In the Garden:
Pacific Northwest
December, 2011
Regional Report

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Blooming out of season, this poinsettia is a real eye-catcher in the garden.

Fiddling in the Garden

One of the more reassuring aspects of gardening is the way plants behave at certain times of the year. I expect chrysanthemums to bloom in the fall, poinsettias at Christmastime, tulips in the spring, and peonies during the summer months. I look forward to these seasonal changes. For me, part of the fun of gardening is the anticipation of spectacular performance from my favorite plants. While I appreciate their reliability, I think it's only natural to wonder what causes plants to flower when they do.

What Triggers Flowering?
Plant maturity really determines when a plant is able to produce flowers. Some plants reach maturity quickly. Annuals, for instance, complete their life cycle in a single growing season, giving them the ability to produce flowers within a few weeks of germination. Other plants can take years to reach maturity. When grown from seed, lilacs, wisteria, and apples can remain in a juvenile, non-blooming state for as long as seven years!

Favorable growing conditions (temperature, light, water, and essential nutrients) are the basic requirements for a plant to reach maturity. Once it reaches a certain minimum size and receives a particular environmental signal, it will develop flower buds.

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. Natural day length fluctuates gradually with the seasons. 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 "day length" is the term you will hear most often, that's what we'll use, just to avoid confusion.

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 are those that 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 is helpful if we want to coax plants to flower out of season. What's wrong with having tulips in September or poinsettias in full color by 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: In early January, cut it back hard by reducing each branch to just 1 inch in length. 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. I 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 the third week of February and the second week of March, take stem-tip cuttings 3 to 5 inches long and root them in a mixture of peat moss and perlite. Repot the cuttings 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 - except when clever gardeners are involved!

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