For many of us gardeners, poinsettia plants are an essential element of our Christmas decorating. I love the huge displays at church and in the stores, even if I don’t have room for that many in my home. I’ve written about how to keep the plants alive (see my 2012 post on “Pretty Poinsettias”). But this year I learned something new.

When I was small (I had my sixth birthday on the trip), my parents and I spent four weeks traveling around Mexico, from early December to early January. It was a wonderful time to visit, with all the Christmas and New Year celebrations. One common sight we couldn’t miss were the gardens full of bright red poinsettias in full bloom.

By the age of six I was familiar with potted poinsettias, so I was astonished to see poinsettias growing as large as trees! They were huge! I assumed they grew that large because they were planted outside in gardens, rather than in a small pot. Now I know there’s more to the story.

Most of us don’t have indoor space for a large shrub or small tree, especially one with thorns. That’s why the entire Christmas poinsettia industry—worth $250 million in sales last year, just in the U.S.—owes its existence to a bacteria. More specifically, it’s a phytoplasma—a particular kind of  bacteria that has lost its cell walls and can only live as a parasite in plant cells. In many ways, these bacteria are similar to viruses.

Most phytoplasmas cause serious diseases, such as aster yellows, and we would consider them “bad” bacteria. Typical symptoms of infection include yellowing and dying stems and foliage, leaves growing in place of flowers (resulting in sterility), green flowers, and witch’s brooms. However, not all phytoplasmas kill their hosts. Phytoplasma PoiBI¹, the kind that infects poinsettias, is one example.

This phytoplasma causes a witch’s broom condition—an increase in the number of side shoots and a decrease in the distance between branches. (There are also many other causes of witch’s brooms. While unattractive in large trees, witch’s brooms are the reason we have dwarf conifers. )

Instead of growing into a tall and scraggly shrub, PoiBI keeps our potted pointsettias short and compact. The plants have more branches, and therefore more “flowers” (actually, the red “petals” are bracts, and the flowers are the tiny yellow structures in the center). They fit on our coffee table or front porch, and we buy them by the millions.²

Next time you admire a vibrant red (or white or peach) poinsettia, think of all the bacteria living in its tissues, making it a suitable houseplant. Or, maybe not. Maybe we should just enjoy the pretty flowers!
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Thanks to The Mad Virologist for inspiring this post.

1. PoiBI stands for Pointsettia Branch-Inducing. No one can claim that scientists aren’t logical.
2. According to the USDA, over 80 million potted poinsettias were sold in 2015—during a six week season!