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The Homeowners Column
How Plants Respond to the Length of Daylight
Extension Educator, Horticulture
Can you feel it? The daylight today is just a little bit longer than last Saturday. Although somedays we wish the days were longer than 24 hours, that part doesn't change but the amount of light and dark does change as the seasons progress. The winter solstice on December 22 marked the shortest length of daylight of the year. The sun was at its farthest distance from the equator. From now on, daylight gradually gets longer until June 22, the longest length of daylight of the year. These days of the year go virtually unnoticed by most people today. However to ancient agrarians and to our plants today, December 22 was and is an important day of light.
Winter solstice has been celebrated for thousands of years by cultures throughout the world. It was a time to celebrate light. The people of ancient Rome celebrated Saturnalia. On December 17, they began a party that lasted seven days. The festival was named in honor of Saturn, the god of agriculture, from whose name we get Saturday.
Those Romans knew how to party. There were processions through the streets, candles were lit to symbolize the rebirth of the year, houses were decorated with greenery and presents were exchanged. Masters gave their slaves gifts, men dressed as women or masqueraded in the hides of animals. A dried bean was hidden in food and whoever found it was elected king of the revels. These traditions found their way to all parts of the Empire, blending with customs that already existed and influencing the way in which people celebrated the end of the year.
As far as we know, plants don't have any wild parties to mark the winter solstice but they do respond to the length of daylight. The response of plants to the relative length of day and night is called photoperiodism. Plants actually "measure" the duration of darkness rather than of light. Plant response to day light length is caused by changes in a pigment called phytochrome. Phytochrome is a light-sensitive, blue-green pigment. It occurs in plant tissues in minute quantities--about one part in 10 million. The pigment acts as an enzyme in other words, it activates certain processes without itself being used up. During daylight, phytochrome is converted to an active form, and in darkness it is converted to an inactive form. If the active form of phytochrome gets to a specific level for that plant, the enzyme can start changes in the plant.
Photoperiodism influences many activities in plants including growth, seed germination, flowering, fruit development and the onset of winter dormancy. Some plants need a specific period of daylight in order to flower. Some plants require short days/long nights to flower. Some of our holiday plants such as poinsettias and kalanchoe require a short day/long night to bloom. In plants referred to as short day, their flower bud formation begins when daylight is relatively short about 10 to 12 hours and flowering is suppressed if daylight is longer. Greenhouse growers manipulate light to get the plants to bloom at the prescribed time.
Long day/short night plants initiate flower bud formation once they are exposed to relatively long days and short nights, about fourteen hours or more and won't flower if daylight length is shorter. Some of our common vegetables are long day plants such as beet, radish, lettuce, spinach and potatoes. Bulbing in onions is initiated under long days. The amount of daylight required depends on the variety. Some plants don't care about day length to flower and are called day neutral plants. Of course all this plant response may change depending on temperature, humidity and maturity of the plants.
The length of daylight is important to all living creatures, so raise your glass of eggnog and cheer the return of light.