The key to
the concept is that the terms long-day and short-day refer to whether the
plant will begin flowering when the days are longer or shorter than their
individual critical day lengths.
So, when the days (number of hours of daylight, that is)
are 14 hours long, both chrysanthemums and spinach will be stimulated to begin flowering.
Chrysanthemums will initiate flowers because the day length is shorter than their critical
day length of 15 hours; spinach will flower because the day length is longer than its
critical day length of 13 hours.
Other common long-day plants include foxglove, lettuce,
petunia, sedum, and hibiscus.
Familiar short-day plants include kalanchoe, onion, and
To make things even more interesting, some plants respond
to photoperiod when they are young, but at maturity are day-neutral!
Remember that not all plants are photoperiodic (initiate
flowers in response to day length). Some plants simply begin flowering once theyve
reached a certain age. These plants are termed day-neutral. Familiar day-neutral plants include
cucumber, tomato, pea, sunflowerand dandelion!
Eventually, scientists discovered that it was actually the
hours of uninterrupted darkness that triggered flowering, rather than the hours of
light. Experiments showed that even brief flashes of light during the dark period of the
cycle could interfere with flower development. Despite this new understanding of
photoperiodism, the terms long- and short-day, which refer to hours of light rather than
darkness, are still commonly used.
Poinsettias are short- day plants native to the mountains of Mexico. In their native
habitat, poinsettias begin initiating flowers when the days are approximately 14 hours
long (or, more accurately, when the nights are about 10 hours long.)
Poinsettias have become popular holiday gift
plants. After the holidays, the flowers drop and the colorful bracts fade back to green.
Gardeners who want to save their poinsettias hoping theyll flower and color up
during the next holiday season need to follow some special procedures, designed to mimic
conditions in the plants native habitat.
Starting in October, the plants should be placed in
a very dark room for 14 hours per day. Streetlight shining in the window, or even turning
on a lamp for a minute, can interfere with the requirement for darkness and delay
flowering. Some gardeners go to the trouble of covering the plants with an opaque bag, or
place them in a sealed closet to ensure adequate darkness.
During the other 10 hours in the day, the plant should receive bright, indirect
light. This procedure should result in colored bracts in time for the December holidays.