Botany Lessons from a Holiday Poinsettia

Learn more about the natural history and botany behind your greenhouse-grown poinsettia.
Botany Lessons from a Holiday Poinsettia - Articles


Photo credit: Alyssa Collins

Red, pink, white, yellow and even speckled poinsettias have become familiar symbols of the long winter holiday season in America. This plant has taken a fascinating journey from Central American forests to our holiday tables and provides some interesting horticultural lessons.

A Not-So-Natural History

Poinsettia’s botanical name Euphorbia pulcherrima means that it is the most beautiful member of the spurge family, a diverse group that includes crown of thorns and castor-oil plant. The foil-covered, potted poinsettia that we purchase in late fall bear little resemblance to its wild ancestor, which grows as a lanky shrub in the coastal forests and hot, dry interior of Mexico. The plant was introduced to America in 1828 by Joel Roberts Poinsett, botanist, physician and the first United States Ambassador to Mexico. However, credit for making the Poinsettia one of the most popular indoor plants belongs to the Ecke family of California. In the early 1900s, Paul Ecke Sr. sold the brightly colored plants from roadside stands in Hollywood. His son Paul Ecke Jr. was responsible for breeding plants that withstood shipping and produced a high number of long-lasting blooms. Ecke Jr. escalated their popularity as a holiday decoration by donating them to fashion magazine shoots and holiday sets for TV broadcasts, such as The Tonight Show and Bob Hope Christmas specials.

Flower Power

The colorful parts of the poinsettia are not flower petals but bracts. These leaf-like structures are similar to the white and pink parts that surround dogwood blossoms. Though not very noticeable, the actual flowers of poinsettias are contained within the small yellow-green globes in the center of the bracts. If you keep an eye on your holiday poinsettia you will see the male flowers emerge first holding small flakes of pollen. As these begin to shrivel, a single female flower emerges in each cluster; its stigma (pollen receptor) resembles a small red hair. The staggered emergence of male and female flowers in poinsettias allows the plants to fertilize each other, but reduces the chance of self-pollination. This is a plant’s way of preserving diversity and disease resistance within its species.

Poinsettias are Infectious

The original potted poinsettias were a single bloom on a compact plant. The beautiful, multi-bloom plants that we know today are the result of phytoplasma—an organism similar to bacteria found in the tissues of some poinsettia plants. Growers discovered that this organism lived in plants with the dense and branching characteristics that were desirable in a potted poinsettia. Today infected plants are grafted with those uninfected to transfer this desirable “disease.”

A Poinsettia’s Inner Clock

Poinsettias are short-day plants, meaning that waning daylight triggers the flowers. They are the opposite of the long-day plants in our gardens such as asters, coneflowers, lettuce, spinach and potatoes. (Plants whose flowering is not dependent on the length of daylight are called day-neutral—they flower when the plant gets large enough or reaches a certain stage of development.) Despite the term “short-day,” research has shown that it is really the length of night, or darkness, that is important in the blooming cycle. In order to flower for the winter holiday season, a poinsettia needs 12-14 hours of darkness each day, beginning around October 1. Even a short light interruption during the dark period can reset the clock and prevent it from blooming.

Poinsettias as House Guests

Poinsettias are not poisonous, despite what you may have heard. They probably acquired this reputation because, as members of the Euphorbiaceae family, they have a milky, latex-like sap. Therefore, those with a sensitivity to latex may also be sensitive to poinsettia sap. Chewing on poinsettias will not kill Fluffy or Fido, but it’s best to keep poinsettias away from new kittens or puppies, as ingested leaves may cause stomach discomfort in a small animal.

As the days get longer, poinsettias’ colorful bracts will fade and green leaves will replace them. They make a nice patio plant, but will be extremely difficult to rebloom in fall, due to reasons described above. When your green and lanky poinsettia has out-stayed its welcome, give yourself permission to compost it and purchase a new one next holiday season. With over 100 poinsettia cultivars to choose from, you might want to explore a different color or form to add to your holiday decorations next year!