LOOK BEYOND POINSETTIA GLITZ

A Harem of Males

Phew, what a year 2020 was! Well, it’s over and, at least at this writing, things look hopeful for the future, at least from my perspective. Except if you live in a tropical or subtropical climate, there’s not much distraction from anything gardenwise, for now, so let’s take a close look at a plant no doubt sitting on many coffee tables and windowsills. Poinsettia. I’m not a big fan of their appearance, but I do like them as botanical curiosities.

Let’s share some botanical lore of this plant by setting your holiday poinsettia on a table in good light for a close look at its flowers. I say “close  ” because the flowers are not those large, red, leaf-like structures. The large, red, leaf-like structures are just that — leaves, albeit modified leaves called bracts. The bracts attract pollinating insects to the plant.

The actual flowers of the poinsettia, which are not very showy at all, originate within the small, greenish, cup-shaped structures you see above the bracts. These cup-shaped structures are “cyathiums.”
poinsettia cyathiums
Each cyathium contains a single female flower surrounded by a harem of males. Pluck a cyathium from the plant, take a sharp knife or razor, and slice it in half from top to bottom. A magnifying glass helps now. You’ll see that the inner wall of the cyathium is lined with numerous tiny flowers which, when the flower is mature, protrude up through the opening of the cup. These are all male flowers.

Look very carefully and you’ll also see a stalk attached to the very bottom of the cyathium, protruding up through the cup opening, and capped by what looks like a turban. That is the single, female flower, which, to promote cross- rather than self-pollination, emerges from the cyathium after male flowers begin to shrivel. Cross-pollination promotes genetic diversity for healthier plants. 

In addition to the male and female flowers, on the outside of the cyanthium is a single yellow gland that looks like the mouth of a fish poised to ingest food.
Cyathium close up
Next time a friend comments on the beauty of your poinsettia flower, take out a magnifying glass and closely examine a few cyathiums before looking up and agreeing nonchalantly.

Fool the Plant

Have you ever wondered how stores always manage to have blooming poinsettias for the holiday season? 

Poinsettia, along with chrysanthemums and most strawberry varieties, is a “short-day” plant. Short-days induce “short-day” plants to form flower buds. (In reality, plants are responding to long nights, but the phenomenon was originally thought to be daylength dependent, and the term “short-day” plants has stuck with us.) Poinsettia can be fooled into blooming at any time of the year merely by exposing it to artificially shortened days. 

To secure blooming plants for this past holiday season, poinsettias grown in large commercial greenhouse ranges were covered with a shade cloth so that they experienced 14 hours of darkness each “night” beginning around September. After 8 weeks of this treatment, they formed flower buds which became fully developed just before the holiday season.

You Do It

You can do this at home to make this year’s poinsettias bloom again whenever you want. Your poinsettias first will need a rest period. Loss of leaves and fading flowers indicate that your plants are entering dormancy. (Even tropical plants, such as poinsettia, do take a short, mild annual rest.) When the plants become dormant, move them to where it is cool. Water them infrequently, just enough to keep their stems from shriveling.

About April, the plants will be ready to start growing again. Cut the stems back to a few strong shoots, each about 6 inches long. Give the plants sun, warmth, and water. New shoots will push forth from dormant buds. When warm weather has settled, you can put the plants outside.

As temperatures cool in late summer, bring the plants indoors to a sunny window. Keep them away from cold drafts, which would cause their leaves to drop.

Now for the photoperiod treatment. Beginning three months before you would like the plants to bloom, make sure the plants get 14 hours of darkness each night. Moving them into a dark closet or covering them with a paper bag are convenient ways to do this.

The dark period each night must be uninterrupted. Even peek at them with a flashlight at midnight will have them acting as if they had a short night, instead of the needed long night.

After 8 weeks of the above treatment, move the plants back to their sunny window. Flower buds should be evident, and you should have blooming poinsettias within a month. If you want bloom for the holiday season next year, start the photoperiod treatment about the middle of September.

Poisonous?

Poinsettia, Euphorbia pulcherrima, is a member of the Euphorbiaceae family, commonly called the Spurge family. A common characteristic of plants in this family — which includes Hevea braziliensis, tapped commercially to make rubber — is the milky sap they exude when cut. The sap has often been considered poisonous and was sometimes used medicinally as a purge (hence “spurge,” from the French word espugier, meaning to purge). But, as Philippus Aureolus Theophrastus Bombastus von Hohenheim, more commonly known as Paracelsus, wrote in the 15th century, “All things are poisons, for there is nothing without poisonous qualities. It is only the dose which makes a thing poison.”

In fact, the sap is only mildly poisonous to humans and other animals. It can cause a rash on the skin or mouth, and stomach upset. So go ahead and nick the stem of your poinsettia and see the milky sap exude. No need to touch it.

8 Comments

  1. Linda Jean
    Posted January 6, 2021 at 9:57 am | Permalink

    How do nurseries start poinsettias? I’m guessing there’s a tiny seed. Are they sown in individual pots and some later weeded out? I love to look at poinsettias set up in church and when they are all prettied up for sale at the florist, but I have no need to have them in my home. They don’t last long and make a mess dropping their leaves and flowers.

    • Posted January 10, 2021 at 8:43 pm | Permalink

      Nurseries grow clones, which are named varieties, to sell. As such they must be propagated asexually, by rooting cuttings in this case.

  2. Diane
    Posted January 9, 2021 at 12:03 pm | Permalink

    Lee, will this extended darkness force Christmas cacti to bloom? I have 2 that have not bloomed since their initial bloom 2 years ago. If not, what is their secret to blooming?

    • Posted January 10, 2021 at 8:39 pm | Permalink

      Christmas cactii also respond to a dry period. Wit that said, you should get blooms if you give the plant enough light and periodic moisture. The blooms just might not be when you want them, though.

  3. Richard Cowhig
    Posted January 10, 2021 at 8:56 am | Permalink

    Just read you article about seed germination
    Nicking or soaking seed in warm water or acid, yes acid is refered to as scarification.
    In addition i ran a poinsettia trial as an accidemic, poinsettias are not poisonous!
    OSU ran an experinent in early 70’s which was published in the J OF AMA.
    Totally disproved this old tale!!!

    • Posted January 10, 2021 at 8:37 pm | Permalink

      I did mention that myth about poinsettias being poisonous in my post.

  4. Richard
    Posted January 13, 2021 at 10:19 am | Permalink

    Hi Lee, I’ve never kept poinsettias after the holidays but I have to say that one in the first picture is beautiful. How old is it? I never thought about them changing shape over time, but that is a stunner.

    • Posted January 13, 2021 at 2:35 pm | Permalink

      I can’t remember how old it is (it’s age changes every year!). Poinsettia is a shrub in the tropics, so develops a more or less permanent trunk over time.

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