Get It Growing: Winter Solstice Marks Turning Point

Daniel Gill, Merrill, Thomas A.  |  11/23/2006 1:40:40 AM

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Get It Growing News For 12/22/06

By Dan Gill
LSU AgCenter Horticulturist

The winter solstice occurred this week, and it marks a turning point in the length of our days and nights.

Why is that important for gardeners? Mainly because it reminds us that changing of the seasons affects the way our plants perform.

To provide the background, the winter solstice occurs in December each year. This year it was on Dec. 21. After the winter solstice, the days will begin to get longer and the nights shorter. That’s just the opposite of what they’ve been doing since the summer solstice on June 21 – with the days getting shorter and the nights getting longer from then until now.

This all stems from the travel of our planet in our solar system. As the Earth travels around the sun, 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, and that means the period between sunrise and sunset is shorter on that day than any other day during the year. It also means that is our longest night or period of darkness during the year.

What this means is that the winter solstice is a turning point toward longer days and shorter nights – a turning point that humans throughout the ages and around the world have marked with various celebrations, festivals and religious rituals.

But let’s get back to what this has to do with gardening.

First, it seems natural to mention that for thousands of years plants have played a roll in the human observances of this time of year. For example, in Europe, plants that stayed green during the 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 during this season. Nowadays, we also include plants that bloom this time of the year in those decorations – to remind us during the depths of winter that life will continue and spring will come again. Blooming poinsettias and holiday cactus can be seen everywhere about now.

While those are some ways it relates to gardening, the winter solstice mostly reminds gardeners it is important for us to understand that the lengths of days and nights change from season to season. Those changes have an effect on the way many 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, so they can prepare. Plants do this two ways – 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 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 cultivar 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 and kalanchoes. That is why these plants bloom in fall and winter.

On the other hand, some plants, such as spinach and radish, flower only after exposure to long days and short nights. So they are long-day plants.

Still others, including many annuals and vegetables, are day neutral. Their flowering is not regulated by photoperiod.

As it turns out, the terms short-day plants and long-day plants are not quite accurate. It is not how long or short the period of light is. Instead, it’s really the length of the darkness in a 24 hour period that’s important.

Photoperiodic plants actually need a sufficiently short or long period of darkness to develop a response. However, once humans start using a term and get familiar with it, it’s hard to get them to change, so 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. It plays a large role in some plants’ abilities to anticipate the coming of the winter and respond. That’s why, for instance, shade trees drop leaves in mid- to late November even though the weather around here generally is not that cold then. Because the nights are getting longer, the 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. The phytochrome, in turn, can trigger the release of various hormones or growth factors, which may cause the plant to bloom and 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 are 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 Earth’s movement around the sun to develop calendars that allow us to determine the seasons, 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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