Timepieces in our Plants

Have you ever wondered how plants tell time?  In order for plants to flower at about the same time every year, go dormant for winter and resume growth in the spring, it is vital for a plant to know the time of year. Without this awareness, plants could get caught fully leafed out, actively growing and too tender to survive the cold. As the days get shorter in the fall, plants get the cue from the change in lighting that they need to begin winter preparations.

This awareness of the time of year results from the plants ability to track daylength or photoperiod and is known as photoperiodism. Plants are grouped roughly as long-day, short-day and day neutral based on their response to photoperiod. Photoperiod is defined as the time that plants are exposed to light during a 24 hour period. A 10 hour photoperiod has 10 hours of light and 14 hours of darkness and would naturally occur twice each year, once in the late winter in early February and again in the Fall at the beginning of November. It’s interesting enough just to know that plants run on a 24 hour cycle. Many plants will go into a night cycle with the lights fully on. Think of these plants as jet lagged.

Japanese Anenome is a late season bloomer.

Long day plants create blossoms as the days get longer. Therefore, these plants set their flower buds as soon as the days get long enough in the spring. Most sunflowers are long day plants, so they bloom in the summer. Other common long-day plants include: Petunia, Snapdragon, Black-eyed Susan, Shasta Daisy, Purple coneflower, African marigold, & Hosta.

Here’s a great demonstration of photoperiod:  Plant nearly any common sunflower in the late summer after about mid-July. They will grow to a very short height and make very tiny blossoms and do it all very quickly as compared to the same seed planted at the end of May.  Since the daylength in mid-late summer is correct for flower development, the small young seedlings produce flowers almost immediately, compared to the plant in the spring where weeks of vegetative growth result in a much larger plant at flowering. To complicate this scenario, there are some day-neutral sunflowers grown for the cut flower trade that are much less sensitive to day length and will look pretty much the same assuming there is enough light and warmth to grow and flower. Many bedding plants are photoperiodic, but have been bred to have a reduced response to photoperiod.

Tricyrtis hirta "Miyazaki" or Toadlily blooms in fall.

Short-day plants have pretty much the opposite reaction to day length as long-day plants. These plants set flower buds and bloom as the days shorten in late summer going into fall. Chrysanthemums (mums), Dahlia and Poinsettia are examples of short-day plants. Generally, sometime after the longest day of the year, these plants begin to set flower buds, but some can set flower buds before if the conditions are right. This makes dahlia a great choice for long-lasting summer through fall color. If you dig sweet potatoes too early in the season, there will be few if any of the swollen roots that we eat since the plants are still actively making more leaves and shoots.

The mums and poinsettias that we purchase at garden centers and greenhouses in the late summer and fall have been manipulated during production to bloom late enough for a great fall and winter display. Mum manipulation consists chiefly of pinching the bloom buds back in summer making the plants not only much thicker and more compact, but delaying flowering. Many varieties of fall mums (there are exceptions) will grow very tall and bloom in late June or early July if planted in your garden and allowed to grow naturally without pinching. In the same manner many varieties of poinsettias are also day length manipulated through the use of blackout curtains.

How do deciduous trees such as Red Maples know when to put on their annual color show? As the days get shorter in the fall, these plants begin to shut down the cellular chemistry that creates the pigment chlorophyll, the green color we see all season. As the chlorophyll breaks down, we can now see the other color pigments that have been there all along, but were masked by the overwhelming amount of green. This same cue tells the maples to create the zipper or abscission layer that eventually lets leaves fall from the trees.

So far, I’ve used the expression short and long day, this is an easier way to talk about this subject, but is inaccurate. Plants really tell time by how long the nights are by the reaction of the red pigment phytochrome in their cells to the dark. Day-neutral plants flower when they have grown large enough to flower.

Why do trees near street lights still change color and lose their leaves? Most street lights are of the wrong color spectrum, direct their light downward, and are too weak to affect trees. Drought and other stresses on plants can affect when they begin going dormant.