I saw that my Christmas cactus was blossoming for the third time this year and wanted to learn what caused a Christmas cactus to bloom. So I turned to the internet to research it.
There are thousands of articles about this cactus so I limited my search to .edu sites. I wanted to see articles that were affiliated with universities with horticulture departments like Cornell, Penn State and Purdue. I found a number of them but what I wasn’t expecting was that these articles would bring into question everything that I thought I knew about Christmas cactus.
First off, what I’ve been growing and most people grow isn’t even a Christmas cactus, it’s a Thanksgiving cactus. There are three so-called holiday cacti – Thanksgiving cactus (Schlumbergera truncate), Christmas cactus (Schlumbergera x buckleyi) and Easter cactus (Rhipsalidopsis gaertneri). The names all relate to the time of year that they blossom. The Easter cactus is easy to tell apart from the other two given when it blooms.
But I was shocked to learn that most of what is sold and grown as Christmas cacti are actually Thanksgiving cacti. Both are of the same genus but are different species and there are two ways to tell them apart. The flattened stem segments of a Thanksgiving cactus have pointed lobes on the margins while the true Christmas cactus has rounded lobes. The second identifying feature is seen in the flowers. The anthers, the pollen bearing structures in the flowers, are yellow in a thanksgiving cactus and a purple color in a Christmas cactus.
The cactus in my window has pointed lobes on its flattened stems (holiday cactus don’t have leaves, they have modified stems) and the anthers are yellow. In other words, my Christmas cactus is really a Thanksgiving cactus! No wonder it blossoms in late November.
But then I looked into the issue of what makes a Thanksgiving cactus blossom. Come to find out there are two factors, and one of them can override the other.
All holiday cactus are photoperiodic plants. Long nights of uninterrupted darkness will cause them to set blooms. One way to get these plants to blossom is to put them into a dark place at 5pm and then bring them back into the light in the morning.
I never worry about the light and my cactus blooms without fail – in fact it blossoms two or three times a winter. It’s never had uninterrupted darkness. But it still blossoms even without addressing its photoperiodic needs because there’s another factor that can override the plants photosensitivity and that’s temperature. If the plant is growing in temperatures between 55 – 60 degrees F, it’ll blossom regardless of the light.
I don’t like a warm house and during the fall and winter the house is usually in the low to mid 60s. I also keep my Thanksgiving cactus near a window so it’s even cooler. Cool temperatures trigger blossoming and this explains why my cactus blossoms every November as well as a few other times during the winter.
There are lots of other interesting facts about this cactus but for now I’m content just sharing these things that I learned. Your Christmas cactus is probably a Thanksgiving cactus. A sustained period of darkness is necessary for blooming if your house is warm. Cool temperatures alone are enough to get a Thanksgiving or Christmas cactus to bloom.
As I write this I think of the scene in the TV movie “The Homecoming” where the mother goes into the basement and brings up a blooming cactus. Her trick might be a good one to get a cactus to bloom if you keep your house warm. If you have an unheated basement, put your Thanksgiving cactus in a window there and instead of worrying about the amount of darkness, let the cool temperatures bring this great houseplant into bloom.
At least now I know what I’m growing and I have an explanation for it keeps blossoming over and over!