Do we know why Christmas cactuses are blooming like crazy right now? Several friends and my sister are experiencing this.
— K.F., Chicago
I was stumped by this question, so I reached out to one of the Chicago Botanic Garden’s floriculturists for an answer. She said the blooming plants may be Thanksgiving cacti (Schlumbergera truncate) that flower earlier than Christmas cacti (Schlumbergera x buckleyi). Thanksgiving cacti typically flower from the middle of October until late December but will also flower at other times especially if they are grown indoors under an artificial light source. The Christmas cactus typically flowers from early December to late January. The Thanksgiving cactus, which is commonly sold, has stem segments or sections that have edges that resemble crab claws. The flowers are more upright. The Christmas cactus has stem segments that are flattened and has smooth, scalloped edges that are not toothed. Its flowers tend to hang down.
It is likely that the plants in question are Thanksgiving cacti. The plants could be flowering earlier than normal due to having ideal night temperatures. Photoperiod, or the duration of light, and temperature control the flowering of these plants. Flower initiation will begin when temperatures drop below 57 degrees, regardless of photoperiod. When temperatures are higher, as in a typical home environment of 60 to 75 degrees, short-day treatment is needed for flower initiation. For plants being grown at 65 to 72 degrees, the critical period of darkness required each night is 12.5 to 14 hours. Interrupting the dark period with even a brief, two-hour light exposure can prevent flowers from developing. To encourage flowering, ensure the plant remains in long nights for about six weeks.
There is also an Easter cactus (Rhipsalidopsis gaertneri) which has rounded stem segments with scalloped edges. The segments have a hairy covering at the tip of each segment and often have a red edge to them. The Easter cactus has flowers that are much smaller than the Thanksgiving cactus and typically blooms from March to April.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.
