In the Garden:
Pacific Northwest
December, 2003
Regional Report

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Whether white, marbled, or traditional red, poinsettias are irresistibly festive.

Plant Foolery 101

After a few seasons in the garden, you come to expect certain plants to bloom at certain times of the year; chrysanthemums in fall, poinsettias at Christmas, tulips in spring, and daisies in summer. I think it's only natural to wonder what causes plants to flower when they do.

What Triggers Flowering?
Certain conditions must be met before a plant is mature enough to produce flowers. Some plants reach maturity quickly, while others remain in the juvenile stage for a very long time. For example, annuals complete their life cycle in a single growing season, and therefore have the ability to bloom within a few weeks of germination. But lilacs, wisteria, and apples, when grown from seed, can remain in a juvenile, non-blooming state for as long as seven years.

Favorable soil and air temperatures are the basic requirements for a plant to reach maturity. Once it reaches a certain minimum size, or receives a particular environmental signal, it will develop flower buds. As an example, hybrid tea roses form flowers after they've developed a minimum number of leaves or nodes on their stems. Anything that makes a hybrid tea rose grow more rapidly (temperature, light, water, and essential nutrients) will bring about earlier and more profuse flowering.

Environmental Signals
The two most important environmental signals that tell plants to form flowers are day length and temperature. Day length is the number of hours of light in a day. In nature, day length fluctuates gradually with the seasons (except at the equator, where day length is constant). Some plants are more sensitive to day length than others, but day-length-sensitive plants will only begin to form flower buds when the days are a certain number of hours long. This is called the "critical day length." Just to set the record straight, plants really measure hours of darkness, not day length. But since the term "day length" is commonly used, we'll continue the custom.

Critical day length varies from plant to plant. Long-day plants begin to form flower buds when the number of hours of daylight meet or exceed their critical day length requirement. These plants usually bloom in the spring and summer when days are long. Short-day plants form flower buds when the hours of daylight meet or are less than the plant's critical day length. These plants generally bloom in the fall as the days gradually shorten. Plants that do not respond to day length as a signal to start forming flower buds are called "day-neutral" plants.

Temperature-sensitive plants behave in the same ways as day length-sensitive plants, but their trigger is a critical temperature. Soil temperature controls the growth of spring-flowering bulbs, but air temperature can play a part, too, as in the case of camellias, where warm temperatures can result in bud drop or failure of flower buds to open.

Fooling Mother Nature
Understanding the environmental conditions required for flower bud formation can be helpful when we want to coax plants to flower out of season. What's wrong with having tulips in September or poinsettias for the Fourth of July?

If you think you're ready for the challenge, here's how to make your holiday poinsettia bloom on the Fourth of July: Early in January, cut back your poinsettia by reducing each branch to just 1 inch. You'll have a main stem with several 1-inch branches jutting out in all directions. Provide full sunlight during the day, and interrupt the hours of darkness by exposing the plant to light for 1 to 2 hours in the middle of the night. (Use a 75-watt incandescent bulb suspended 3 feet away from the plant.). Interrupting the hours of darkness will reset the plant's internal clock. Continue this treatment until the end of April.

Between Feb. 25 and March 11, take stem-tip cuttings 3 to 5 inches long and root them in a mixture of peat moss and perlite. Repot the cuttings in 3 to 4 weeks after they have rooted, and remember to continue exposing the plants to 1 to 2 hours of light in the middle of the night.

Beginning the last week of April, stop interrupting the nights. Instead, allow the plants to receive only 8 to 9 hours of daylight, covering them completely to eliminate any light for a period of 15 to 16 hours. Repeat this process once every 24 hours. Continue to treat plants in this manner until color shows in the bracts. When color begins to show, allow plants to experience natural day and night lengths. Your poinsettias should be fully colored by the Fourth of July.

To everything there is a season – but a single light bulb can change it all!


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