What are short day and long day plants?

CORVALLIS - Ever wonder why you have trouble getting your Christmas cactus or poinsettia to bloom again? Or have trouble with bolting spinach and lettuce in your summer garden?

To understand plant flowering, you need to get a handle on "photoperiodism," or amount of light and darkness a plant is exposed to. The amount of uninterrupted darkness is what determines the formation of flowers on most types of plants, explained Ann Marie VanDerZanden, horticulturist with the Oregon State University Extension Service.

Botanists used to think that the length of daylight a plant was exposed to determined whether a plant would form flowers. But experiments proved otherwise. It is the length of darkness that a plant experiences that plays the most crucial role.

A plant that requires a long period of darkness, is termed a "short day" (long night) plant. Short-day plants form flowers only when day length is less than about 12 hours. Many spring and fall flowering plants are short day plants, including chrysanthemums, poinsettias and Christmas cactus. If these are exposed to more than 12 hours of light per day, bloom formation does not occur.

Other plants require only a short night to flower. These are termed "long day" plants. These bloom only when they receive more than 12 hours of light. Many of our summer blooming flowers and garden vegetables are long day plants, such as asters, coneflowers, California poppies, lettuce, spinach and potatoes. These all bloom when the days are long, during our temperate summers.

And some plants form flowers regardless of day length. Botanists call these "day neutral" plants. Tomatoes, corn, cucumbers and some strawberries are day-neutral. Some plants, such as petunias defy categorization, said VanDerZanden.

"They flower regardless of day length, but flower earlier and more profusely with long days," she said.

Horticulturists and home gardeners manipulate the day and night length (indoors with lights) to get plants to bloom at times other than they would naturally.

For example, chrysanthemums, short day plants, naturally set flower and bloom with the long nights of spring or fall. But by making the days shorter by covering the chrysanthemums for at least 12 hours a day for several weeks over the late spring and early summer, you can simulate the light and darkness pattern of spring or fall, thereby stimulating summer blooming.

Or you can bring a long-day plant into bud formation and eventual bloom early before our day lengths surpass 12 hours. Put the plant under grow lights for a few hours a day beyond natural daylength for a few weeks. Adding supplemental day length to stimulate early blooming is a common practice in the nursery and fresh flower industry, especially this time of year, for Valentine's Day and Easter flowers.

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Ann Marie VanDerZanden
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