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The poinsettia: taking cues from daylight

For those with holiday lights hung all year and trees still decorated, perhaps pots of poinsettias, with their red, pink or white bracts, still liven up the house. These bracts (the modified leaves often mistaken for flowers) turn vibrant colors — traditionally a bright red — in midwinter, making an excellent example of the importance of photoperiods. 

Photoperiods, or the lengths of daylight, are important environmental signals for plants. They can allow developmental processes, such as flowering time, to line up with specific times of the year to maximize growth and reproductive success.  

Poinsettias are classified as short-day plants. Short-day plants require long nights to trigger certain physiological processes, such as flowering and bract color change. Native to Mexico and Central America, poinsettias need about 12 hours or more of darkness each day to initiate these changes. 

The poinsettia’s reliance on the length of daylight to initiate its color change makes it a perfect example for students learning about the effect of photoperiods. Applied Plant Science, a class offered every fall by the �鶹���� Fairbanks natural resource management program, uses poinsettias to demonstrate this concept. 

Meriam Karlsson, professor of horticulture, teaches this class and grows poinsettias from the previous year’s cuttings in the Agricultural and Forestry Experiment Station greenhouse. She regiments their photoperiod to over 12 hours of darkness starting in mid-September every year. By the end of the semester, students can see how the previously green leaves have all turned red, white, pink or even orange. 

There are hundreds of varieties of these plants in many different colors. An orange variety called “Autumn Leaves” was bred for Thanksgiving, Karlsson said. 

While plant breeders are still developing new varieties of poinsettias in different colors, the color shift for these plants in the wild is not just an aesthetic feature. It signals to pollinators, such as bees or birds, that the tiny yellow and red flowers housed at the center of the bracts are blooming. Since flower production is energetically expensive, the poinsettia conserves resources by producing pigment in existing leaves instead of creating large, colorful flowers each year. 

The flowers may be the less flashy part of the plant, but they draw the eye once noticed. Protruding like yellow lips from one side of the flower are large nectaries — glands that produce nectar. The nectar attracts and rewards pollinators with a sweet and nutritious snack. 

Poinsettias have a reputation for being toxic, but this is a widespread misconception. They are not poisonous or toxic. The white milky sap, released when parts of the plant are broken or damaged, contains a compound similar to that found in rubber latex that can cause a reaction in people with a latex allergy. 

Once removed from a short-day photoperiod, the bracts will eventually turn back to green. Flowers will die if they’re not pollinated or will start developing seeds if they are. As we move from holiday parties to spring celebrations, we, like the poinsettia, will take our cues from the cycle of light. 

ADDITIONAL CONTACT: Meriam Karlsson, mgkarlsson@alaska.edu, 907-474-7005