Christmas cactus is one of my favorite holiday plants. It’s easy to grow but has special needs in order to flower.
In order to cause a Christmas cactus to flower satisfactorily, you must carefully mange the temperature and/or the amount of light it receives. Christmas cactus, being a short-day plant like poinsettia and chrysanthemum, responds to the shorter days and cooler temperatures of fall in order to stimulate flower production.
The flower buds will form regardless of day length when nighttime temperatures are 50 to 55 degrees F. But when night temperatures are 60 to 65 degrees F, the flower buds seldom form at night temperatures above 70 degrees F. After flower buds are well developed, they will bloom at normal room temperature. Temperatures above 90 degrees F may cause flower buds to drop from the plant.
To assure lots of flowers, I have found it easiest to simply place the plant outdoors during fall so it will be exposed to cooler temperatures. Outdoors, place the plant in a lightly shady area. Provide protection or temporarily bring the plant indoors during high winds or heavy rain. But most importantly, do not leave your plant outside when a frost or freeze is expected. I made that mistake once and lost a nice Christmas cactus as a result
If you have to keep your Christmas cactus indoors exposed to normal room temperature, you can force flower buds by providing the correct amount of dark period. If the night temperature is 55 to 70 degrees F, your cactus will need 13 hours of uninterrupted darkness per day to set buds. But if you keep you house toasty at night (70 to 85 degree F), your cactus will require more darkness — about 15 hours in every 24
To provide this extra long night treatment, place your plant in a closet or underneath a box, large bag, etc. It’s important that the closet or covering is lightproof and avoid a closet that is opened frequently. Any interruption in this dark treatment may result in poor flower bud initiation for the holidays. It’s also important to not leave the plant in the dark constantly. It will need some light during each 24-hour period. Consistently provide a longer night period.
Temperature control and short day treatment should be continued until buds are well developed — for six to nine weeks.