Get It Growing: How Do Plants Know When Seasons Change?

Daniel Gill, Merrill, Thomas A.  |  10/24/2006 12:36:06 AM


Get It Growing News For 12/19/03

By Dan Gill
LSU AgCenter Horticulturist

Have you ever wondered why poinsettias bloom now rather than for the Fourth of July? And have you ever thought about how plants know when the seasons are changing?

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 (which occurs each year around Dec. 21-22 – and occurs this year at 1:05 a.m. on Dec. 22). That means the period between sunrise and sunset is shorter on the winter solstice than any other day of the year, and it is our longest night or period of darkness of the year.

It also marks a turning point. Beginning with the summer solstice (the longest day of the year) in June, the days have been getting shorter and shorter and the nights longer and longer. Now, after the winter solstice, the days gradually will begin to get longer and the nights shorter. From ancient times until today, humans around the world have marked this time of year with various celebrations, festivals and religious rituals.

It is important for gardeners to understand how the length of days and nights from season to season affects the way many plants grow and what they do throughout the year. Plants living in temperate 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. Two ways that plants do this are by measuring hours of darkness that occur in a 24-hour period and by measuring how much cold they have experienced.

The fact that seasonal changes in the length of light and darkness in a 24-hour period have an effect on plants was researched thoroughly back in the early 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).

In 1920 two employees of the U. S. Department of Agriculture discovered a mutation in tobacco – a variety called Maryland Mammoth – that prevented the plant from flowering in the summer as normal tobacco plants do. Maryland Mammoth would not bloom until late December. Experimenting with artificial lighting in winter and artificial darkening in the summer, they found that Maryland Mammoth was affected by the relative length of light to darkness in a day. Because it would flower only when exposed to the short day lengths that naturally occur in winter, they called it a short-day plant.

Once this behavior was discovered, it was found to take place in many kinds of plants. Other short-day plants include chrysanthemums, poinsettias, Christmas cactuses, camellias and kalanchoes.

On the other hand, some plants, such as spinach and radish, flower only after exposure to long days and short nights and so are called long-day plants. Still others, including many annuals and vegetables, are day neutral – their flowering is not regulated by photoperiod but by growing conditions and maturity.

As it turns out, the terms short-day and long-day are not really accurate. We now know it is not how long or short the period of light is but rather the number of hours of darkness during a 24-hour period. Photoperiodic plants actually need a sufficiently short or long period of darkness to develop a response. But you know how it is. Once we start using a term and get familiar with it, it’s hard to get us to change, so we still use the terms short-day and long-day plants.

Plants don’t just measure the length of night to determine when to bloom. It also plays a large role in some plants’ abilities to anticipate the coming of the winter and respond appropriately. It is why, for instance, shade trees drop their leaves in November even though the weather in Louisiana then generally is mild. Because the nights get longer in the fall, trees 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 in a 24-hour period. The phytochromes, in turn, can trigger the release of various hormones or growth factors, which may cause a plant to bloom, drop its leaves or begin forming a bulb.

How do plants know when spring is arriving? Some are photoperiodic and can perceive the shortening of the nights. Many others (such as azaleas and peach trees) are able to measure the amount of cold that has occurred, and when a sufficient number of chilling hours accumulates, they are triggered to bloom or send out new growth.

It is important for gardeners to understand 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. And just as we have used Earth’s movement around the sun to develop calendars that allow us to do this, many plants also can determine the time of year based on similar perceptions.

Get It Growing is a weekly feature on home lawn and garden topics prepared by experts in the LSU AgCenter. For more information on such topics, contact your parish LSU AgCenter Extension office or visit our Web site at A wide range of publications and a variety of other resources are available.


Contact: Dan Gill at (225) 578-2222 or
Editor: Tom Merrill at (225) 578-2263 or

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