As the earth travels around the sun in its orbit, the length of our days and nights varies from season to season. This is because the earth’s axis of rotation is tilted in respect to its plane of orbit around the sun.
Here in the Northern Hemisphere, we are tilted farthest away from the sun on the winter solstice. On that day, the period between sunrise and sunset is shorter than any other day during the year, and it is our longest night or period of darkness during the year. The winter solstice occurred on December 21.
The solstice also marks a turning point. Beginning with the summer solstice, the longest day of the year on June 21, the days begin getting shorter and shorter and the nights longer and longer. After the winter solstice, the days will begin to get longer and the nights shorter. From ancient times until today, human cultures worldwide in the Northern Hemisphere have marked this time of year with various celebrations, festivals and religious rituals.
What does this have to do with gardening? Well, I could mention that for thousands of years plants have played a role in human observances of this event. In Europe, plants that stay green during winter often had special significance. They were a reminder of life in the midst of freezing cold and leafless, dormant trees and shrubs.
Evergreen plants such as holly, English ivy and mistletoe and conifers such as fir, spruce, cedar and pine are still used today to decorate our homes. And we now also include plants that bloom this time of the year. They remind us in the depths of winter that life will continue and spring will come again. Blooming poinsettias and Christmas cactus plants can be seen everywhere.
What I’d like to focus on, however, is the lengths of days and nights changing from season to season. Why? Because gardeners need to understand that it has an effect on the way many of our plants grow and what they do throughout the year.
Just like us, plants living in climates where major temperature changes occur during the year and cold winters are typical need to be able to tell when the seasons are changing. Plants do this two ways, by measuring hours of darkness that occur in a 24-hour period and measuring how much cold they have experienced.
The fact that the length of daylight and darkness in a 24-hour period has an effect on plants was researched thoroughly back in the 1900s, and the term “photoperiodism” was created to describe the phenomenon. Animals, such as certain birds, insects and mammals, also respond to changing photoperiods during the year.
The discovery of photoperiodism happened in 1920 when two employees of the U. S. Department of Agriculture discovered a genetic mutation in tobacco. Plants possessing the mutant gene bloomed in December instead of flowering in the summer as normal tobacco plants do. Experimenting with artificial lighting in winter and artificial darkening in the summer, they found that the mutant plants would flower only when exposed to the long nights that naturally occur in winter. They called them “short-day” plants.
Once this behavior was discovered, photoperiodism was found to take place in many kinds of plants. Other short-day plants include chrysanthemum, poinsettia, Christmas cactus and kalanchoe. That is why these plants bloom in fall and winter.
Some plants, such as spinach and radish, flower only after exposure to long days and short nights of summer and are called long-day plants.
Still others, including many annuals and vegetables, are day neutral; flowering is not regulated by photoperiod.
As it turns out, it is not how long or short the period of light is, but rather knowing the length of darkness in a 24-hour period. Photoperiodic plants need a sufficiently short or long period of darkness to develop a response. But we still use the terms short-day and long-day plant.
Plants don’t just measure the length of night to determine when to bloom. Darkness also plays a large role in some plants’ ability to anticipate the coming of the winter and respond. It is why, for instance, shade trees drop leaves in mid- to late November, even though the weather is generally still mild. Because the nights are getting longer, they know winter is on the way.
What mediates this remarkable response are various pigments, called phytochromes, which allow photoperiodic plants to measure how many hours of light or dark they receive. The phytochromes, in turn, can trigger the release of various hormones or growth factors that may cause the plant to bloom or drop its leaves or begin forming a bulb.
How do plants know when spring is arriving? Some can perceive the shortening of the nights. Many others are able to measure the amount of cold that has occurred, and when a sufficient number of chilling hours accumulate, they’re triggered to bloom or send out new growth.
The winter solstice reminds us how remarkable plants are. They have abilities to sense the world around them and respond to it in ways that many gardeners are unaware of.
It might not have occurred to you that it is just as important for a plant to know when it is time to bloom or drop its leaves, as it is for a farmer to know when it’s time to plant a crop. Just as we have used the Earth’s movement around the sun to develop calendars that allow us to determine the seasons, many plants can also determine the time of year based on similar perceptions.
Dan Gill is a horticulturist with the LSU AgCenter. He can be reached at DGill@agcenter.lsu.edu