Two things set buds on a holiday cactus, and a warm room with a lamp on kills both of them. The plant needs 14 to 16 hours of unbroken darkness every night, plus nights that stay below 70°F (21°C). Miss either one and you get healthy green segments and nothing else. A Christmas cactus bloom is a response to fall conditions, not a reward for feeding or repotting.
The spare room at the back of my house smells like cold plaster from October on, because the radiator in there has been shut off for years. Three weeks after a leggy plant went in there, I noticed dozens of pink buds on segments that had sulked in the warm kitchen for two seasons. Nothing in my own routine had changed except the air around the pot. Iowa State University Extension puts the ceiling at 70°F (21°C), above which flowering stops whatever the day length is doing, and calls 60 to 70°F (16 to 21°C) the range you want.
Holiday cacti are short-day plants. The signal is the length of the night rather than the length of the day, so the dark stretch has to run unbroken. Most need 14 to 16 hours of it, though some cultivars set buds on as little as 12 hours. A hallway light flicked on for one minute at 10 p.m. resets the count, and a week of that keeps the plant blank while it still looks perfectly healthy.
You do not need a grow tent for any of this. A spare room with the door shut works, and so does a cupboard, as long as nobody opens it at night. Watch the segment tips while you wait, because a Christmas cactus bloom always starts there. Those flat green pads are segments called phylloclades, they do the plant's photosynthesis, and flowers come from nowhere else.
Cold does part of the job for you. Some Christmas and Thanksgiving cacti flower at 55°F (13°C) in fall no matter how long the nights run. That is why a plant on a chilly landing blooms while the one beside the television sits there green. Easter cactus asks for more. It wants 14 to 16 hours of darkness at 60 to 70°F (16 to 21°C) for 3 to 4 weeks, then a cool spell at 47 to 55°F (8 to 13°C) for 4 to 6 weeks, then longer days again.
Good holiday cactus care splits off from desert cactus care right at the watering can. Desert types want a dry rest. These do not, and the soil should stay evenly moist while the plant is budding and in flower. Let the pot go bone dry in December and you can lose the flowers you already grew.
Once buds appear, stop handling the plant. Bud drop is what you get for carrying it to the front window so guests can see it, and the buds land on the carpet within a few days. Leave it exactly where it set buds until the first flowers open. After that you can put it wherever you like.
Hold off on any change of position until the first flowers open. Moving a budding plant early is the usual cause of bud drop on a cactus that was doing fine a week ago.
Check what you actually own before you blame yourself. Most plants sold as Christmas cactus are Thanksgiving cactus or hybrids of the two. Thanksgiving segments carry 2 to 4 upward-pointing teeth along each edge, while true Christmas cactus segments are scalloped and smooth. Match the routine to the plant in your hands, give it cool dark nights, and you get the Christmas cactus bloom you waited all fall for.
Read the full article: Cacti Are Not Hard to Kill, Just Hard to Read
