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The Journal Gazette

Saturday, August 18, 2018 1:00 am

Name tells how light influences flowers

Ricky Kemery

Q. I am a little confused about the difference between short-day and long-day plants.

A. In the early 1920s, USDA scientists W.W. Garner and H.A. Allard were the first to recognize the role of light duration on flowering in plants. They wondered why “Maryland Mammoth” tobacco failed to flower in the summer even though it grew to 10 feet in height, but flowered in the greenhouse in winter at less than 5 feet. After ruling out factors such as temperature and nutrition, they discovered that it was the length of the day that was influencing flowering.

Garner and Allard named this response of plants to day length photoperiodism. After a study on numerous plants, they classified plants into three categories based on day length response for flowering: 1) short-day plants, 2) long-day plants and 3) day-neutral plants.

It was later discovered that it wasn't the length of the day that plants were perceiving, but the length of the night – or darkness.

Perennial chrysanthemums are a classic example of a short-day plant. These plants, if left on their own, will normally flower sometime in the fall, as the days became shorter. Greenhouse growers, looking for a plant to add color late in summer and fall, manipulate the length of the day using covers to fool the plants into thinking the days are shorter than they really are. In this way the mums produce flowers just in time for us to plant them in landscapes in August and September.

Long-day plants really want shorter nights and longer day periods in order to flower.

Most spring flowers are long-day plants.

Some plants, like certain summer vegetables, do not respond to day length. These are called day-neutral plants.

Goldenrod is a short-day plant. It begins to flower late in the summer, when butterflies are beginning their long migrations south. Their pollen is a primary food source for that long journey.

Many folks think that goldenrod is the culprit for summer allergies, but in fact those allergies are caused by common and giant ragweed – both short-day plants. The small ragweed flowers produce tremendous amounts of pollen that irritate our breathing. If you spot it in your landscape, cut it back and pull it to prevent it from flowering.

Tobacco, soybeans, corn and rice are short-day plants. When you think about it, they all mature late in the season – just like they should.

One of my favorite annual flowers is cosmos, which is also a short-day plant. It produces its flowers late in the season when other flowers are fading. Many sunflower varieties are also short day plants.

Poinsettia is perhaps the most famous short-day plant. Growers manipulate the day length so the bracts of poinsettias turn color – just in time for the holidays.

The Plant Medic, written by Ricky Kemery, appears every other Saturday. Kemery retired as the extension educator for horticulture at the Allen County branch of the Purdue Extension Service.