Autumn creeps slowly into the mountain valleys of West Virginia.
Cool, crisp evenings signal the end of summer and our gradual march toward winter. Colors slowly appear, then radiate, in the trees hugging the sides of the mountains.
Summer-visiting birds pack up and move south, preferring to winter in warmer locations. And I find it harder and harder to remove myself from the embrace of my warm, comfortable bed. Autumn is here!
These cooler temperatures signal the end of summer, but the beginning of fall colors and wildflowers.
Though even if temperatures were to remain warm, many of the plants would still know that it is time for fall. How? It all comes down to how their internal clocks are set. For many plants, signals come from light and not temperature.
Did you ever wonder how chrysanthemums know to wait until fall to bloom? Or how about Thanksgiving and Christmas cacti knowing when their respective holidays are around the corner?
It’s because they look to the sky and set their clocks by the amounts of light and dark during the day.
As autumn marches ever onward, the length of daylight dwindles and the nights become longer. On the autumnal equinox — which is Monday, by the way — the sun will shine directly on the equator and day and night will be equal in length.
Afterward, the days will grow shorter and we will have less daylight than dark until the shortest day on the winter solstice.
For example, the change in leaf color from green to fall’s vibrant oranges, reds and yellows occurs because the tree perceives both the shortening days and the cooler temperatures to break down the green pigment chlorophyll to save energy for winter.
Where light can really make a difference, though, is in giving plants a signal to initiate flowering.
Plants exhibit what is called a photoperiod. This photoperiodicity (now that’s a $50 word) means that when it comes to flowering, plants can either be long-day plants, short-day plants or day-neutral plants.
As the name suggests, day-neutral plants will flower at any time without regard to the length of light or dark. Good examples of these plants are the annuals we plant in late spring that flower all the way until frost kills them, and several of the plants in the vegetable garden such as tomatoes. Daylight makes no difference to these plants.
But light does matter to short- and long-day plants.
Long-day plants will only bloom when the amount of daylight is longer than the period of darkness. Depending on the plant, there is a set amount of light that the plant must receive before it will flower.
Plants such as petunia, daylily and iris are long-day and require at least 12 hours of light to bloom. Sometimes this day length isn’t required for blooming, but will make blooming much faster in plants like sunflowers and snapdragons.
Short-day plants need a period of at least a few weeks where the daylight is short and the period of darkness is long. Research actually shows that it is the long period of darkness that is most important rather than the length of light. So if the darkness is interrupted by a flash of light during the night, even for just a few seconds, the flowering of that plant can be interrupted.
If the length of darkness does not reach the length that the plant needs to initiate flowers, or if it continually gets interrupted by flashes of light, the plant may never bloom.
Many of our fall-blooming plants, such as chrysanthemums and those bluish-purple asters sold beside them, are short-day plants and will not bloom if they do not have enough uninterrupted darkness.
Those Thanksgiving and Christmas cacti also are dependent on long nights to bloom.
Another well-known short-day plant is the poinsettia. Right now is about the time that you need to give it at least 14 hours of uninterrupted darkness so that it will start to change color. You can do this by putting a box over the plant, or moving it to a completely darkened room for 14 hours a day. You’ll need to continue this routine until about the middle of November.
This little light phenomenon is often a surprise to students when I talk about it in the botany class for the Master Gardener program. Sometimes you get that “aha!” moment on their face when they realize that the reason their plants don’t bloom is because of either too much or not enough light.
One of the most common problems my students face is with getting hydrangeas to bloom. While not necessarily a short-day plant, most hydrangeas do not bloom if they get too much light (say, more than 14 hours). The illumination from a porch light, solar light or dusk-till-dawn light that bathes the plant well into the night may just be keeping it from blooming.
Next time you plant something near an outdoor light source, be sure to do a little research to check out its photoperiod. Otherwise, you might end up with a late bloomer.