Coles County Yard and Garden

Coles County Yard and Garden




By Lucille Saunders, Coles County Master Gardener

Have you ever heard the word Photoperiodism and, if so, do you know what it means?

Photoperiodism is the effect of the relative length of daylight exposure upon the growth and flowering (reproduction) of plants. It has its effect on mammals-deer reproducing in the spring, bears hibernating in the winter, lack of sunny days on humans, but today we are concerned about plants.

Flowering plants fall into four groups: 1) Long-Day Plants (see below), 2) Short-Day Plants (see below), 3) Indeterminate Plants (such as cucumbers, roses, tomatoes) which bloom with more or less readiness under all light conditions, and 4) Intermediates Plants whose flowering is initiated by a zone or band of day lengths of median duration, but inhibited by day lengths either above or below this band. There are other features of plant growth, such as stature, bulk, production of seeds or oil, etc. influenced by the photoperiod, although, other factors such as temperature, moisture, humidity and food supply have a bearing upon the behavior of all plants.

Long-Day Plant applies to plants in which the flower bud formation is initiated by exposure to relatively long day lengths (14 hours or more) and this same bud formation is suppressed by exposure to shorter day lengths. Commonly known plants in this category are Hollyhock, Timothy, Coreopsis, Marigold, Rudbeckia, Bee Balm, Rose of Sharon, Clematis and many others. To bring long-day plants into bloom at a time of year when short days are the rule, artificial lighting has to be used to extend the amount of light each day, so basically under greenhouse conditions.

Short-Day Plant applies to plants in which the flower bud formation is normally initiated by relatively short day lengths (10 to 12 hours) and suppressed by long day light exposure. Typical plants are Chrysanthemum, Poinsettia, Cosmos (late varieties), certain varieties of Dahlias and African Marigolds, Ageratum, most Goldenrods, most native Asters, etc. Again, such plants can be forced into earlier bloom by shading them at an appropriate hour in the evening until nightfall with opaque black cloth (setting in a dark closet late afternoon until morning) and the shading should be started prior to the time of normal bud-setting and has to be continued for several weeks. A practice many of us might be familiar with if we want last year's poinsettia to bloom again around Christmas.

It should be emphasized that photoperiodic effects relate directly to the timing of both the light and dark periods. So the change to Daylight Savings Time does have effect on plants. Research has determined the length of the dark period is the main factor in whether or not a plant flowers. Even a few minutes of light interrupting the dark period will prevent a Short-Day (requiring a long night) from flowering while an interruption of the daylight with a few minutes of darkness has no effect at all.

An understanding of this phenomenon is important in determining whether a particular plant will bloom if taken to a new region -or even a new location in the garden that might get more or less light, like from a street lamp. The importance in genetics is that is has been possible by varying the length of day to secure more than one generation of certain plants within a year. By manipulating the length of the day (or night), it is possible to bring into bloom simultaneously varieties or species that normally bloom in widely separated periods and this is the reason that there have been successful crosses that have permitted an extension of the blooming periods of many of today's plants giving them much longer seasons.

Here is another reminder about University of Illinois Master Gardeners of Coles County's SPRING INTO GARDENING. It's coming up on Saturday morning, February 27, 2016. Topics will include the plight of the Monarch butterflies, creating pollinator pockets, and getting started with backyard water gardens. Register online at or call the Extension office at 217-345-7034.

If you have other questions about your garden or landscape, feel free to contact a master gardener at the University of Illinois Extension office in Charleston at 345-7034. You can also check out the many horticulture webpages at the U of I Extension's website by visiting . And be sure to like the Master Gardeners' new Facebook page, at

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